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thesis entitled

POSTEMERGENCE WEED CONTROL IN SOYBEAN
[Glycine max (L.) Merr] WITH IMAZAMOX, IMAZETHAPYR,
OXASULFURON, AND CLORANSULAM-METHYL

 

presented by

Kelly Allan Nelson

has been accepted towards fulfillment
of the requirements for

Master's Crop and Soil Sciences

degree in

 

 

KW am

Major professor

Date affix 291/997

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POSTEMERGENCE WEED CONTROL IN SOYBEAN
[Glycine Max (L.) Merr] WITH IMAZAMOX, IMAZETHAPYR,
OXASULFURON, AND CLORANSULAM-METHYL
By

Kelly Allan Nelson

A THESIS
Submitted to
Michigan State University
in partial fulfillment of the requirements
for the degree of

MASTER OF SCIENCE

Department of Crop & Soil Sciences

1997

ABSTRACT
POSTEMERGENCE WEED CONTROL IN SOYBEAN
[Glycine Max (L.) Merr] WITH IMAZAMOX, IMAZETHAPYR,
OXASULFURON, AND CLORANSULAM-METHYL
By

Kelly Allan Nelson

Imazamox, imazethapyr, oxasulfuron, and cloransulam—methyl controlled
velvetleaf, Wild mustard, and common chickweed. Imazamox and imazethapyr
controlled redroot pi gweed and eastern black nightshade. Irnazamox controlled common
lambsquarters, While cloransulam-methyl controlled common ragweed. Selected tank
mixtures enhanced grass and broadleaf weed control by these herbicides, but antagonized
weed control or increased soybean response in certain instances. Diphenyl ether tank
mixtures With imazamox and imazethapyr increased common ragweed control, but
increased giant foxtail tillering and antagonized giant foxtail control by imazethapyr more
than imazamox. Grass antagonism was species and graminicide dependent for
oxasulfuron. Giant foxtail control by clethodim was antagonized by imazethapyr.
Methylated seed oil increased weed control with imazamox and imazethapyr compared to
nonionic surfactant. Imazamox at 45 g/ha plus a methylated seed oil c0ntrolled a broad
spectrum of weeds in narrow and Wide row soybeans. Herbicide treatments reduced

weed biomass more in narrow than wide row soybeans.

ACKNOWLEDGMENTS

An enormous thank you is extended to Dr. Karen Renner for making this degree
possible, guiding my thoughts, and challenging me With new ideas and research
perspectives. I would like to thank Dr. Donald Penner for his help, support, and Wisdom.
I am grateful for the motivation and friendship fiom Dr. Larry Olsen and want to thank
Dr. J irn Kells for the chance to teach a wonderful class of students.

My family and friends deserve my praise for their love and support. An
extraordinary thanks goes to my wife, Teak, for her confidence, encouragement,
dedication, and enthusiasm. My parents deserve a particular thanks for all of the
experiences they challenged me With as well as their assistance, support, and motivation.
I would like to thank Ken and Barbara Nelsen for their encouragement, Gary Powell for
all of his technical help, and Mike Partika for all of his assistance. A special thanks is
extended to Andy Chomas, James Mickelson, Julie Lich, Jason Fausey, Karen Novosel,
Brent Tharp, Corey Ransom, Christy Sprague, Matt Rinella, Paul Knoerr, Eric Spandl,
John Burk, Jim Sherman, Marla VanWormer, Kyle Fiebig, and Angie Eichom for their

assistance and support through the various aspects of my graduate research career.

iii

TABLE OF CONTENTS

LIST OF TABLES .................................................... vii

LIST OF FIGURES ..................................................... x

INTRODUCTION ...................................................... 1
CHAPTER 1

REVIEW OF LITERATURE ............................................. 3

INTRODUCTION ................................................. 3

HERBICIDE NflXTURES ........................................... 5

ALS-INHIBITING HERBICIDES .................................... 7

Background ................................................ 7

Irnidazolinone .............................................. 8

Sulfonylurea ................................................ 9

Triazolopyrimidine sulfonanilide ............................... 11

HERBICIDES ................................................... 1 1

Imazethapyr ............................................... 1 1

Irnazamox ................................................. 14

Irnazamox and Imazethapyr Comparison ........................ 17

Oxasulfuron ............................................... 19

Cloransulam-methyl ......................................... 21

ACCase-Inhibitors .......................................... 23

Diphenyl ethers ............................................ 25

HERBICIDE ANTAGONISM ...................................... 27

Background ............................................... 27

Diphenyl ether Antagonism ................................... 29

ALS-Inhibitors and Graminicide Antagonism ..................... 33

ADJUV ANT S ................................................... 39

ENVIRONMENTAL EFFECTS ON HERBICIDE EFFICACY ............ 40

ROW SPACING AND WEED COMPETITION ........................ 43

LITERATURE CITED ............................................ 46

iv

TABLE OF CONTENTS (cont)

CHAPTER 2
POSTEMERGENCE WEED CONTROL WITH OXASULFURON AND
CLORANSULAM-METHYL IN SOYBEAN S [Glycine max (L.) Merr] ......... 66
ABSTRACT ..................................................... 66
INTRODUCTION ................................................ 69
MATERIALS AND METHODS ..................................... 71
Field Experiments .......................................... 71
General Methods for Greenhouse Experiments .................... 73
Broadleaf weed sensitivity study ......................... 74
Giant foxtail antagonism study .......................... 74
Large crabgrass, j ohnsongrass, and corn interaction study ..... 75
Statistical Protocol .......................................... 75
RESULTS AND DISCUSSION ..................................... 76
Field Experiments .......................................... 76
Broadleaf weed sensitivity study ............................... 77
Giant foxtail antagonism ..................................... 78
Large crabgrass, johnsongrass, and corn interaction study ........... 78
LITERATURE CITED ............................................ 80
CHAPTER 3
WEED CONTROL IN SOYBEAN [Glycine max (L.) Merr] WITH IMAZAMOX
AND IMAZETHAPYR ................................................. 89
ABSTRACT ..................................................... 89
INTRODUCTION ................................................ 92
MATERIALS AND METHODS ..................................... 96
Field Experiments .......................................... 96
General Methods for Greenhouse Experiments .................... 98
Weed species sensitivity and adjuvant study ................ 99
Influence of adjuvants on giant foxtail control ............. 100
Giant foxtail antagonism studies ........................ 100
Clethodim antagonism by imazamox and imazethapyr . 101
Irnazarnox and imazethapyr antagonism by the diphenyl
ether herbicides ......................... 101
Adj uvant effects on diphenyl ether antagonism ....... 102
Sodium acetate effects on giant foxtail control With
imazethapyr and imazamox ................ 102
Soybean injury study ................................. 102
Statistical Protocol ......................................... 103
RESULTS AND DISCUSSION .................................... 104
Field experiments .......................................... 104

TABLE OF CONTENTS (cont)

Weed species sensitivity and adjuvant study ..................... 107
Influence of adjuvants on giant foxtail control ................... 108
Clethodim antagonism by imazamox and imazethapyr ............. 108
Imazarnox and imazethapyr antagonism by the diphenyl ether
herbicides .......................................... 109
Adj uvant effects on diphenyl ether antagonism ................... 109
Sodium acetate effects on giant foxtail control with imazethapyr and
imazamox .......................................... 1 10
Soybean injury study ....................................... 111
LITERATURE CITED ........................................... 1 13
CHAPTER 4
WEED CONTROL IN WIDE AND NARROW ROW SOYBEAN [Glycine max (L.)
Merr] WITH IMAZAMOX, IMAZETHAPYR, AND OXASULFURON ....... 134
ABSTRACT .................................................... 134
INTRODUCTION ............................................... 136
MATERIALS AND METHODS .................................... 138
RESULTS AND DISCUSSION .................................... 142
LITERATURE CITED ........................................... 148
SUMMARY ......................................................... l 56

vi

LIST OF TABLES

CHAPTER 1
REVIEW OF LITERATURE

Table I: The ALS-inhibiting herbicides are classified by their chemical family
and a description of their major use. .................................. 63

CHAPTER 2
POSTENIERGENCE WEED CONTROL WITH OXASULFURON AND
CLORANSULAM-METHYL IN SOYBEAN [Glycine max (L.) Merr]

Table I . Weed height and leaf number for field experiments conducted in 1995
and 1996. ....................................................... 83

Table 2. Broadleaf weed control in the field with oxasulfuron and cloransulam-
methyl alone and with selected tank mixtures ............................ 84

Table 3. Giant foxtail control by oxasulfuron and cloransulam-methyl alone and
tank mixed with clethodim and quizalofop .............................. 85

Table 4. Weed control by oxasulfuron and cloransulam—methyl in greenhouse
experiments 28 days after treatment. .................................. 86

Table 5. Giant foxtail control by oxasulfuron alone and tank mixed with

grarninicides 21 days after treatment. .................... f ............. 87

Table 6. J ohnsongrass, large crabgrass, and corn control by oxasulfuron,

quizalofop, and clethodim alone and in tank mixtures. .................... 88
CHAPTER 3

WEED CONTROL IN SOYBEAN [Glycine max (L.) Merr] WITH IMAZAMOX
AND IMAZETHAPYR

Table 1. Weed height and leaf number at the time of herbicide application for
field experiments conducted in 1995 and 1996. ........................ 117

vii

LIST OF TABLES (cont)

Table 2. Soybean injury 7 and 14 days afier treatment by imazamox and
imazethapyr alone and when tank mixed with other herbicides in 1995
and 1996. ...................................................... 118

Table 3. Common ragweed control by imazamox and imazethapyr alone and
when tank mixed with other herbicides. .............................. 119

Table 4. Giant foxtail control by imazamox and imazethapyr alone and when tank
mixed with other herbicides. ....................................... 120

Table 5. Common lambsquarters control by imazamox and imazethapyr alone
and when tank mixed with other herbicides. ........................... 121

Table 6. Yellow nutsedge control by imazamox and imazethapyr 8 and 56 weeks
after treatment. .................................................. 122

Table 7. Soybean yield in plots treated with imazamox or imazethapyr alone and
when tank mixed with other herbicides in 1995 and 1996. ................ 123

Table 8. Soybean injury, common ragweed control, giant foxtail control, common
lambsquarters control, and soybean yield in the field following an application of
imazamox and imazethapyr alone and tank mixed with the diphenyl ether
herbicides in 1996. ............................................... 124

Table 9. Common ragweed, common lambsquarters, velvetleaf, and yellow
nutsedge control by imazamox and imazethapyr applied nonionic surfactant and

methylated seed oil in the greenhouse 28 days after treatment .............. 125

Table 10. The effect of 28% urea ammonium nitrate or nonionic surfactant on
giant foxtail control by imazamox and imazethapyr 21 days after treatment. . . 126

Table 11. Giant foxtail control by imazamox and imazethapyr tank mixed with
clethodim 21 days afier treatment. ................................... 127

Table 12. Giant foxtail control by imazamox and imazethapyr tank mixed with
the diphenyl ethers 21 days after treatment. ........................... 128

Table 13. The effects of adjuvants on giant foxtail control by imazamox and
imazethapyr tank mixed with the diphenyl ethers 21 days after treatment. . . . . 129

Table 14. The effect of sodium acetate on giant foxtail control with imazamox
and imazethapyr. ................................................ 130

viii

LIST OF TABLES (cont)

CHAPTER 4
WEED CONTROL IN WIDE AND NARROW ROW SOYBEAN [Glycine max (L.)
Merr] WITH IMAZAMOX, IMAZETHAPYR, AND OXASULFURON

Table I . Weed height, leaf number, and population density for field experiments
conducted at East Lansing and Clarksville, MI. ........................ 150

Table 2. Soybean injury by imazamox, imazethapyr, and oxasulfirron tank
mixtures 7 and 14 days after treatment at East Lansing and Clarksville, MI. . . 151

Table 3. Common ragweed, common lambsquarters, giant foxtail, and eastern
black nightshade control at East Lansing and Clarksville 28 days after
treatment. ...................................................... 152

Table 4. Redroot pigweed control 28 and 56 days after treatment at East Lansing
and Clarksville in wide and narrow row soybeans. ...................... 153

Table 5. Common ragweed and common larnbsquarters control, total weed

biomass reduction, soybean biomass reduction, and soybean yield in Wide and
narrow row soybeans. ............................................ 154

ix

LIST OF FIGURES

CHAPTER 1
REVIEW OF LITERATURE

Figure 1. The imidazolinone skeleton consists of an acid, backbone, and
imidazolinone ring. ............................................... 64

Figure 2. The sulfonylurea structure includes a phenyl ring, sulfonylurea bridge,

and heterocycle moeity ............................................. 64
Figure 3. The triazolopyrimidine sulfonanilide herbicide skeleton ........... 65
CHAPTER 3

WEED CONTROL IN SOYBEAN S [Glycine max (L.) Merr] WITH IMAZAMOX
AND IMAZETHAPYR

Figure 1 a. Soybean injury by imazamox and imazethapyr applied with a nonionic

surfactant or a methylated seed oil 7 DAT ............................. 131

Figure I b. Soybean dry weight reduction by imazamox and imazethapyr applied

with a nonionic surfactant or a methylated seed oil 21 DAT. .............. 132

Figure 1c. Soybean leaf area reduction by imazamox and imazethapyr applied

with a nonionic surfactant or methylated seed oil 21 DAT. ............... 133
CHAPTER 4

WEED CONTROL IN WIDE AND NARROW ROW SOYBEAN S [Glycine max
(L.) Merr] WITH IMAZAMOX, IMAZETHAPYR, AND OXASULFURON

Figure I . The percent of total weed biomass 56 days after treatment for wide and
narrow row herbicide treatments ..................................... 155

INTRODUCTION

Imazamox, imazethapyr, oxasulfuron, and cloransulam-methyl are selective
herbicides that inhibit acetolactate synthase (ALS) in susceptible weed species. ALS is
an enzyme necessary for the synthesis of leucine, isoleucine, and valine. Imazamox,
oxasulfuron, and cloransulam—methyl are new herbicides for postemergence weed control
in soybeans. Irnazarnox is an imidazolinone herbicide that was introduced by American
Cyanamid in 1994. Oxasulfiiron is a sulfonylurea herbicide that was discovered by Ciba-
Geigy in the early 1990's. Cloransulam-methyl is a triazolopyrimidine sulfonanilide
herbicide that was introduced by DowElanco in 1994.

Postemergence herbicides can provide effective weed control in soybeans.
Postemergence herbicides have facilitated the increase in conservation tillage programs
that reduce soil erosion and the transition ficm wide to narrow row soybeans that
efficiently harvest sunlight for crop production. Tank mixture decisions for
postemergence herbicides depend primarily upon weed pressures, weed species and size,
resistance management strategies, rotational crop restrictions, interactions between
herbicides, and cost effectiveness. Ideally, a herbicide tank mixture broadens or
complements the weed control spectrum of one or more of the herbicides present, but

may increase crop injury or antagonize weed control by one or more of the herbicides in

2

the mixture. Antagonistic mixtures reduce weed control, reduce crop yields, and increase
weed seed production. A correct adjuvant selection can increase weed control and reduce
antagonistic herbicide interactions.

Imazethapyr was applied to 44% of the total soybean acres in the United States in
1995; however, rotational crop restrictions prevent its use in many geographical areas.
Current rotational crop restrictions for sensitive crop species like sugarbeets are 12, 18,
and 24 months for oxasulfuron, imazamox, and cloransulam-methyl, respectively. These
herbicides may offer postemergence weed control Options for producers with such crops
in their rotation. Field and greenhouse research were initiated: to determine soybean and
weed sensitivity to postemergence applications of these ALS-inhibiting herbicides
applied alone and with selected tank mixtures, to determine the influence of adjuvants on
soybean injury and weed control, to evaluated monocot antagonism with selected tank

mixtures, and to evaluate these herbicides in narrow and wide row soybeans.

CHAPTER 1

REVIEW OF LITERATURE

INTRODUCTION

Soybeans [Glycine max (L.) Merr] are a major food and oil-bearing seed crop.
Soybeans were first domesticated in the Orient and have been used as a food source for
centuries. In 1995 and 1996, Michigan produced 59.6 and 46.7 million bushels of
soybeans from 1.5 and 1.6 million planted acres, respectively (Michigan 1996; Michigan
1997). Illinois, the leading soybean producer in the United States, planted over 9.7
million acres (USDA 1996). Weed control is essential for efficient and profitable
soybean production. Postemergence herbicides have facilitated conservation tillage
programs as an effective method to reduce soil erosion.

Herbicides were used on 51.8 million soybean acres in 1995 which accounted for
approximately 97% of the soybean acres planted in the major soybean producing states of
the United States (USDA 1996). The top five soybean herbicides in the order of
popularity for the eight major soybean producing states in 1995 were imazethapyr (2-
[4,5-dihydro-4-methyl-4-(l-methy1ethyl)-5-oxo-1H-imidazol-Z-yl]-5-ethyl-3-
pyridinecarboxylic acid), pendimethalin (N-(l-ethylpropyl)-3,4-dimethyl-2,6-

dinitrobenzenamine), glyphosate (N-(phosphonomethyl)glycine), trifluralin (2,6-dinitro-

4
N,N-dipropyl-4-(trifluoromethyl)benzenamine), and irnazaquin (2-[4,5-dihydro-4-methyl-
4-(1-methylethyl)-5-oxo-1H-imidazol-Z-yl]-3-quinolinecarboxylic acid) (USDA 1996).
Imazethapyr was applied to 37, 42, and 44 percent of the total treated acres in 1993, 1994,
and 1995, respectively (USDA 1996). A total of 2.3 million pounds active ingredient of
the acetolactate synthase (ALS)-inhibiting herbicides were applied in sixteen major
soybean producing states in 1995 (USDA 1996). The introduction of crops resistant to
non-selective herbicides and increased weed resistance will revolutionize herbicide usage
patterns for soybeans as well as other crops in the next few years.

The ALS-inhibiting herbicides have provided effective weed control at low
herbicide application rates. Herbicide mixtures have been used to broaden the spectrum
of weeds controlled by the ALS-inhibiting herbicides. These mixtures have been both
beneficial and detrimental in the case of herbicide antagonism.

The number of herbicide mixtures available to soybean producers is numerous. A
mix and match approach can be quite effective depending upon weed pressures, weed
species, weed sizes, and rotational crops. The introduction of new herbicides provides a
producer another opportunity and decision regarding weed management. These decisions
depend upon effective screening and recommendations from industry researchers,
university extension, and university research. The major decisions for an integrated weed
management program should include cost effectiveness, convenience, crop and herbicide

rotation strategies, and environmental consciousness.

5

HERBICIDE MIXTURES

A herbicide mixture is a combination of two or more components which retain
their own characteristics while inhibiting or ending plant growth. There are several
benefits and risks associated with herbicide mixtures. The dilemma for researchers is the
interaction between benefits and risks. For instance, herbicide tank mixtures have been
used to broaden or enhance the weed control spectrum, avoid herbicide resistance, reduce
the amount of one or both of the herbicide components, and ultimately reduce production
costs. Nonetheless, there is a potential for increased crop injury, weed control
antagonism by either herbicide, and increased selection for herbicide resistance when
similar modes of action are utilized. Tank mixture evaluations are utilized to determine
optimum weed control and solve our dilemma so producers will have a net positive
benefit from these mixtures.

An ideal herbicide mixture fills the gaps of an imperfect weed control system.
Postemergence tank mixtures have reduced production costs by decreasing herbicide use
rates. These mixtures reduced the number of field passes and herbicide applications
compared to preplant incorporated or split applications which caused less soil compaction
and crop damage. An ideal herbicide mixture not only increases weed control but also
reduces crop injury (Friesen 1979; Malefyt and Duke 1981).

Several herbicides have been used to enhance the control of specific weed
problems. For instance, thifensulfuron (methyl 3-[[[[(4-methoxy-6-methyl-1,3,5-triazin-
2-yl)amino]carbonyl]amino]sulfonyl]-2-thiophenecarboxylic acid) has been tank mixed

with several postemergence herbicides for common larnbsquarters (Chenopodium

6

album L.) control (Fielding and Stoller 1990; Green 1991; Monks et al. 1993). Common
ragweed (Ambrosia artemisiifolia L.) has been controlled with a diphenyl ether herbicide
mixture (Hoverstad 1996a, 1996b; Leif and Taylor 1993; Monks et a1. 1993; Wilcut
1991). Gaeddert et a1. (1996) tank mixed lactofen ((i)-2-ethoxy-1-methyl-2-oxoethy1 5-
[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoate) with imazethapyr to control
ALS-resistant palmer amaranth (Amaranthus palmeri S. Wats.). However, herbicide
mixtures may increase soybean injury (Bauman et a1. 1996; Geier and Stahlrnan 1996).
Simpson and Stoller (1996) observed increased soybean injury when imazethapyr was
tank mixed with thifensulfuron at 4.4 g ai/ha on sulfonylurea tolerant soybeans compared
to imazethapyr or thifensulfuron alone. The combination of a graminicide with a
broadleaf herbicide should provide broad spectrum weed control, but numerous mixtures
have resulted in weed control that was less than when the graminicide was applied alone
(Cantwell et a1. 1989; Ditrnarsen et a1. 1997; Harvey et a1. 1995; Myers and Coble 1992).
In addition, the diphenyl ether herbicides have antagonized giant foxtail (Setariafaberi
Herrm.) control by imazethapyr (Bauman et a1. 1996; Cantwell et a1. 1989; Geier and
Stahlman 1996; Hoverstad 1996a, 1996b). An antagonistic mixture is expensive. It
reduces weed control, reduces crop yield, and increases weed seed production. Critical
evaluations of herbicide mixtures are needed for Wise weed control management
decisions that avoid increased soybean injury and antagonistic herbicide mixtures.

The correct surfactant or adjuvant selection can affect soybean injury, weed

control, and help overcome herbicide interactions. This important component of the

7

herbicide rrrixture needs continual evaluation for the most beneficial combination. It
provides another factor to the weed control design and options.

New herbicides and herbicide resistant crops have exponentially increased the
number of herbicide combinations available. Extensive research has been compiled on
the interaction of sethoxydim (2[ l -(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-
hydroxy-2-cyclohexen-l-one) with postemergence soybean herbicides, but the
introduction of sethoxydim resistant corn has challenged researchers to keep in mind the
possibilities for cross-crop herbicide combinations. This technological option provides

greater herbicide and crop rotation options as well as improved management decisions.

ALS-INHIBITING HERBICIDES
Background. The ALS-inhibiting herbicides include the imidazolinone, sulfonylurea,
triazolopyrimidine sulfonanilide, and pyrimidinyl thiobenzoate herbicide families. These
herbicides are used for weed control in several crops and non-crop settings (Table l).
The ALS-enzyme catalyzes the condensation of two pyruvate molecules to form a-aceto-
lactate and the condensation of one pyruvate and one a-ketobutyrate to form a-aceto-a-
hydroxybutyrate which are necessary for leucine, isoleucine, and valine synthesis
(Schloss 1990). Leucine and valine are derivatives of a-aceto-lactate, and isoleucine is a
derivative of a-aceto-a-hydroxybutyrate (Schloss 1990). ALS-inhibiting herbicides
block the synthesis of branched chain amino acid synthesis which disrupts and prevents

cell division in the meristematic tissue of susceptible plants and results in slow plant

8

death. Low mammalian toxicity has been related to the incapability of mammals to
synthesize branched chain amino acids since they lack ALS-enzymes.

Metabolic detoxification is the primary selection difference between susceptible
and non-susceptible species for ALS-inhibiting herbicides developed for crop production.
The metabolic detoxification mechanism is herbicide and plant species dependent (Brown
1990; Brown et al. 1990). However, absorption, translocation, and sensitivity of the site
of action can affect herbicidal activity on sensitive and marginally controlled plant
species (Hinz and Owen 1996). Slight structural changes, isomeric or substitution, have
drastically affected the herbicidal activity of a compound (Hill et al. 1996; Ladner 1991).
This has been an important feat for researchers to identify and determine the most active
and safest compounds.

Weed resistance has plagued the ALS-inhibiting herbicides. Imazethapyr
resistance has been reported in smooth pigweed (Amaranthus hybridus L.), kochia
(Kochia scoparia (L.) Schrad.), common waterhemp (Amaranthus rudis Sauer), palmer
amaranth, prickly lettuce (Lactuca serriola L.) (F riesen et a1. 1993; Gaeddert et al. 1996;
Lovell et al. 1996; Mallory-Smith et al. 1990; Schmenk et al. 1996), and numerous other
plant species. Some weed species have cross-resistance to several sulfonylurea
herbicides (Friesen et al. 1993; Gaeddert et a1. 1996; Lovell et a1. 1996; Mallory-Smith et
al. 1990; Schmenk et al. 1996).

Imidazolinone. The imidazolinone herbicides include irnazamethabenz ((:)-2-[4,S-
dihydro-4-methyl-4—(1-methylethyl)-5-oxo-1H—imidazol-2-yl]-4(and 5)-methylbenzoic

acid (3:2)), imazamox (2-(4-isopropyl-4-methyl-5-oxo-2-imidazolin-2-yl)-5-

9
(methoxymethyl) nicotinie acid), irnazapyr((:)-2-[4,5-dihydro-4-methyl-4-(1-
methylethyl)-5-oxo-lH—imidazol-Z-yl]-3-pyridinecarboxylic acid), irnazaquin, and
imazethapyr. Ladner (1991) described the imidazolinone herbicide structure as a three
part entity consisting of an acid equivalent, backbone, and imidazolinone ring.
Imidazolinone ring termination or the conversion to an immobile structure, and alkyl
hydroxylation followed by glucose conjugation are the primary mechanisms of
metabolism (Figure 1) (Berhane et al. 1993; Shaner and Mallipudi 1991). Wild oats
(Avenafatua L.) must de-esterfy irnazamethabenz to the acid equivalent before it has
herbicidal activity (Shaner and Mallipudi 1991). Typical use rates for the imidazolinones
range from 35 to 530 g ai/ha depending upon the weed species, application timing, and
crop or industrial use.

Slight changes in the structural configuration have affected the efficacy of the
irnidazolinones to sensitive plant species. The methyl and isopropyl isomers of irnazapyr
have nearly a two-fold difference in herbicidal activity (Ladner 1991). The
imazamethabenz-methyl formulation is a mixture of the meta and para 5-methyl. The
meta isomer is very active on wild oat, while the para isomer is very active on Wild
mustard (Sinapis arvensis L.) (Ladner 1991; Shaner and Mallipudi 1991).
Sulfonylurea. The sulfonylurea herbicides include bensulfuron (2-[[[[[(4,6-dimethoxy-
2-pyrirnidinyl)amino]carbonyl]amino]sulfonyl]methyl]benzoic acid), chlorimuron (2-
[[[[(4-chloro-6-methoxy-2-pyrimidiny1)amino]carbonyl]amino]sulfonyl]benzoic acid),
chlorsulfuron (2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-

yl)amino]carbonyl]benzensulfonamide), ethametsulfiiron (2-[[[[[4-ethoxy-6-

10
(methylamino)-1,3,5-triazin-2-yl]amino]carbonyl]amino]sulfonyl]benzoic acid),
metsulfuron (2-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-
y1)amino]carbonyl]amino]sulfonyl]benzoic acid), nicosulfuron (2-[[[[(4,6.dimethoxy-2-
pyrimidinyl)amino]carbonyl]amino]sulfony1]-N,N-dimethyl—3-pyridinecarboxarnide),
oxasulfuron, (2[[[[(4,6-dimethyl—2-pyrimidinyl)-amino]-carbonyl]-amino]sulfonyl]
benzoic acid, 3-oxetanyl ester (CAS)), primisulfuron (2-[[[[[4,6-bis(difluoromethoxy)-2-
pyrimidinyl]amino]carbonyl]amino]sulfonyl]benzoic acid), sulfometuron (2-[[[[(4,6-
dimethyl-2-pyrimidinyl)amino]carbonyl]amino]sulfony1]benzoic acid), thifensulfuron,
triasulfuron (2-(2-chloroethoxy)-N-[[(4-methoxy-6-methyl-1,3,5-triazin-2-
yl)amino]carbonyl]benzenesulfonamide), tribenuron (2-[[[[(4-methoxy-6-methyl-l,3,5-
triazin-2-y1)methylamino]carbonyl]amino]sulfonyl]benzoic acid), and triflusulfiiron (2-
[[[[[4-(dimethylamino)-6-(2,2,2-trifluoroethoxy)-1 ,3 ,S-triazin-Z-
yl]amino]carbonyl]amino]sulfonyl]-3—methylbenzoic acid). The sulfonylureas are used
for weed control in numerous crops and non-crop settings (Table 1). The sulfonylurea
herbicide structure consists of a phenyl ring, sulfonylurea bridge, and heterocycle moiety
(Figure 2) (Hay 1990). Metabolism depends on the plant species and the herbicide. The
sulfonylureas have been metabolized through aryl and aliphatic hydroxylation followed
by subsequent glucose conjugation, homoglutathione conjugation, de-esterification,
oxidative O-demethylation, and hydrolysis or cleavage of the bridge (Brown 1990;
Brown et al. 1990). In water, the sulfonylureas may undergo hydrolysis which cleaves
the bridge and heterocycle which then releases carbon dioxide, or form an anionic salt in

a neutral or basic solution (Hay 1990). At a higher pH, there is less hydrolysis and the

11

sulfonylurea remains in a net anionic form (Hay 1990). This process is important in soil
persistence since hydrolysis is greater in acidic soils (Hay 1990). Typical use rates for
the sulfonylureas range from 4.2 to 420 g ai/ha depending upon the weed species,
application timing, and crop or industrial use.

Triazolopyrimidine sulfonanilide. The triazolopyrimidine sulfonanilides include
flumetsulam (N-(2,6-difluorophenyl)-5-methyl[ l ,2,4]triazolo[1,5—a]pyrimidine—2-
sulfonamide) and cloransularn-methyl (2-[[(5-ethoxy-7-fluoro[l,2,4]triazolo[1,5-
c]pyrimidin-2y1)sulfonyl]amino]-3-cloro-benzoic acid, methyl ester). Rapid metabolism
of the triazolopyrimidine sulfonanilides by grass species allows them to be less
susceptible than broadleaf plants due to homoglutathione conjugation (Hodges et al.
1990; J achetta et a1. 1995). Triazolopyrimidine sulfonanilides are primarily metabolized
by hydroxylation followed by subsequent glucose conjugation of the methyl substituent
or the aniline ring (Figure 3) (Hodges et al. 1990). Typical use rates range fi'om 17.5 to

78 g ai/ha depending upon the crop, weed species, and application timing.

HERBICIDES
Imazethapyr. Imazethapyr is an imidazolinone herbicide for preplant incorporated,
preernergence, and postemergence grass and broadleaf weed control in soybeans.
Postemergence applications have been recommended at 70 g ai/ha (Anonymous 3).
Extensive research has been conducted on imazethapyr with respect to rotational crop

safety, weed control, herbicide mixtures, and weed resistance.

12

Imazethapyr is a residual herbicide and has injured sugarbeets (Beta vulgaris L.)
up to 2 years after postemergence applications (Renner and Powell 1991). Moyer and
Esau (1996) reported that there was a yield loss potential for sugarbeet and potatoes
(Solanum tuberosum L.) planted 3 years after a postemergence application of imazethapyr
to dry beans (Phaseolus vulgaris L.). A 40 month rotational crop restriction is currently
recommended for sugarbeets afier a postemergence application of imazethapyr (Crop
Protection Reference 1996).

Imazethapyr controls a large spectrum of annual weeds (Anonymous a), but
marginally controlled species and resistant species must be taken into account for weed
control management decisions. Hager and Renner (1994) reported similar common
ragweed dry weight reduction by bentazon and imazethapyr 14 days after treatment
(DAT) in the field, but greater control was observed by bentazon in the greenhouse
compared to imazethapyr. Common ragweed is partially susceptible to postemergence
imazethapyr applications. Lateral and axillary buds initiate regrowth approximately 21
DAT (Ballard et al. 1996). Ballard et a1. (1996) reported that the relative growth rate of
common ragweed treated with imazethapyr decreased at a slow rate, but by 56 DAT it
increased. The relative growth rate and metabolism of imazethapyr was greater in
common ragweed than giant ragweed (Ambrosia trifida L.), but basipetal translocation
was less (Ballard et al. 1995). This explains the primary difference in control of these
two species by imazethapyr. Soil applied imazethapyr was more effective for common
lambsquarters control than postemergence applications, as postemergence applications

did not adequately control this problematic weed (Cantwell et al. 1989; Hoverstad 1996c;

13

Wilcut 1991). Nonetheless, smooth pigweed, palmer amaranth, prostrate pigweed
(Amaranthus blitoides S. Wats.), and redroot pigweed (Amaranthus retroflexus L.) were
controlled with imazethapyr (Arnold et al. 1993; Cantwell et al. 1989; Cole et al. 1989;
Klingaman et al. 1992). Cole et al. (1989) reported that the half-life of imazethapyr in
redroot pigweed was longer compared to soybean, peanut (Arachis hypogaea), sicklepod
(Cassia obtusifolia L.), and Florida beggarweed [Desmodium tortuosum (Sw.) DC.], and
control was related to this value. However, other Amaranthus species like common
waterhemp can tolerate several times the recommended normal use rates of imazethapyr
(Lovell et al. 1996). Velvetleaf (Abutilon theophrasti Medik.) was controlled by
imazethapyr and 70 g/ha was needed to maintain control (Cantwell et al. 1989;
Klingaman et a1. 1992). In addition, imazethapyr has suppressed yellow nutsedge
(Cyperus esculentus L.) growth in field and greenhouse studies. Yellow nutsedge
suppression has been evaluated up to 142 days after treatment (DAT) by Grichar et al.
(1992). Ackley et a1. (1996) evaluated ALS-inhibiting herbicides in the greenhouse and
had greater yellow nutsedge control with imazethapyr in the greenhouse than in the field.
This study compared ALS-inhibitors, but the surfactants were different depending on the
ALS-inhibiting herbicide used. Chlorimuron and halosulfuron (methyl 5- {[(4,6-
dimethoxy-2-pyrimidinyl)amino]carbonylaminosulfonyl}-3 -chloro-l-methyl-1-H-
pyrazole-4-earboxylate) controlled yellow nutsedge in the field, but there was no reported
difference between control by chlorimuron and imazethapyr in the greenhouse (Ackley et
al. 1996). However, Derr and Wilcut (1993) reported that chlorimuron reduced yellow

nutsedge fresh weight more than imazethapyr. Richburg et al. (1993) determined that

l4

foliar and soil or soil treatments alone were more effective in controlling nutsedge than
foliar applications only.

Imazamox. Imazamox (AC 299,263) is an imidazolinone herbicide for postemergence
grass and broadleaf weed control in soybeans (Anonymous 1995b). The structure of
imazamox was revealed in 1994 and the recommended postemergence rate was
established at 35 and 45 g ai/ha (Quakenbush et al. 1994). There has been a tendency for
greater control of common ragweed and common lambsquarters at the 45 than 35 g/ha
rate in Wide row soybeans (Ballard and Hellmer 1995).

Rapid metabolism of imazamox by soybeans is the basis for selectivity
(Anonymous 1995b). At 140 g ha, soybean heights were reduced and lower yields
observed compared to the 35 g/ha rate (Lueschen et a1. 1995). Microbes are primarily
responsible for the degradation of this herbicide in the soil (Quakenbush et al. 1994).
Current rotational crop restrictions recommend 4 months for small grains; 10 months for
corn (Zea maize L.), tobacco (Nicotiana tabacum), sunflower (Helianthus annuus L.),
sorghum (Sorghum bicolor), rice (Oryza sativa L.), and potato; and 18 months for
sensitive crops like sugarbeet and canola (Brassica napus L.) prior to planting
(Anonymous 1995b; Hayden et al. 1995).

Soybean injury and weed control differences with imazamox have been attributed
to the adjuvant used in the herbicide mixture. A methylated seed oil (MSO) increased
weed control by imazamox on several species compared to a nonionic surfactant (NIS)
(Nalewaja 1994). McMullan (1994a) reported greater wild oat and volunteer barley

(Hordeum vulgare L.) control when a MSO was added compared to other adj uvants and

15

the use of 28% urea ammonium nitrate (UAN) was beneficial. Gednalski et al. (1995)
had more soybean injury with a MSO compared to NIS, but weed control averaged over
weed species was greater with a MSO. Kleven and Zollinger (1995) reported greater dry
edible bean injury with imazamox plus a MSO compared to NIS.

Busse and Hartberg (1994) reported that imazamox controlled panicum spp.,
foxtail spp., wild mustard, Pennsylvania smartweed (Polygonum pensylvanicum L.),
eastern black nightshade (Solanum ptycanthum Dun.), common sunflower (Helianthus
annuus L.), redroot pigweed, velvetleaf, and common lambsquarters over several
locations. In addition, early postemergence applications of imazamox on common
ragweed, johnsongrass [Sorghum halepense (L.) Pers.], giant ragweed, wild buckwheat
(Polygonum convolvulus L.), jirnsonweed (Datura stramonium L.), common purslane
(Portulaca oleracea L.), barnyardgrass [Echinochloa crus-galli (L.) Beauv.], wild oat,
and quackgrass (Agropyron repens (L.) Beauv.) provided 85~100% control of these
species (Anonymous 1995b). Other studies have shown that imazamox plus a MSO
controlled wild mustard (N alewaja 1994), common cocklebur (Xanthium strumarium L.)
(Hoverstad 1996a; Kapusta et a1. 1995; Lueschen and Getting 1996; Owen et al. 1996b),
redroot pigweed (Harvey et al. 1994; Hoverstad 1996c; Kapusta et al. 1995; Nalewaja
1994), rox orange (Peterson and Regehr 1995), velvetleaf (Kapusta et al. 1994, 1995;
Owen et al. 1995b; Peterson and Regehr 1996; Wax et al. 1995), giant foxtail (Harvey et
al. 1994; Hoverstad 1996a, 1996c; Kapusta et a1. 1994, 1995; Owen et al. 1996b; Wax et
al. 1995), common ragweed (Kapusta et a1. 1994), giant ragweed (Hayden et al. 1995),

common lambsquarters (Harvey et al. 1994; Hoverstad and Gunsolus 1996; Lueschen and

16

Getting 1996; Wax et al. 1995), smooth pigweed (Hayden et a1. 1995; Wax et al. 1995),
palmer amaranth (Peterson and Regehr 1995, 1996), ivyleaf morningglory (Ipomoea
hederacea (L.) Jacq.) (Kapusta et al. 1994), and wild-proso millet (Panicum miliaceum
L.) (Mickleson et al. 1996b) at a standard postemergence application timing. However,
inadequate control of common waterhemp (Krausz and Kapusta 1996; Owen et al. 1996b;
Peterson and Regehr 1996), common lambsquarters (Hoverstad 1996c; Nelson and
Renner 1995a; Owen et al. 1995b, 1996b), common ragweed (Dobbles and Loux 1996;
Hoverstad 1996a, 1996c; Hoverstad and Gunsolus 1996; Nelson and Renner 1995a),
woolly cupgrass (Eriochloa villosa (Thunb.) Kunth) (Owen et al. 1996a), wild-proso
millet (Mickelson et al. 1996b), yellow nutsedge (Kapusta et al. 1995), ivyleaf
morningglory (Kapusta et al. 1995; Peterson and Regehr 1996), giant foxtail (Dobbles
and Loux 1996; Kapusta et al. 1994; Peterson and Regehr 1996), large crabgrass
(Digitaria sanguinalis (L.) Scop.) (Peterson and Regehr 1995, 1996), and quackgrass
(Frasier et al. 1995) has been reported. These studies did not specify whether this was
due to regrowth or new weed emergence.

Tank mixtures with imazamox have been evaluated to broaden the weed control
spectrum and maintain consistent control of marginally or inadequately controlled
species. Several tank mixtures have broadened the weed control spectrum of imazamox,
but some have antagonized the control of other weed species. For example, the diphenyl
ether herbicides have been tank mixed with imazamox to increase common ragweed
(Dobbles and Loux 1996; Hoverstad 1996a; Nelson and Renner 1995b) and common

waterhemp control (Peterson and Regehr 1996). Dobbles and Loux (1996) had greater

17

common ragweed control when imazamox was tank mixed with lactofen than acifluorfen
(5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic acid). Lactofen or acifluorfen
reduced giant foxtail control with imazamox compared to imazamox alone (Dobbles and
Loux 1996; Hoverstad 1996a; Lueschen and Getting 1996; Nelson and Renner 1995a,
1995b; Owen et al. 1995b; Peterson and Regehr 1996; Wax et al. 1995). Acifluorfen
antagonism of giant foxtail control by imazamox was greater as the acifluorfen rate
increased (Hoverstad 1996a), but Peterson and Regehr (1995, 1996) observed no
difference in antagonism as the rate of acifluorfen increased. Peterson and Regehr (1995,
1996) also reported that mixtures of imazamox with lactofen or acifluorfen antagonized
large crabgrass control, while acifluorfen reduced ivyleaf morningglory control compared
to imazamox alone. However, fall panicmn (Panicum dichotomiflorum Michx.) control
by imazamox was not affected by bentazon, fomesafen (5—[2-chloro-4-
(trifluoromethyl)phenoxy]-N-(methylsulfonyl)-2-nitrobenzamide), or lactofen tank
mixtures, and there was no difference in johnsongrass or giant foxtail control between
imazamox alone and imazamox tank mixed with clethodim ((E,E)-2-[1-[[3-chloro-2-
propenyl)oxy]imino]propyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen—1-one)
(Hayden et al. 1995; Mickelson et al. 1996a).

Imazamox and Imazethapyr Comparison. Irnazamox and imazethapyr are
imidazolinone herbicides that differ structurally by a single oxygen molecule. This slight
difference has affected the total amount of active ingredient applied, soil persistence,
rotational crop restrictions, and weed control alone or tank mixed.

Herbicide carryover into sensitive crops has prevented the use of several

l8

herbicides in certain soybean cropping systems. The recommended use rate for
imazamox is one half that for imazethapyr. Imazamox has had less reported carryover
than imazethapyr due to a slight chemical structural difference and reduced active
ingredient applied per hectare. Dexter et al. (1994) reported that sugarbeets were injured
by imazethapyr, but not imazamox 1 year after a soybean application at two locations
with a soil pH of 8.0 to 8.1. Imazamox applied at four times the recommended field rate
injured sugarbeets (Lueschen et a1. 1997). Carryover was greater when the soil had a low
pH (1997). Imazamox had better sugarbeet safety than imazethapyr (Lueschen et al.
1995)

With a more active herbicide, there is usually greater crop injury associated;
however, injury by imazamox at 35 g/ha and imazethapyr at 70 g/ha was similar (Geier
and Stahlrnan 1996; Lueschen et al. 1995; Owen et al. 1996a). Researchers have shown
that weed control with imazamox has been greater than imazethapyr on common ragweed
(Hoverstad 1996c; Hoverstad and Gunsolus 1996; Kapusta et al. 1994; Nelson and
Renner 1995a), common lambsquarters (Hoverstad 1996c; Nalewaja 1994; Nelson and
Renner 1995a, 1995b; Owen et al. 1995b), giant foxtail (Hoverstad 1995, 1996b; Kapusta
et al. 1994; Nelson and Renner 1995a, 1995b; Owen et al. 1995b; Wax et a1. 1995), and
yellow nutsedge (Kapusta et al. 1995) at 45 g/ha. Other studies have shown there was no
difference in common lambsquarters, velvetleaf, ivyleaf morningglory, giant ragweed,
Pennsylvania smartweed, giant foxtail, stinkgrass (Eragrostis cilianensis (All.)

E.Mosher), or green foxtail control between imazamox and imazethapyr (Bauman et al.

19

1996; Geier and Stahlman 1996; Getting and Gunsolus 1996; Hoverstad 1995; Hoverstad
and Gunsolus 1996).

Oxasulfuron. Oxasulfuron (CGA-277476) is a postemergence herbicide for selective
grass and primarily broadleaf weed control in soybeans. It was discovered in the early
1990's by Ciba Crop Protection now Novartis (Anonymous 1995a; Brooks et al. 1995).
Limited soil residual and no rotational crop restrictions have been reported (Anonymous
1995a).

Oxasulfirron at 79 g ai/ha rate has controlled velvetleaf (Anderson et al. 1996a;
Anonymous 1995a; Brooks et al. 1995; Gonzini et al. 1995b; Hoverstad 1996b;
Hoverstad and Gunsolus 1995; Kidder and Porpiglia 1996; Owen et al. 1995a; Peterson
and Regehr 1995; Sander et a1. 1995; Wax et a1. 1996), common cocklebur (Anonymous
1995a; Brooks et a1. 1995; Lueschen and Getting 1995; Owen et al. 1995a; Ritter and
Menbere 1997; Sander et a1. 1995), Pennsylvania smartweed (Anonymous 1995a; Getting
and Gunsolus 1996; Gonzini et al. 1995a; Lueschen and Getting 1995; Owen et al. 1995a;
Sander et al. 1995), common sunflower (Anderson et al. 1996a; Anonymous 1995a;
Brooks et al. 1995; Sander et a1. 1995), common ragweed (Anonymous 1995a; Brooks et
al. 1995; Hoverstad 1996b; Kidder and Porpiglia 1996; Ritter and Menbere 1997; Sander
et a1. 1995; Wax et al. 1996), giant ragweed (Ritter and Menbere 1997), morningglory
spp. (Anonymous 1995a; Brooks et al. 1995; Kidder and Porpiglia 1996; Smith et a1.
1997; Wax et al. 1996), hemp sesbania (Sesbania exaltata (Raf) Rydb. ex A.W.Hill)
(Anonymous 1995a; Brooks et al. 1995; Smith et al. 1997), shattercane (Sorghum bicolor

(L.) Moench) (Kidder and Porpiglia 1996), rox orange (Peterson and Regehr 1995),

20

sicklepod (Cassia obtusifolia L.) (Smith et al. 1997), burcucumber (Sicyos angulatus L.)
(Ritter and Menbere 1997) common lambsquarters (Getting and Gonsolus 1996; Gonzini
et al. 1995a; Hoverstad 1996b; Hoverstad and Gunsolus 1995, 1996; Kidder and
Porpiglia 1996; Wax et al. 1996), pigweed spp. (Brooks et a1. 1995; Wax et al. 1996), and
barnyardgrass (Kidder and Porpiglia 1996). Kidder and Johnson (1997) reported
barnyardgrass, broadleaf signalgrass, johnsongrass, shattercane, foxtails, and volunteer
corn was either supressed or controlled by oxasulfuron alone. Bruns and Nalewaja
(1995) reported soybean injury and common lambsquarters control was less with a
nonionic surfactant compared to a methylated vegetable oil or petroleum oil adjuvant.
Other researchers have reported that eastern black nightshade (Anonymous 1995a; Owen
et a1. 1996d), giant foxtail (Gonzini et al. 1995a, 1995b, 1996), common waterhemp
(Krausz and Kapusta 1996; Owen et al. 1996d), common ragweed (F ausey and Renner
1996; Hoverstad 1995; Hoverstad and Gunsolus 1995, 1996), common lambsquarters
(F ausey and Renner 1996; Gonzini et al. 1995b, 1996; Lueschen and Getting 1995; Owen
et al. 1995a, 1996d; Ritter and Menbere 1997), redroot pigweed (F ausey and Renner
1996; Lueschen and Getting 1995), pahner amaranth (Peterson and Regehr 1995),
pigweed spp. (Owen et al. 19953), velvetleaf (Gonzini et al. 1995a, 1996), and large
crabgrass (Gonzini et al. 1995b; Owen et a1. 1996d; Peterson and Regehr 1995) were not
controlled by oxasulfuron alone.

Tank mixtures have been used for the control of eastern black nightshade and
enhanced control of common lambsquarters or pigweed spp. with oxasulfuron

(Anonymous 1995a; Gonzini et al. 1995b, 1996; Hoverstad 1996b; Kidder and Porpiglia

21

1996; Owen et al. 1996d; Sander et a1. 1995). Oxasulfuron tank mixed with lactofen
provided better common ragweed control than the tank mixture with acifluorfen
(Hoverstad 1996b). CGA-248757 (methyl [[2-chloro-4-fluoro-5-[(tetrahydro-3-oxo-
1H,3H-[l ,3,4]thiadiazolo[3,4—a]pyridazin—l-y1idene)amino]phenyl]thio]acetate) has
enhanced common ragweed, pigweed spp., and common lambsquarters control with
oxasulfuron (F ausey and Renner 1996; Hoverstad 1995; Kidder and Porpiglia 1996;
Lueschen and Getting 1995; Owen et al. 1995a), but reduced common cocklebur and
common ragweed control in other studies (Hoverstad 1996b; Hoverstad and Gunsolus
1995; Lueschen and Getting 1995). Redroot pigweed and giant foxtail control was
increased, but velvetleaf control was not increased when ALS-inhibiting herbicides like
oxasulfirron were tank mixed with glyphosate on Roundup Ready" soybeans (Gonzini et
al. 1996; Lich et al. 1997). Oxasulfuron has reduced grass control of some
postemergence graminicides (Kidder and Johnson 1997; Ritter and Menbere 1997);
therefore, graminicide applications should be made 1 day before or 7 days after an
oxasulfuron application (Anonymous 1995a; Kidder and Johnson 1997). Increased
graminicide rates have helped overcome grass antagonism by oxasulfuron (Kidder and
Johnson 1997). Kidder and Johnson (1997) reported that grass antagonism was weed,
herbicide, and solution mixture pH related. Preemergence grass herbicides followed by
oxasulfuron have provided broad spectrum weed control (Bauer et a1. 1995, 1996;
Hoverstad 1996b; Kidder and Johnson 1997; Wax et a1. 1996).

Cloransulam-methyl. Cloransulam-methyl is a triazolopyrimidine sulfonanilide applied

at 26 to 44 g ai/ha early preplant, preplant incorporated, or preemergence timings, and at

22

17.5 g/ha postemergence for broadleaf weed control in soybeans. Its structure was
revealed in 1994 (J achetta et al. 1994). A ten month rotational crop restriction was
recommended for cereal crops, rice, peanuts, and cotton (Glossypium hirsutum L.);
however, sugarbeets and sunflowers require twenty-four months (Anonymous b; J achetta
et al. 1995). Soybeans metabolize cloransulam-methyl through homoglutathione
conjugation of the 7-fluorine of the molecule (J achetta et al. 1995).

Postemergence applications of cloransulam-methyl controlled common cocklebur
(Anonymous b; Hunter et al. 1994; J achetta et a1. 1995; Menbere and Ritter 1997; Owen
et al. 19950; Schultz et al. 1996; Thompson et al. 1995), velvetleaf (Anderson et al.
1996b; Anonymous b; Hoverstad 1996b; Hoverstad and Gunsolus 1996; Hunter et al.
1994; Jachetta et al. 1995; Owen et al. 1995c, 1996c; Schultz et a1. 1996), common
ragweed (Anonymous b; Hoverstad 1996b; Hoverstad and Gunsolus 1996; Hunter et al.
1994; J achetta et al. 1995; Menbere and Ritter 1997; Schultz et al. 1996; Thompson et al.
1995), giant ragweed. (Anonymous b; Hunter et al. 1994; Jachetta et al. 1995; Menbere
and Ritter 1997; Schultz et al. 1996; Thompson et al. 1995), cormnon sunflower
(Anderson et al. 1996b; Anonymous b; Hunter et al. 1994; J achetta et al. 1995; Schultz et
a1. 1996; Thompson et al. 1995), jimsonweed (Anonymous b; Jachetta et al. 1995;
Schultz et al. 1996), morningglory spp. (Anonymous b; Hunter et al. 1994; J achetta et al.
1995; Oliver and Gander 1997; Owen et al. 1996c; Thompson et al. 1995), burcucurnber
(Menbere and Ritter 1997), and spurred anoda (Anoda cristata (L.) Schlecht.) (Menbere
and Ritter 1997). Common lambsquarters (Anonymous b; Menbere and Ritter 1997;

Owen et al. 1995c), pigweed spp. (Anonymous b; Smith et al. 1997; Sweat et al. 1995;

23

Thompson et al. 1995), eastern black nightshade (Anonymous b; Jachetta et al. 1995),
purple moonflower(1pomoea alba L.) (Oliver and Gander 1997), hemp sesbania (Smith
et al. 1997), sicklepod (Smith et al. 1997), and grass spp. (Jachetta et al. 1995) were not
controlled with cloransulam-methyl applied postemergence.

Acifluorfen or thifensulfiiron have been tank mixed to increase pigweed spp. and
common lambsquarters control (Anderson et al. 1996b; Hoverstad et al. 1996; Owen et al.
1995c, 1996c; Thompson et al. 1995). These tank mixtures caused greater soybean injury
than cloransulam-methyl alone (Anderson et al. 1996b; Owen et al. 1995c; Thompson et
al. 1995). Ditrnarsen et al. (1997) reported grass antagonism by cloransulam-methyl was
weed species related when tank mixed with the aryloxyphenoxypropionates, but no
antagonism was indicated when tank mixed with the cyclohexanediones. Preemergence
acetaarrilide followed by cloransulam-methyl postemergence weed management programs
have been evaluated for broad spectrum weed control (Anderson et al. 1996b; Hoverstad
1996b; Schultz et al. 1996).

ACCase-Inhibitors. ACCase (acetyl-CoA carboxylase) is responsible for catalyzing
acetyl-CoA into malonyl-CoA which is the first step in fatty acid synthesis. Fatty acids
are essential for the formation of phospholipids which are the building blocks for plant
cell membranes. Cyclohexanedione (clethodim and sethoxydim) and
aryloxyphenoxypropionate (fenoxaprop ((:)-2-[4-[(6-chloro-2-
benzoxazolyl)oxy]phenoxy]propanoic acid) and quizalofop ((;|-_)-2-[4-[(6-chloro-2-
quinoxalinyl)oxy]phenoxy]propanoic acid)) herbicides are ACCase inhibitors in monocot

species (Rendina and Felts 1988; Sasaki et al. 1995). Sasaki et al. (1995) reported that

24

there were a prokaryotic and eukaryotic forms of ACCase. The prokaryotic form of
ACCase in pea (Pisum sativum L.) plants was insensitive to the grarninicides, but the
eukaryotic form was sensitive due to the lack of the ach gene in the plastid genome
(1995). Dicots like soybeans have the eukaryotic form in the cytosol and the prokaryotic
form in the plastids; however, monocots like giant foxtail have the eukaryotic form of
ACCase in the plastids and cytosol. In addition, Bamwell and Cobb (1994) have reported
that the cyclohexanedione absorption was less and slower than the
aryloxyphenoxypropionate graminicides in grass species, but translocation was similar.
Enhanced absorption was related to the ester formulation of the
aryloxyphenoxypropionates (1994). Once in the plant, it is quickly converted to the acid
form. The R(:)-entaniomer of the aryloxyphenoxypropionates is responsible for grass
phytotoxicity (1994).

Derr et al. (1985) has noted that differences between giant foxtail, large crabgrass,
and goosegrass (Eleusine indica (L.) Gaertn.) control with fluazifop-P (R)-2-[4-[[5-
trifluoromethyl)-2—pyridinyl]oxy]phenoxy]propanoic acid) could not be explained by
uptake, retention, or absorption differences alone. Differences in graminicide tolerance
by tall (F estuca arundinacea Schreb.) and red (F estuca rubra L.) fescue to sethoxydim
and haloxyfop ((:)-2-[4-[[3-chloro-5-(trifluoromethyl)-2-
pyridinyl]oxy]phenoxy]propanoic acid) were due to ACCase differences between the
species which allowed for differential metabolism of these grarninicides (Stolenberg et al.

1989)

25

The graminicides have been used to effectively control monocot weeds in
soybeans (Jordan et al. 1993; Krausz et al. 1993; Myers and Coble 1992). Krausz et al.
(1993) evaluated quizalofop, fenoxaprop, fluazifop-P, sethoxydim, clethodim, and
haloxyfop for control of giant foxtail at four heights from 7 to 60 centimeters. There was
no difference in control between graminicides treated on foxtail that was sixty
centimeters tall, but under drought stress clethodim and haloxyfop had the most
consistent control (Krausz et al. 1993). Large crabgrass control by quizalofop and
clethodim was similar at field rates, but under stressed conditions or a half-rate clethodim
had greater control (Myers and Coble 1992). However, Jordan et al. (1993) had greater
large crabgrass control with quizalofop in the field at two locations than clethodim. A
half rate of quizalofop had greater control than clethodim (1993). Increasing the
graminicide rate increased the control of large crabgrass for all of the graminicides
(1993). Seedling johnsongrass was controlled by fluazifop-P, quizalofop, sethoxydim,
and clethodim at the field and 1.5 times the field rate (1993). Minton et al. (1989b) had
superior red rice (Oryza sativa L.) control by quizalofop in the field compared to
fluazifop-P, sethoxydim, or haloxyfop.

Diphenyl ethers. The diphenyl ether herbicides (acifluorfen, fomesafen, and lactofen)
inhibit the porphyrinogen oxidase (Protox) enzyme. This causes an increase in
protoporphyrin IX which is a product of protox (Duke et al. 1991; Lee and Duke 1994;
Nandihalli et al. 1992; Sherman et a1. 1991). Protoporphyrin IX accumulates in the

cellular membranes and subsequently peroxidizes the cellular membrane upon exposure

26

to light by generating singlet oxygen (Duke et al. 1991; Lee and Duke 1994; Nandihalli et
a1. 1992; Sherman et al. 1991).

Soybean injury by the diphenyl ether herbicides causes bronzing of the leaves,
necrosis, and leaf crinkling (Harris et al. 1991; Lee and Oliver 1982; Wichert and Talbert
1993). Harris et al. (1991) compared soybean injury between fomesafen, acifluorfen, and
lactofen, and soybean injury was greater with lactofen or acifluorfen than fomesafen.
Lactofen at 220 or 440 g ai/ha caused 20-37% visual injury 8 DAT (days after treatment)
and reduced soybean dry weights 16 and 26 DAT compared to the untreated soybeans,
but soybean yield was not affected (W ichert and Talbert 1993). Common ragweed was
controlled by acifluorfen (Monks et a1. 1993; Ritter and Cable 1981b). Ritter and Coble
(1981b) reported that common ragweed was more susceptible to acifluorfen than
soybeans due to slower metabolism.

Environmental conditions can affect weed control with the diphenyl ether
herbicides (Lee and Oliver 1982). Acifluorfen provided better control of common
ragweed and common cocklebur at 85% relative humidity compared to 50% relative
humidity (Ritter and Coble 1981a). Wichert et al. (1992) reported that the control of
prickly sida (Sida spinosa L.), common cocklebur, and pitted (Ipomoea lacunosa L.) or
entireleaf morningglory (Ipomoea hederacea var. integriuscula Gray) by all of the
diphenyl ethers was enhanced at a high relative humidity. Lactofen was the least

sensitive of the diphenyl ethers to reduced control at 50% relative humidity (1992).

27

HERBICIDE ANTAGONISM

Background. Herbicide interactions define a situation, process, and result. Synergism,
additivity effect, and antagonism have been used to describe weed control interactions by
herbicide mixtures (Hatzios and Penner 1985; Putnam and Penner 1974). A correct
evaluation of an interaction depends on an accurate comparison between the observed
control and the expected control. The Weed Science Society of America has defined
synergism as “an interaction of two or more factors such that the effect when combined is
greater than the predicted effect based on the response to each factor applied separately”
(Ahrens 1994). An additive effect described interactions that did not affect the biological
activity of the most active herbicide (Colby 1967). Possibly a more appropriate term for
an additive effect is an independent one since some combinations, herbicide-fungicide or
grass-broadleaf herbicide, would not imply an interaction (Putnam and Penner 1974).
However, generalized grass or broadleaf herbicide classifications would not do justice to
an independent effect label since several broadleaf herbicides have biological activity on
grasses. Antagonism is an “interaction of two or more chemicals such that the effect
when combined is less than the predicted effect based on the activity of each chemical
applied separately” (Ahrens 1994). Several reviews on herbicide interactions have been
published (Bamwell and Cobb 1994; Green 1989; Hatzios and Penner 1985; Putnam and
Penner 197 4).

Antagonism may take place at munerous locations between the herbicide mixture
in the tank and the site of action in the plant. Hatzios and Penner (1985) classified

herbicide antagonism into four mechanisms: 1) chemical, a transformation of a herbicide

28

structure into a less active or ineffective form; 2) biochemical, a biological reduction in
herbicide absorption into or translocation within the plant; 3) competitive, an interaction
that depends on the relative concentration of two or more herbicides for the target enzyme
or site of action; and 4) physiological, one herbicide’s activity negates the separate and
opposite properties of the other herbicide.

Several researchers have proposed methods for analyzing herbicide interactions
(Colby 1967; Morse 1978; Nash 1981; Nash and Harris 1973). Putnam and Penner
(1974) and more recently Hatzios and Penner (1985) reviewed the methodology for
analyzing herbicide interactions. Colby (1967) developed a simplistic equation to
calculate the expected control of a herbicide mixture based on the control of the
herbicides in the mixture applied alone: E = XY / 100. An expected control value (E) was
calculated by taking the product of the percent of control by herbicide X and Y divided
by 100 (1967). According to this equation, when the expected percent of control was
greater than the observed percent of control then synergism resulted; however, the
opposite was necessary for antagonism. Fisher’s protected LSD value has been used as a
conservative method to compare the significance between the expected value and
observed value. When there was no significant difference between these values, the
interaction was classified as additive or there was no net effect on the herbicide with the
most biological activity. The percent dry weight reduction has been used by numerous
researchers to quantitatively evaluate the relative weed control by herbicides. A percent
dry weight reduction must be converted to a percent of control. One hundred minus the

percent dry weight reduction gives the percent of control for herbicides X and Y. These

29

numbers are inserted into Colby’s equation and the expected value calculated. The
expected value is subtracted from 100 to get the expected percent dry weight reduction.
The herbicide combination is antagonistic when the observed percent dry weight
reduction is less than the expected percent dry weight reduction.

Herbicide mixtures need to be evaluated for interactions to ensure crop safety and
weed control. Early detection of synergism, additive effects, or antagonism is the key for
quality herbicide recommendations as well as efficient and effective weed management.
Antagonism has been regarded as a negative aspect of herbicide mixtures. However, such
mixtures can be utilized to evaluate and understand herbicide modes of action, promote
crop safety, and predict future problems. For instance, bentazon (3-(1-methylethyl)-(1H)-
2,1,3-benzothiadiazin—4(3H)-one 2,2-dioxide) reduced l“C-irnazethapyr absorption and
translocation by pinto beans, and safened them fi'om imazethapyr injury (Bauer et al.
1995a). Herbicide rates, formulation, mode of action, adjuvant selection, target species,
time of application, plant growth stage, and environmental conditions can help overcome
or avoid antagonistic interactions.

Diphenyl ether Antagonism. The diphenyl ether herbicides have antagonized grass
and/or broadleaf weed control by several postemergence herbicides. As a result, soybean
yields have been reduced in some situations (Whitwell et al. 1985). The mechanism of
antagonism for this interaction has not been completely defined. This discussion
evaluates diphenyl ether antagonism of grass control by the graminicides (Godley and
Kitchen 1986; Holshouser and Coble 1990; Minton et al. 1989a; Whitwell et al. 1985;

Wilhm et al. 1986) and some broadleaf weed control (Shaw and Wesley 1993; Sorensen

30

et al. 1987; Wesley and Shaw 1992; Westberg and Coble 1992). Recent studies have
reported grass antagonism with the ALS-inhibiting herbicides (Bauman et a1. 1996; Geier
and Stahlman 1996; Hoverstad 1996a, 1996b).

Acifluorfen has antagonized johnsongrass, fall panicum, large crabgrass, red rice,
and goosegrass control with sethoxydim (Holshouser and Cable 1990; Minton et al.
1989b; Vidrine 1989; Whitwell et al. 1985), large crabgrass and bamyardgrass control
with fluazifop (Godley and Kitchen 1986; Minton et al. 1989a), barnyardgrass control
with haloxyfop and fenoxaprop (Minton et al. 1989a), and quackgrass control with
haloxyfop (Whitwell et al. 1985). Acifluorfen reduced haloxyfop absorption by
quackgrass (1985) and sethoxydim absorption by one-third in large crabgrass (Holshouser
and Coble 1990). Minton et al. (1989a) reported that the control of barnyardgrass by
quizalofop and sethoxydim was antagonized by lactofen. Similarly, fomesafen
antagonized bamyardgrass control by quizalofop and fluazifop (Minton et al. 1989a) as
well as goosegrass control by sethoxydim (Holshouser and Coble 1990). Sethoxydim
absorption was reduced by acifluorfen more than fomesafen (Holshouser and Coble
1990). Fomesafen and lactofen had less graminicide antagonism than acifluorfen
(Holshouser and Coble 1990; Minton et al. 1989a). J ohnsongrass control with
sethoxydim was reduced by acifluorfen and lactofen, but not fomesafen (V idrine 1989).
However, Vidrine (1989) reported that lactofen caused greater antagonism of
johnsongrass control by fluazifop-P and quizalofop than acifluorfen or fomesafen. On
the contrary, johnsongrass control by haloxyfop was not antagonized by lactofen,

fomesafen, or acifluorfen (V idrine 1989).

31

Grass antagonism by the diphenyl ethers is weed and herbicide dependent. Chen
and Penner (1985) observed synergism between acifluorfen and sethoxydim for
barnyardgrass control, but acifluorfen had no effect on barnyardgrass control by diclofop
((i)-2-[4-(2,4-dichlorophenoxy)phenoxy]propanoic acid). Minton et al. (1989b)
observed enhanced control of red rice by acifluorfen tank mixed with fluazifop-P,
haloxyfop, or sethoxydim while fomesafen did not influence red rice control in the field.
Greenhouse studies indicated lactofen antagonized red rice control by fluazifop-P and
fomesafen antagonized red rice control by sethoxydim, but no interactions were observed
with acifluorfen (Minton et al. 1989b). Godley and Kitchen (1986) had no reduction in
itchgrass (Rottboellia exaltata L.f.) control when acifluorfen was tank mixed with
fluazifop-P, but observed antagonism of large crabgrass control. The difference between
the antagonistic response was because itchgrass was more susceptible to fluazifop-P than
large crabgrass (Godley and Kitchen 1986). Under adequate moisture and ideal
environmental conditions, there has been no diphenyl ether antagonism observed (Godley
and Kitchen 1986; Holshouser and Coble 1990; Vidrine 1989). As discussed earlier, the
diphenyl ethers have antagonized grass control by the ALS-inhibiting herbicides in
several instances (Bauman et al. 1996; Cantwell et al. 1989; Geier and Stahlman 1996;
Hoverstad 1996a, 1996b). Grass suppression by acifluorfen has been reported from 0 to
74% depending on the grass species and the environmental conditions at the time of
application (Chen and Penner 1985; Godley and Kitchen 1986; Holshouser and Coble
1990; Minton et al. 1989a). Lactofen and fomesafen have some activity on grass species,

but to a much lesser extent (Holshouser and Coble 1990; Minton et al. 1989a).

32

Wesley and Shaw (1992) found that fomesafen and acifluorfen antagonized
common cocklebur control by chlorimuron, but no interaction was observed with
lactofen. Antagonism was more evident on larger common cocklebur (1992). Common
cocklebur, pitted morningglory, and prickly sida absorbed more l‘C-chlorimuron in the
presence of acifluorfen or lactofen than when applied alone (Shaw and Wesley 1993).
However, in whole plant treatments conducted by Westberg and Coble (1992),
chlorimuron absorption by common cocklebur was reduced when tank mixed with
acifluorfen. This resulted in reduced translocation of chlorimuron to the meristematic
tissue (W estberg and Coble 1992). There was no difference in the absorption or
translocation of chlorimuron in sicklepod, a species more tolerant to acifluorfen
(W estberg and Coble 1992). Metabolism experiments could not explain the antagonism
observed (W estberg and Coble 1992).

Cantwell et a1. (1989) reported that bentazon tank rrrixed with imazethapyr
antagonized giant foxtail and smooth pigweed control. The absorption and translocation
of "C-imazethapyr by redroot pigweed was reduced by bentazon in greenhouse studies
by Bauer et al. (1995b). Numerous studies have evaluated antagonism between bentazon
and sethoxydim on several grass species (Gerwick 1988; Holshouser and Coble 1990;
Lassiter and Coble 1987; Minton et al. 1989; Rhodes and Coble 1984a, 1984b;
Wanamarta et al. 1989; Whitwell et al. 1985; Wilhm et al. 1986). Wanamarta et al.
(1989) determined that sodium and other cations antagonized sethoxydim uptake by
quackgrass similar to a bentazon tank rrrixture with sethoxydim. Sodium ions in the

bentazon formulation complexed with sethoxydim to form Na—sethoxydim (W anamarta et

33

al. 1989). In the presence of organic acids, sethoxydim antagonism was reduced

(W anamarta et al. 1989). In addition, the ammonium ion in ammonium phosphate,
ammonium nitrate, or ammonium sulfate successfully competed with the sodirun ion;
therefore, alleviating the antagonism of Na-bentazon on sethoxydim absorption (Jordan
and York 1989; Jordan et a1. 1989; Wanamarta et al. 1993). Similar ionic complexes
with glyphosate have resulted in reduced weed control in hard water situations (N alewaja
and Matysiak 1993; Thelen et al. 1995).

The formulation of fomesafen and acifluorfen includes a combination of the acid
and sodium salt similar to the bentazon formulation (Ahrens 1994). The interaction
between the ALS-inhibiting herbicides and diphenyl ethers with respect to giant foxtail
antagonism could be due to the formation of cationic complexes. However, Godley and
Kitchen (1986) as well as Westberg and Coble (1992) have proposed that the diphenyl
ethers cause wide spread membrane disruption of the epidermal cells which interferes
with normal absorption and translocation of the systemic herbicide resulting in
antagonism. This appears to be the most logical explanation for the diphenyl ether
antagonism.

ALS-Inhibitors and Graminicide Antagonism. Tank mixtures between the ALS-
inhibiting herbicides and graminicides would allow consistent broad spectrum
postemergence weed control; however, grass antagonism has been reported in corn
(Harvey et al. 1995), cotton (F erreira et al. 1995; Ferreira and Coble 1994), soybeans
(Cantwell et al. 1989; Holshouser and Coble 1990; Myers and Coble 1992), and small

grains (Baerg et al. 1996; Devinc and Rashid 1993). Herbicides in the triazolopyrimidine

34

sulfonanilide (Ditrnarsen et al. 1997), imidazolinone (Bjelk and Monaco 1992; Croon and
Merkle 1988; Vidrine 1989), sulfonylurea (Hall et al. 1982; O’Sullivan and Kirkland
1984), and pyrimidinyl thiobenzoate (F erreira et al. 1995; Ferreira and Coble 1994)
families have antagonized the control of several grass species by the graminicides. The
mechanism of antagonism between these herbicides has not been completely understood
or explained. Some researchers have observed reduced graminicide absorption (Chow
1988; Devinc and Rashid 1993) and translocation (Baerg et al. 1996; Chow 1988; Croon
and Merkle 1988; Ferreira et al. 1995) when an ALS-inhibiting herbicide was tank mixed
with the graminicides. Gerwick et al. (1988) postulated that antagonism was based on a
physiological interaction between the ALS-inhibitors and the graminicides. Further work
on the interaction between the substrates of ACCase-inhibitors and ALS-inhibitors could
bring greater understanding of plant biochemistry and help alleviate grass antagonism
between these herbicide mixtures.

Ditrnarsen et al. (1997) evaluated antagonism between cloransulam-methyl and
several graminicides. Antagonism observed with the aryloxyphenoxypropionates was
species dependent, but no antagonism was observed with the cyclohexanediones
(Ditrnarsen et al. 1997).

Imazethapyr antagonized giant foxtail (Cantwell et al. 1989) and woolly cupgrass
(Owen et al. 1996a) control by sethoxydim. Imazethapyr also antagonized broadleaf
signalgrass (Brachiaria platyphylla (Griseb.) Nash) control by fluazifop and sethoxydim
as well as large crabgrass and fall panicum control by fluazifop, clethodim, quizalofop,

and sethoxydim (Myers and Coble 1992). Myers and Coble (1992) reported severe

35

antagonism under dry conditions and a sequential graminicide application overcame
antagonism by imazethapyr. Grass control has been reported with imazethapyr (Cantwell
et al. 1989; Myers and Coble 1992), but limited grass activity was reported by
Holshouser and Coble (1990) and Minton et al. (1989).

The control of fall panicum, large crabgrass, and goosegrass by sethoxydim
(Holshouser and Coble 1990), johnsongrass control by fluazifop and haloxyfop (Croon
and Merkle 1988; Vidrine 1989), and red rice control by quizalofop, fluazifop, haloxyfop,
fenoxaprop, clethodim, and sethoxydim (Minton et al. 1989) was antagonized when tank
mixed with imazaquin. Antagonism of johnsongrass was dependent on seedling or
rhizome growth habit (V idrine 1989). Croon et al. (1989) observed no chemical
interaction between haloxyfop and imazaquin. The absorption of haloxyfop or
sethoxydim by sorghum or large crabgrass was not affected by imazaquin (Croon et a1.
1989; Holshouser and Coble 1990); however, imazaquin reduced haloxyfop translocation
in sorghum (Croon et al. 1989). Imazaquin antagonism of red rice control by the
graminicides was greater than chlorimuron (Minton et al. 1989). Neither imazaquin, nor
chlorimuron affected sethoxydim absorption by large crabgrass (Holshouser and Coble
1990). Inadequate rainfall before application, weed size, and graminicide rate affected
grass antagonism by imazaquin or chlorimuron and grass control by the graminicides
(Croon and Merkle 1988; Vidrine 1989).

Chlorimuron antagonized fall panicum and large crabgrass control by sethoxydim
(Holshouser and Coble 1990); goosegrass control in the field, but not in the greenhouse

by sethoxydim (1990); johnsongrass control by fluazifop (Croon and Merkle 1988;

36

Vidrine 1989), haloxyfop (1989), and fenoxaprop (Croon and Merkle 1988; Vidrine
1989); and broadleaf signalgrass control by quizalofop (Bj elk and Monaco 1992).
Chlorimuron had no reported grass activity (Holshouser and Coble 1990; Vidrine 1989);
however, Bj elk and Monaco (1992) reported broadleaf signalgrass was suppressed for 3
days after treatment and then quickly resumed growth. Croon et al. (1989) found that the
absorption of l“C-haloxyfop by sorghum was not affected by chlorimuron, but
translocation was reduced. At the biochemical sites of action, Bj elk and Monaco (1992)
determined that elevated pyruvate levels caused by the inhibition of the ALS-enzyme
were not related to the antagonism of quizalofop on broadleaf signalgrass.

Kidder and Johnson (1997) reported that oxasulfuron antagonized grass control by
certain graminicides. This antagonism was grass species and graminicide related (Kidder
and Johnson 1997). Ritter and Menbere (1997) reported that oxasulfuron tank mixed
with clethodim antagonized giant foxtail control.

Wild oat control by diclofop and flamprop (N-benzoyl-N-(B-chloro-4-
fluorophenyl)-DL-alanine) was antagonized by chlorsulfuron (Chow 1988; O’Sullivan
and Kirkland 1984). However, Hall et al. (1982) had conflicting results in the growth
room and observed no antagonism between diclofop and chlorsulfuron. Devinc and
Rashid (1993) reported slight antagonism of wild cat control by tralkoxydim from
chlorsulfuron. Chow (1988) related increased diclofop antagonism to an increase in the
rate of chlorsulfirron. Chlorsulfuron also reduced the absorption and translocation of 1“C-

diclofop in a localized treatment study (Chow 1988).

37

Tribenuron antagonized diclofop control of wild oat (Baerg et al. 1996; Eberlein
et al. 1988). Eberlein et al. (1988) had no physical or chemical, inert ingredient, ACCase-
inhibitor, or spray retention interactions between tribenuron and diclofop. The absorption
and metabolism of MC-diclofop by wild oat was not affected by tribenuron; however,
reduced translocation was observed (Baerg et al. 1996). A slight separation of time
between tribenuron and diclofop applications greatly reduced antagonism (Baerg et al.
1996)

Several other sulfonylureas have reduced grass control when tank mixed with a
graminicide. Sethoxydim-resistant corn has brought new opportunities for
postemergence grass control in corn. Harvey et a1. (1995) found that nicosulfuron had no
additive effect on giant foxtail control by sethoxydim, but primisulfuron antagonized
giant foxtail control by sethoxydim. Langton and Harvey (1997) reported that the ALS-
inhibiting herbicides reduced sethoxydim absorption and translocation. Young et a].
(Young et al. 1996) reported antagonism by ALS-inhibiting herbicides tank mixed with
sethoxydim was related to reduced absorption of “C-sethoxydim in certain instances.
Primisulfirron antagonized giant foxtail control by sethoxydim more than large crabgrass
or shattercane (Young et al. 1996). Wild oat control by tralkoxydim in the growth
chamber was antagonized by metsulfirron-methyl (Devinc and Rashid 1993).
Metsulfuron-methyl reduced l“C-tralkoxydim absorption, but had no effect on
translocation or ACCase activity (Devinc and Rashid 1993). Since metsulfuron-methyl

injured wild oat, Devinc and Rashid (1993) associated this injury with antagonism since

38

they observed no injury to wild oat by chlorsulfuron and slight antagonism when
chlorsulfuron was tank mixed with tralkoxydim.

DPX-PE350 or pyrithiobac, sodium 2-chloro-6-(4,5-dimethoxypyrimidin—2-yl-
thio)benzoate), is an ALS-inhibitor and a member of the pyrimidinyl thiobenzoate family.
DPX-PE350 antagonized large crabgrass control by fluazifop, sethoxydim, quizalofop,
and clethodim (F erreira et al. 1995; Ferreira and Coble 1994; Jordan et al. 1993);
broadleaf signalgrass control by quizalofop and fluazifop, but not sethoxydim or
clethodim (Jordan et al. 1993); and johnsongrass control by sethoxydim and fluazifop,
but not quizalofop (F erreira and Coble 1994). Jordan et al. (1993) reported antagonism of
sethoxydim by DPX-PE350, but not with fluazifop, clethodim, or quizalofop. DPX-
PE350 antagonism with fluazifop was the most consistent (F erreira and Coble 1994;
Jordan et al. 1993). Both Jordan et al. (1993) as well as Ferreira and Cable (1994)
reported some large crabgrass and johnsongrass suppression by DPX-PE350. Sequential
applications of DPX-PE350 and increasing graminicide rates helped overcome
graminicide antagonism (F erreira et al. 1994; Jordan et al. 1993). DPX-PE350 had no
effect on ACCase inhibition by fluazifop or the absorption and metabolism of fluazifop-P
(F erreira et al. 1995). Fluazifop-P antagonism by DPX-PE350 on large crabgrass control
was related to reduced translocation (F erreira et a1. 1995).

Environmental conditions, sequential applications, and increased graminicide
rates have reduced ALS-inhibiting herbicide antagonism of the ACCase-inhibiting
herbicides. Careful selection of tank mixtures can help producers avoid losses due to

antagonistic herbicide mixtures. Bamwell and Cobb (1993) recently reviewed the effects

39

of the graminicides on cellular proton gradients. These effects may help explain

antagonism between the auxin herbicides and graminicides (Bamwell and Cobb 1994).

ADJUV ANT S

The addition of adjuvants to a herbicide mixture has reduced antagonism
(Gerwick et al. 1990; Hart et al. 1992), enhanced weed control (F oy 1993; F oy and Witt
1993; Gednalski et a1. 1995; Gronwald et al. 1993; Nalewaja et al. 1990), and affected
chemical stability (Bridges et al. 1992; Falb et al. 1990; McMullan 1996). Adjuvants
enhance absorption by increasing spray retention and foliar penetration (Gronwald et a1.
1993; Penner 1989).

Sun-It II is a modified vegetable oil containing surfactant manufactured by Agsco
Inc. (F oy 1993). Foy and Witt (1993) reported that soybean injury increased when
imazethapyr was applied to soybeans with adjuvants. Kochia control by imazethapyr
(Nalewaja et a1. 1990); giant foxtail control by primisulfuron (Hart et a1. 1992); and weed
control by imazethapyr (Ogg et al. 1988) or imazamox (Gednalski et al. 1995) was
greater with a methylated seed oil than a nonionic surfactant. Thompson et al. (1996)
reported greater absorption of l‘C-imazethapyr by leafy spurge (Euphorbia esula L.)
when a methylated seed oil was applied instead of a nonionic surfactant when 28% UAN
was present or absent. Hager and Renner (1994) increased common ragweed control with
imazethapyr by adding 28% UAN to imazethapyr plus nonionic surfactant. Hart et al.
(1992) reported enhanced l“C-primisulfuron absorption by giant foxtail and less tank

mixture antagonism when a methylated seed oil was included.

40

AMS (ammonium sulfate) improved the uptake of imazethapyr in quackgrass
compared to imazethapyr with surfactant alone. AMS has been used to overcome salt
antagonism with several herbicides. The ammonium ion was important for reducing Na-
sethoxydim antagonism by competing with sodium for sethoxydim (Wanamarta et al.
1993). Thelen (1994) reported the nitrate from UAN was not as effective as the
ammonium ion in ammonium sulfate for overcoming hard-water antagonism of

glyphosate.

ENVIRONMENTAL EFFECTS ON HERBICIDE EFFICACY

Temperature, relative humidity, rainfall, and light affect weed control and crop
response to a herbicide (Hammerton 1967). Successful herbicide product development
strategies strive to provide weed control over a variety of environmental conditions. It is
important to understand environmental effects before or when unique problems,
situations, or concerns arise.

The amount and type of crop injury as well as the ability of the plant to
metabolize or compensate for the injury affects the yield potential of that crop. Ateh and
Harvey (1997) reported that soybean injury by contact and translocated postemergence
herbicides reduced soybean yields. Soybean injury was related to the environmental
conditions at the time of application (Ateh and Harvey 1997). Soybean area 22 days after
treatment had a direct affect on soybean yield (yield reduction = 4 + (0.3 x area reduction
22 DAT) (Ateh and Harvey 1997). Soybean area was the product of soybean height and

width. Visual injury at 8 DAT was the best indicator of yield loss (yield reduction = 4 +

41

0.2 x visual injury 8 DAT) (Ateh and Harvey 1997). Soybeans are more sensitive to
ALS-inhibiting herbicides at low temperatures (Malefyt and Quakenbush 1991; Reed et
al. 1992). Some soybean varieties have greater herbicide tolerance than others (Reed et
a1. 1992; Simpson and Stoller 1996). Temperature exhibited no effect on translocation or
absorption of imazethapyr, but soybean metabolism was halved as the temperature
decreased fi'om 27/33 to 21/27 C and from 21/27 to 15/21 C (Malefyt and Quakenbush
1991). Soybean recovery fi'om thifensulfuron was greater under ideal growing conditions
(Reed et al. 1992). Under higher temperatures (30 C) and low relative humidity (35%),
thifensulfuron had increased ivyleaf morningglory control compared to low temperature
(15 C) (Reed et al. 1992), but temperature had a limited effect upon velvetleaf or
common lambsquarters control by thifensulfuron (Reed et al. 1992) or pitted
morningglory control by imazethapyr (Kent et al. 1991a, 1992b). Temperature (18 to 35
C) had no effect on the absorption or translocation of 1“C-imazethapyr (Kent et al. 1992b;
Malefyt and Quakenbush 1991), but a greater distribution of l“C-imazethapyr in pitted
morningglory was noted at 35 C compared to 18 C (Kent et al. 1991b). However, Harker
and Dekker (1988) reported more translocation l“C-cloproxydim ((E,E)-2-[l-[[(3-chloro-
2-propenyl)oxy]imino]butyl]-5-[2-(ethylthio)propyl]-3 -hydroxy-2-cyclohexen-1-one) to
the shoot by quackgrass at high temperatures (fi'om 10/5 to 20/15 C or fi'om 20/ 15 to
30/25 C).

Relative humidity affects weed control more than temperature (Kent et al. 1991a,
1992b; Malefyt and Quakenbush 1991; Reed et a1. 1992). The relative humidity is the

amount of moisture in the air relative to the greatest amount that can be present at that

42

temperature. At a higher relative humidity, greater weed control of several weed species
has been noted (Malefyt and Quakenbush 1991; Nalewaja et al. 1990; Reed et al. 1992).
Pitted morningglory absorbed and translocated greater amounts of l“C-imazethapyr at
100% compared to 40% relative humidity (Kent et al. 1991a, 1992b).

Drought stressed plants have a higher herbicide tolerance level than actively
growing plants (Bridges 1989; Kells and Wanamarta 1987). As discussed earlier,
herbicide antagonism may be greater or more severe under reduced water availability
depending on the mixture and weed species evaluated. Reynolds et al. (1988) reported
that the effects of induced water stress on basipetal translocation in grain sorghum was
dependent on the graminicide. For instance, increased acropetal translocation of
sethoxydim or fluazifop by grain sorghum was noted while there was less acropetal
translocation of quizalofop (Reynolds et al. 1988). Reduced quackgrass control by
fluazifop or sethoxydim was related to water stress at the time of application by Kells and
Wanamarta (1987).

Plant life depends on light to generate chemical energy. Light is important for the
activation of the protox herbicides, but can increase degradation of others (Bridges et al.
1992). McMullan (1994b, 1996) showed that barley control with clethodim was greater
in the absence of ultraviolet light. This was partially accounted for by the pH of the
solution and the selected adjuvant. As the pH decreased, there was a decrease in
photodegradation (F alb et al. 1990). A narrow pH window (5 to 7) was established by
McMullan (1996) for the best control of barley by clethodim. Since quizalofop and

fenoxaprop are formulated as esters, they are not affected by pH like clethodim

43

(McMullan 1994b, 1996). It was shown that an adjuvant could affect clethodim uptake
and reduce photodegradation on johnsongrass (Bridges 1989).

There are several environmental factors that interplay and affect herbicide injury
of crops and weed control. These factors may influence absorption, penetration,
metabolism, and the activation or degradation of herbicides. The interplay of these
factors must be considered when evaluating problems or interactions between herbicides

for weed control.

ROW SPACING AND WEED COMPETITION

Soybean yields are reduced when weeds are not controlled due to competition for
essential plant growth factors. Coble et al. (1981) found that as few as four common
ragweed plants per 10 meter of row reduced soybean yields, and if common ragweed was
not controlled before it was 6 weeks old, soybean yields decreased. Similarly, Knake and
Slife (1962) showed that fifty-four giant foxtail plants per foot of row reduced soybean
yields 28%. The harvested giant foxtail dry weight in this study was proportional to the
reduction in soybean dry weight (Knake and Slife 1962). A comprehensive review of
weed interference in soybeans has been published by Stoller et al. (1987).

Narrow (usually less than or equal to 25 cm) row soybeans yielded more and
decreased weed yields more than similar wide row treatments (Burnside and Colville
1964; McWorter and Sciumbato 1988; Mickelson and Renner 1997; Patterson et al. 1988;
Peters et al. 1965; Wax and Pendleton 1968). Soybeans grown in narrow row spacings

were more profitable than wide row soybeans for most of the post emergence herbicide

44

applications (Mickelson and Renner 1997). Taylor (1980) reported that wide row
soybean yield may equal narrow row soybean yield under severe water stressed
conditions because water deficits occur first in narrow row soybeans. Patterson et a1.
(Taylor 1988) found that soybean yield was related to the soybean biomass in wide and
narrow soybean row spacings. Soybean yield was inversely related to weed biomass
yield (Burnside and Moomaw 1977). The total plant biomass by weeds and soybeans
was greater than the weed free soybean biomass (Weber and Staniforth 1957). However,
Legeré and Schreiber (1989) measured equal total biomass between handweeded
soybeans compared to the total biomass of the untreated control.

The introduction of postemergence herbicides has facilitated the transition of
producers fi'om wide to narrow row soybeans. Limited research has evaluated
postemergence herbicides in narrow row compared to wide row soybeans. Mickelson and
Renner (1997) evaluated weed control in narrow and Wide row soybeans using reduced
herbicide rates. Postemergence herbicide treatments in narrow row soybeans had 30%
less total weed biomass than the same postemergence treatments in wide row soybeans
(Mickelson and Renner 1997). Split applications of postemergence herbicides at a one-
fourth the recommended field rate had similar weed control compared to the
recommended field rate (Mickelson and Renner 1997). Burnside and Moomaw (1977)
evaluated reduced herbicide rates in wide and narrow soybeans. Several tank mixtures
provided weed control that was adequate in narrow row soybeans and did not require a
cultivation (Burnside and Moomaw 1977). Early timings of reduced rate herbicides were

needed for adequate weed control in narrow row soybeans (Burnside and Moomaw

45

1977). Weeds suppressed by chlorarnben (3-amino-2,5-dichlorobenzoic acid) were
controlled to a greater extent in narrow than wide row soybeans (Peters et al. 1965).
Faster canopy closure helped control weeds that were partially controlled by a herbicide
treatment.

All other factors being equal, light competition by soybean is greater in narrow
than wide row soybeans. Redroot pigweed total leaf area was less in narrow (25 cm) row
soybeans than wide (75 cm) (Legeré and Schreiber 1989). Canopy closure for 25, 51, 76,
and 102 cm row spacing soybeans ranged from 35-36, 47-50, 56-65, and 67-80 days after
planting, respectively (Burnside and Colville 1964; Wax and Pendleton 1968).
Mickelson and Renner (1997) reported that the canopy closure for narrow row soybeans
was 45 days earlier than wide row soybeans. Shibles and Weber (1966) reported that
plants in narrow rows (25 cm) reached 95% solar interception before wide row (102 cm)
soybeans. The implementation of simple adjustments in cultural practices like narrow
row soybeans can enhance postemergence weed control of marginally controlled weeds,

save input costs, and reduce soil erosion.

46

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55

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56

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57

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58

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59

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60

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62

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63

Table I : The ALS-inhibiting herbicides are classified by their chemical family and a

description of their major use.

 

 

Chemical Family Herbicide Industrial or Major Crop Use
Inridazolinone irnazarneth peanuts
irnazarnethabenz cereal crops
imazamox soybeans
irnazapyr industrial
imazaquin soybeans
imazethapyr soybeans
Triazolopyrimidine sulfonanilide cloransulam-methyl soybeans
flumetsulam soybeans, corn
Sulfonylurea bensulfirron rice
chlorimuron soybeans
chlorsulfuron cereal crops
etharnethsulfuron canola
halosulfuron corn
metsulfuron cereal crops, pastures
nicosulfuron corn
oxasulfuron soybeans
primisulfuron corn
prosulfuron corn
rimsulfirron corn
sulfometuron industrial
sulfosulfirron cereal crops
thifensulfuron soybeans
triasulfirron cereal crops
tribenuron cereal crops
triflusulfuron sugarbeet
Pyrimidinyl thiobenzoate pyrithiobac cotton

 

64

Acid

N

Backbone an \

o
Imidazolinone ring

Figure I . The imidazolinone skeleton consists of an acid, backbone, and imidazolinone
ring. Imidazolinone ring termination or the conversion to an immobile structure, and

alkyl hydroxylation followed by glucose conjugation are the primary mechanisms of
metabolism.

Pheny] ring 3’ 1 Heterocycle moiety
1‘
7 / \

Sulfonylurea bridge

IO

Figure 2. The sulfonylurea structure includeds a phenyl ring, sulfonylurea bridge, and
heterocycle moeity. Metabolism of the sulfonylurea herbicides includes (1) de-
esterification, (2 and 6) hydrolysis or cleavage of the bridge, (3) homoglutathione

65

O/ \0

Figure 3. The triazolopyrimidine sulfonanilide herbicide skeleton. The
triazolopyrinridine sulfonanilide herbicides are metabolized primarily through

hydroxylation and subsequent glucose conjugation of the herbicide structure denoted with
an asterisk.

CHAPTER 2

POSTEMERGENCE WEED CONTROL WITH OXASULFURON
AND CLORANSULAM-METHYL IN SOYBEAN S [Glycine max (L.) Merr]l

Kelly A. Nelson and Karen A. Renner”-
Abstract. Field and greenhouse experiments were conducted to evaluate postemergence
soybean injury and weed control by oxasulfuron and cloransulam-methyl alone and in
tank rrrixtures. Visual soybean injury was 12 to 14% fi'om oxasulfuron and 9 to 13%
from cloransulam-methyl 7 days after treatment in the field. Tank mixtures of either
herbicide with acifluorfen or acifluorfen plus thifensulfirron were more injurious than
oxasulfuron or cloransulam-methyl applied alone. Both oxasulfuron and cloransularn-
methyl controlled velvetleaf, while cloransulam-methyl also controlled common ragweed.
Adding acifluorfen to the spray solution enhanced common ragweed, common
lambsquarters, redroot pigweed, and eastern black nightshade control with oxasulfuron
and enhanced common lambsquarters, redroot pigweed, and eastern black nightshade
control with cloransulam-methyl. Tank mixing thifensulfirron with oxasulfuron or
cloransulam-methyl increased common lambsquarters and redroot pigweed control. In

the greenhouse, oxasulfuron controlled velvetleaf at 20 g ai/ha and common ragweed

 

‘Received for publication and in revised form

 

 

2Grad. Res. Asst. and Prof., Mich. State Univ., East Lansing, MI 48824-1325.

66

67

at 79 g/ha. The dry weight of common lambsquarters was reduced 60% by oxasulfuron at
79 g/ha. Cloransulam-methyl controlled velvetleaf at 4.4 g ai/ha and common ragweed at
8.8 g/ha. Chlorimuron reduced yellow nutsedge dry weight more than oxasulfirron or
cloransulam-methyl. Oxasulfuron reduced giant foxtail control by clethodim, but not by
quizalofop in the field. In the greenhouse, large crabgrass height was 42% greater when
oxasulfuron was tank mixed with quizalofop compared to quizalofop alone. No reduction
in johnsongrass or corn control was observed when oxasulfuron was tank mixed with
quizalofop or clethodim. Grass antagonism by oxasulfuron was grass species and
graminicide related. Cloransulam-methyl tank mixed with clethodim or quizalofop
controlled giant foxtail.

Nomenclature: Acifluorfen, 5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic
acid; chlorimuron, 2-[[[[(4-chloro-6-methoxy-2-
pyrimidinyl)amino]carbonyl]amino]sulfonyl]benzoic acid; clethodim, (E,E)-(_-i;)-2-[1-
[[(3—chloro-2-propenyl)oxy]imino]propyl]—5-[2-(ethylthio)propyl]-3-hydroxy-2-
cyclohexen—l-one; cloransulam-methyl, N-(2-carboxymethyl-6-chlorophenyl)-5-ethoxy-
7-fluoro(1,2,4)triazolo-(1,5c)-pyrimidine-2-sulfonamide; oxasulfuron (CGA-277476), 2-
[[[[(4,6-dimethyl-2-pyrimidinyl)-amino]carbonyl]amino]sulfonyl]benzoic acid, 3-
oxetanyl ester; quizalofop, (:)-2-[4-[(6-chloro-2-quinoxalinyl)oxy]phenoxy]propanoic
acid; thifensulfuron, 3-[[[[(4-methoxy-6-methyl-1,3,5-triazin-2-

yl)amino]carbonyl]amino]sulfonyl]-2-thiophenecarboxylic acid; common lambsquarters,

68

Chenopodium album L. #3 CHEAL; common ragweed, Ambrosia artemisiifolia L. #
AMBEL; eastern black nightshade, Solanum ptycanthum Dun. # SOLPT; giant foxtail,
Setariafaberi Herrm. # SET'FA; johnsongrass, Sorghum halepense (L.) Pers. # SORHA;
large crabgrass, Digitaria sanguinalis (L.) Scop. # DIGSA; redroot pigweed, Amaranthus
retroflexus L. # AMARE; velvetleaf, Abutilon theophrasti Medik. # ABUTH; volunteer
com, Zea mays L. # ZEAMX; yellow nutsedge, Cyperus esculentus L. # CYPES;
soybean, Glycine max (L.) Merr.

Additional index words: Herbicide antagonism, interaction, tank mixture.

 

3Letters following this symbol are WSSA-approved computer code from Composite List
of Weeds, Revised 1989. Available fiom WSSA, 1508 West University Ave.,
Champaign, IL 61821-3133.

69

INTRODUCTION

Oxasulfirron is a sulfonylurea herbicide for postemergence control of broadleaf
and some grass weed species in soybeans (Anonymous 1995; Brooks et al. 1995; Kidder
and Johnson 1997; Ritter and Menbere 1997). Cloransulam-methyl is a
triazolopyrimidine sulfonanilide herbicide for preplant incorporated, preemergence, and
postemergence broadleaf weed control in soybeans (J achetta et a1. 1995; Menbere and
Ritter 1997; Oliver and Gander 1997).

Herbicide interactions have been described as synergistic, additive, or antagonistic
(Hatzios and Penner 1985; Putnam and Penner 1974). Herbicides tank mixtures may
enhance weed control (Green 1991; Monks et a1. 1993; Sander et al. 1995), but they may
increase soybean injury (Simpson and Stoller 1996; Thompson et al. 1995) or result in
antagonism (Ditrnarsen et al. 1997; Ritter and Menbere 1997; Wesley and Shaw 1992).
Herbicides in the triazolopyrimidine sulfonanilide (Ditrnarsen et al. 1997), imidazolinone
(Bjelk and Monaco 1992; Croon and Merkle 1988), sulfonylurea (Ritter and Menbere
1997; Young et a1. 1996), and pyrimidinyl thiobenzoate (F erreira et al. 1995; Ferreira and
Coble 1994; Jordan et al. 1993) families have antagonized the control of several grass
species by the graminicides. Gerwick et a1. (1988) postulated that this antagonism was
based on a physiological interaction between the ALS‘-inhibitors (EC. 4.1.3.18) and
graminicides. Tank mixtures with oxasulfuron or cloransulam-methyl have been

evaluated for soybean injury and enhanced broadleaf weed control (Bauer et al. 1995;

 

4Abbreviation: ALS, acetolactate synthase (EC. 4.1.3.18); UAN, 28% urea ammonium
nitrate; NIS, nonionic surfactant; and DAT, days after treatment.

70

Kidder and Porpiglia 1996; Sander et al. 1995; Thompson et al. 1995). Graminicide
antagonism by oxasulfuron was species and graminicide dependent (Kidder and Johnson
1997; Ritter and Menbere 1997). Ditrnarsen et al. (1997) reported that cloransulam-
methyl antagonized grass control by the aryloxyphenoxypropionate herbicides, but not
the cyclohexanedione herbicides. Antagonistic mixtures are expensive with respect to
reduced weed control, reduced crop yield, and increased weed seed production.

The objectives of this research were: to evaluate soybean injury and weed control
from postemergence applications of oxasulfirron and cloransulam-methyl applied alone
and in selected tank mixtures; to determine the sensitivity of various weed species to
oxasulfirron and cloransulam-methyl; and to identify interactions of oxasulfuron with

graminicides on selected monocot species.

71

MATERIALS AND METHODS
Wm. Field experiments were conducted in 1995 and 1996 at the
Michigan State University Research Farm at East Lansing, MI. The soil was a Capac
loam (fine-loamy, mixed, mesic Aerie Ochraqualfs) with 2.7% organic matter and a soil
pH of 7.2 in 1995. This site was chisel plowed in the fall, and disked and field cultivated
twice in the spring. In 1996, the soil was a Capac sandy clay loam with 3.4% organic
matter and a soil pH of 7.8. This site was moldboard plowed in the fall and cultimulched
twice in the spring.

‘Conrad’ soybeans were planted in 76 cm rows at 366,000 seeds per hectare in
1995 and 359,000 seeds per hectare in 1996 with a John Deere 7200 Max-Emerge“?
planters. Plots were 3 meters wide and 9.1 meters long in 1995, and 3 meters wide and
12.2 meters long in 1996.

The weed control treatments included an untreated control, acifluorfen at 280 g
ai/ha, thifensulfuron at 2.2 g ai/ha, and acifluorfen plus thifensulfuron which were each
tank mixed with oxasulfirron at 79 g ai/ha and cloransulam-methyl at 17.5 g ai/ha (Table
2). Clethodim at 140 g ai/ha and quizalofop (1996 only) at 50 g tha were included in
the factorial design (Table 3). All herbicide treatments were applied with 2.6% (v/v) 28%

urea ammonium nitrate (U AN)‘ and 0.25% (v/v) nonionic surfactant (NIS)"°.

 

5Deere and Co., 501 River Drive, Moline, IL 61265-1100.

6Nonionic surfactant was Activator-90, a mixture of alkyl polyoxyethylene ether and
free fatty acids, from Loveland Industries, Inc., PO. Box 1289, Greeley, CO 80632.

72

Herbicide applications were made with a tractor mounted compressed air sprayer
traveling at 6.3 krn/h and delivering 178 L/ha at 207 kPa of pressure on June 9, 1995 and
June 23, 1996 (28 and 25 days after planting) at a standard postemergence timing.
Treatments were made with 8003 flat-fan nozzles7 on a 51 cm spacing at 48 cm above the
crop and weed canopy. Air temperature was 22 C with 70 percent relative humidity in
1995, and 20 C with 62 percent relative humidity in 1996 at the time of application.
Rainfall ten days prior to herbicide application was 3.1 cm in 1995 and 9.8 cm in 1996,
while rainfall from one to ten days after herbicide application totaled 0.5 cm in 1995 and
0.7 cm of rain in 1996. Soybeans were at the V2 stage of development in 1995 and 1996.
Weed heights, leaf number, and weed density are presented in Table 1.

Plots were evaluated for soybean injury at 3, 7, 14, and 21 days after treatment
(DAT)‘. Soybean injury was based on chlorosis, necrosis, and stunting. Injury is
presented for 7 DAT. Weed control was evaluated at 7, 14, 21, and 56 DAT. Evaluations
taken at 21 and 56 DAT were the most representative of herbicide effectiveness. Weed
control was evaluated visually and as a percent reduction in dry weight. Visual ratings
were based upon a scale of 0 (no effect) to 100 (weed or crop death) percent. Percent dry
weight reduction was determined 21 DAT by harvesting the above ground plant matter of
three common ragweed and three velvetleaf plants marked with plastic garden stakes8 1

day prior to herbicide application in each plot. Percent dry weight reduction was

 

7Teejet flat fan tips. Spraying Systems Co., North Ave. and Schmale Road, Wheaton, IL
601 88.

8Pylon Plastics, Inc. 2111 Ogden Ave., P.O. Box 505, Lisle, IL 60532.

73

calculated as 100*[1-(plant dry weight/untreated plant dry weight)]. Giant foxtail was
harvested fipm an 840 cm2 quadrat. Weight per plant was first calculated and then the
percent dry weight reduction was calculated as previously described.
WWW. Velvetleaf, common ragweed, yellow
nutsedge, common lambsquarters, giant foxtail, johnsongrass, large crabgrass, and corn
seeds were planted in BACCTO’ potting soil in 946 ml plastic pots. Environmental
conditions were maintained at 27 j; 5 C. Plants were grown in a 16 h photoperiod of
natural and supplemental sodium vapor lighting that provided a photosynthetic photon
flux density10 of 120 uE/mZ/s. Midday photosynthetic photon flux density was
approximately 1000 uE/m’ls. Plants were fertilized with 0.1 g of water soluble fertilizer
(20% N, 20% P202, 20% K20) and watered as needed.

Herbicide applications were made with a continuous belt-link sprayer traveling at
1.5 km/hr and equipped with an 80017 even flat fan nozzle calibrated to deliver 234 L/ha
at 193 kPa of pressure. All treatments included 2.5% (v/v) UAN and 0.25% (v/v) NIS.
The soil was moist and soil temperature was 26 i 1 C at the time of herbicide application.
Treatments were applied in the evening fi'om 3:00 pm. to dusk when the relative
humidity was low (32 to 42%). Photosynthetic photon flux density was 110 to 400
uE/mzls at the conclusion of the herbicide treatments. Percent dry weight reduction of

the oven dried top plant growth was calculated as previously described.

 

’Baccto is a product of Michigan Peat Co. Houston, TX 77098.

mLI-COR. 4421 Superior Street, Lincoln, NE 68504.

74

Broadleaf weed sensitivity study. Velvetleaf, common ragweed, and common
lambsquarters seed were purchased from V & J Seed Farms“ and thinned to one plant per
pot prior to herbicide application. Yellow nutsedge tubers were purchased from Valley
Seed Service12 and selected for plants with two shoots that arose fiom one tuber prior to
herbicide application. Velvetleaf was 5 to 8 cm tall with three leaves, common ragweed
was 3 to 5 cm tall with four to six leaves, yellow nutsedge was 10 cm tall with four to six
leaves, and common lambsquarters were 3 to 5 cm tall with six leaves at the time of
herbicide application. Plant top growth was harvested 28 DAT.

Velvetleaf, common ragweed, and yellow nutsedge were arranged in the
experimental design described in the statistical protocol. Oxasulfuron and cloransulam-
methyl were applied at three rates to velvetleaf and common ragweed (Table 4). In the
yellow nutsedge study, chlorimuron was included as an ALS-inhibiting yellow nutsedge
conuol treatment. Cloransulam-methyl was not included in the common lambsquarters
experiment since no control was observed in the field; therefore, this study was arranged
as a single—factor factorial experiment in a randomized complete block design.
Thifensulfuron at 2.2 g ai/ha was included as an ALS-inhibiting common lambsquarters
control treatment.

Giant foxtail antagonism study. Giant foxtail was thinned to ten plants per pot when
plants reached 3 cm. At application, giant foxtail was 8 to 13 cm tall with four to six

leaves. Plant top growth was harvested 21 DAT. Herbicide treatments consisted of an

 

”PO. Box 82, Woodstock, IL 60098.

‘2 PO. Box 9335, Fresno, CA 93791.

75

untreated control, oxasulfuron, and four graminicides applied at three rates alone and in a
tank mixture with oxasulfuron (Table 5). The mathematical method described by Colby
(1967) was used to determine antagonistic interactions. An LSD at the 5% level was used
to compare the observed and expected dry weight reduction.

Large crabgrass, johnsongrass, and corn interaction study. Seedling johnsongrass
and large crabgrass were thinned to five plants were pot, and corn ‘Pioneer 3573' was
thinned to two plants per pot prior to herbicide application one and a half weeks after
planting. Herbicides were applied to johnsongrass that was 13 to 15 cm tall and had four
leaves, large crabgrass 5 to 8 cm tall with three leaves, and com 15 cm tall at the two-
collar growth stage. J ohnsongrass and large crabgrass top growth was harvested 21 DAT
while corn was harvested 14 DAT. Heights were measured by averaging the height of all
the plants in the pot and oven dry weights were recorded. Percent height reduction was
calculated as 100*[1-(plant height/untreated plant height)]. Herbicide treatments
consisted of an untreated control, oxasulfirron, quizalofop, and clethodim applied alone
and in a tank mixture with oxasulfuron (Table 6).

W All of the research was conducted as factorial experiments arranged
in a randomized complete block design with four replications. An analysis of variance
was conducted and experiments were combined over time. The percent data was
transformed to the are sine for an analysis of variance and means were separated with an
LSD at the 5% level. Original means are reported. Soybean injury and common
lambsquarters control in the field for 1995 and 1996 are presented separately due to

interactions between years.

76

RESULTS AND DISCUSSION
Field Experiments. Soybean injury fiom oxasulfirron was 12 to 14%, and 9 to 13%
from cloransulam-methyl alone 7 DAT (Table 2). Tank mixtures of acifluorfen or
acifluorfen plus thifensulfuron with oxasulfuron or cloransulam-methyl injured soybeans
more than oxasulfuron or cloransulam-methyl alone, but were no more injurious than
acifluorfen or acifluorfen plus thifensulfuron alone. Thifensulfirron plus cloransulam-
methyl caused more injury to soybeans than cloransulam-methyl or thifensulfuron alone
in 1995, but no difference in soybean injury was detected in 1996. All treatments had
less than 10% soybean injury by 14 DAT (data not presented).

Velvetleaf was controlled by oxasulfuron and cloransulam-methyl applied alone
and with any tank mixture 21 DAT (Table 2). There was a 21% greater reduction in
velvetleaf dry weight when cloransulam-methyl was tank mixed with thifensulfirron
compared to thifensulfirron alone. Oxasulfirron reduced common ragweed dry weight
66%, and tank mixtures with acifluorfen reduced common ragweed dry weights by 98%
21 DAT. Common ragweed dry weight was reduced more when oxasulfuron was tank
mixed with thifensulfuron compared to oxasulfuron and thifensulfuron applied alone.
Common ragweed was controlled by cloransulam-methyl.

Common lambsquarters control by oxasulfuron alone was 81% in 1995 and 70%
in 1996 21 DAT (Table 2). All tank mixtures containing oxasulfuron provided greater
control of common lambsquarters than oxasulfirron alone except oxasulfuron plus
acifluorfen in 1996 (Table 2). Cloransularn-methyl did not control common

lambsquarters alone, but tank mixing cloransulam-methyl with thifensulfirron controlled

77

common lambsquarters. Acifluorfen, thifensulfuron, and acifluorfen plus thifensulfuron
tank mixtures provided greater control of redroot pigweed than oxasulfirron or
cloransulam-methyl alone 21 or 56 DAT in 1996. Redroot pigweed control from
oxasulfuron decreased by 56 DAT due to regrowth. Eastern black nightshade was not
controlled by oxasulfuron or cloransulam-methyl, and required acifluorfen for adequate
control (data not shown).

Oxasulfuron reduced giant foxtail dry weight by 46% 21 DAT (Table 3). Tank
mixing oxasulfirron or cloransulam-methyl with clethodim did not affect giant foxtail dry
weight reduction compared to clethodim alone 21 DAT. However by 56 DAT, giant
foxtail regrew and control was 36 to 49% less when oxasulfuron was tank mixed with
clethodim compared to clethodim applied alone. Ritter and Menbere (1997) also reported
that giant foxtail control by clethodim was antagonized by oxasulfuron. In 1996, giant
foxtail control by oxasulfuron tank mixed with quizalofop 56 DAT was similar to
quizalofop applied alone. Tank mixtures of cloransulam-methyl with clethodim or
quizalofop controlled giant foxtail.

Broadleaf weed sensitivity study. Velvetleaf was controlled by oxasulfuron and
cloransulam-methyl at 25% of the recommended field use rate (Table 4). Common
ragweed regrowth fi'om the lateral and axillary buds was observed following the
application of oxasulfuron at 20 and 40 g/ha. Ideal growing conditions and smaller
common ragweed may have improved common ragweed control by oxasulfuron
compared to field experiments. Cloransularn-methyl at 8.8 and 17.5 g/ha reduced

common ragweed dry weight 94 and 96%, respectively. Regrowth fi'om the lateral and

78

axillary buds was observed only at the 4.4 g/ha rate. Chlorimuron reduced yellow
nutsedge dry weights more than oxasulfuron or cloransulam-methyl. Applications of
cloransulam-methyl caused crinkling of the newly emerging yellow nutsedge leaves
(personal observation 3 to 7 DAT). Susceptibility of yellow nutsedge to cloransulam-
methyl or oxasulfuron has not been previously reported. Thifensulfuron reduced
common lambsquarters dry weight 22% more than oxasulfuron at 79 g/ha. Common
lambsquarters regrew fiom axillary buds and numerous narrow leaves formed following
the application of oxasulfuron at 70 g/ha (personal observation).

Giant foxtail antagonism. All rates of quizalofop, sethoxydim, fluazifop, and clethodim
controlled giant foxtail (Table 5). Oxasulfuron reduced giant foxtail dry weight by 26%.
Giant foxtail control was antagonized when oxasulfuron was tank mixed with any
graminicide regardless of the graminicide application rate according to Colby’s method.
Quizalofop was not antagonized by oxasulfirron to the extent that sethoxydim, fluazifop,
or clethodim were antagonized. Giant foxtail regrowth in the form of new tillers or
continued apical meristrnatic growth was not observed in the tank mixture treatments, but
the grass did not die.

Large crabgrass, johnsongrass, and corn interaction study. Oxasulfuron reduced
seedling johnsongrass height and dry weight by 80% and 91%, respectively (Table 6).
Quizalofop or clethodim alone or tank mixed with oxasulfuron controlled johnsongrass.
Oxasulfuron had limited activity on large crabgrass. However, when oxasulfuron was
tank mixed with quizalofop or clethodim, large crabgrass height increased compared to

quizalofop or clethodim applied alone. Large crabgrass dry weights were reduced more

79

by quizalofop alone than quizalofop tank mixed with oxasulfirron. Large crabgrass
tillered and regrew when oxasulfuron was tank mixed with quizalofop (personal
observation). Oxasulfuron reduced corn height by 65% and dry weight by 75%. Control
of corn by quizalofop and clethodim was not inhibited when oxasulfuron was tank mixed.
Our research indicated that tank mixtures with oxasulfuron or cloransulam-methyl
were required for enhanced grass and broadleaf weed control. Grass antagonism was
species and herbicide dependent. Grass species and herbicide selection should be
carefully evaluated before tank mixing an ALS-inhibiting herbicide with a graminicide.
None of the postemergence treatments evaluated adequately controlled both grass and
broadleaf weeds. A preemergence acetanilide application followed by postemergence
application of oxasulfuron or cloransulam-methyl offer broadspectrum weed control
(Bauer et al. 1996; Kidder and Johnson 1997; Schultz et al. 1995). Further investigations

should evaluate three-way postemergence tank mixtures for broad spectrum weed control.

80

LITERATURE CITED

Anonymous. 1995. CGA—277476 herbicide. Technical information bulletin. Ciba Crop
Protection, Greensboro, NC. 4 p.

Bauer, T. A., S. W. Pruss, T. R. Dill, and D. Kidder. 1996. Sequential soybean herbicide
programs including CGA—277476. Proc. North Cent. Weed Sci. Soc. 51 :in press.

Bauer, T., T. Threewitt, L. Stahlberg, A. Moses, K. Jones, D. Kidder, and P. Porpiglia.
1995. Weed management programs in soybeans with CGA-277476. Proc. North Cent.
Weed Sci. Soc. 50:140.

Bjelk, L. A. and T. J. Monaco. 1992. Effect of chlorimuron and quizalofop on fatty acid
biosynthesis. Weed Sci. 40: 1-6.

Brooks, R. L, P. J. Porpiglia, J. W. Peek, W. Meyer, K. Gutbrod, and H. Gut. 1995.
CGA-277476, a new postemergence herbicide for soybeans. Abstr. Weed Sci. Soc. Am.
35:8.

Colby, S. R. 1967. Calculating synergistic and antagonistic responses of herbicide
combinations. Weeds 15:20-22.

Croon, K. A. and M. G. Merkle. 1988. Effects of bentazon, imazaquin, or chlorimuron
on haloxyfop or fluazifop-P efficacy. Weed Technol. 2:36-40.

Ditrrrarsen, S. C., R. A. Dorich, L. B. Rowland, and M. E. Schultz. 1997. Efficacy and
antagonism of tank mixtures of cloransulam-methyl with postemergence graminicides in
soybeans. Abstr. Weed Sci. Soc. Am. 37 :3.

Ferreira, K. L., J. D. Burton, and H. D. Coble. 1995. Physiological basis for antagonism
of fluazifop-P by DPX-PE350. Weed Sci. 43:184-191.

Ferreira, K. L. and H. D. Coble. 1994. Effect of DPX-PE350 on the efficacy of
graminicides. Weed Sci. 42:222-226.

Gerwick, B. C., P. Thompson, and R. Noveroske. 1988. Potential mechanisms in
antagonism with aryloxyphenoxypropionate herbicides. Abstr. Weed Sci. Soc. Am.
28:100.

Green, J. M. 1991. Maximizing herbicide efficiency with mixtures and expert systems.
Weed Technol. 5:894-897.

Hatzios, K. K., and D. Penner. 1985. Interactions of herbicides with other agrochemicals
in higher plants. Rev. Weed Sci. 1:1-63.

81

Jachetta, J. J ., J. C. Van Heertum, and B. C. Gerwick. 1995. Cloransulam-methyl: a
new herbicide for soybeans. Abstr. Weed Sci. Soc. Am. 35:8.

Jordan, D. L., R E. Frans, and M. R. McClelland. 1993. Interactions of DPX-PE350
with fluazifop-P, sethoxydim, clethodim, and quizalofop-P. Weed Technol. 7:605-610.

Kidder, D. and M. D. Johnson. 1997. Grass control in soybeans with CGA—277476;
alone, in sequential, and in tankrnix programs. Abstr. Weed Sci. Soc. Am. 37:1.

Kidder, D. W. and P. J. Porpiglia. 1996. CGA-277476 and CGA-248757 for broad
spectrum, postemergence weed control in soybeans. Abstr. Weed Sci. Soc. Am. 36:12.

Menbere, H. and R. L. Ritter. 1997. Use of cloransulam-methyl in soybeans. Abstr.
Weed Sci. Soc. Am. 37 :2.

Monks, C. D., J. W. Wilcut, and J. S. Richburg, III. 1993. Broadleaf weed control in
soybean (Glycine max) with chlorimuron plus acifluorfen or thifensulfirron mixtures.
Weed Technol. 7:317-321.

Oliver, L. R. and J. R. Gander. 1997. Momingglory spectrum for cloransulam
(FirstRate). Abstr. Weed Sci. Soc. Am. 37:2.

Putnam, A. R. and D. Penner. 1974. Pesticide interactions in higher plants. Residue
Review. 50:73-110.

Ritter, R. L. and H. Menbere. 1997. Use of CGA-277476 in soybeans. Abstr. Weed Sci.
Soc. Am. 37:1.

Sander, K. W., T. R. Dill, D. W. Kidder, and P. J. Porpiglia. 1995. Update on the
herbicidal activity of CGA-277476 and CGA-248757 in soybeans in the midwest. Proc.
North Cent. Weed Sci. Soc. 50:139-140.

Schultz, M. E., D. L. Grant, V. B. Langston, and R. A. Dorich. 1995. Cloransularn-

methyl performance when applied postemergence in US. soybeans. AbStr. Weed Sci.
Soc. Am. 36:13.

Simpson, D. M. and E. W. Stoller. 1996. Thifensulfuron and imazethapyr interaction at
the ALS enzyme in sulfonylurea-tolerant soybean (Glycine max). Weed Sci. 44:763-
768.

Thompson, L. G., J. J. Velovitch, A. Haack, and R. A. Dorich. 1995. Tank-mix
alternatives with cloransulam-methyl for postemergence control of common
lambsquarters and pigweed species. Proc. North Cent. Weed Sci. Soc. 50:124.

82

Wesley, M. T. and D. R. Shaw. 1992. Interactions of diphenyl ether herbicides with
chlorimuron and imazaquin. Weed Tech. 6:345-351.

Young, B. G., S. E. Hart, and L. M. Wax. 1996. Interactions of sethoxydim and corn
(Zea mays) broadleaf herbicides on three annual grasses. Weed Technol. 10:914-922.

83

Table l . Weed height and leaf number at the time of application for field experiments
conducted in 1995 and 1996.

 

 

---------- 1995---------- ----------1996----------
Weed Species height leaf height leaf

cm number cm number

Giant foxtail 3-8 3 3-8 3

Velvetleaf 3-5 1-3 3-5 1-3
Common ragweed 3-6 4-6 3-6 4-6
Common lambsquarters 3-4 4-6 3-4 4-6
Redroot pigweed --- --- 3-4 2-5

Eastern black nightshade --- --- 1-2 cota-2

 

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85

Table 3. Giant foxtail control by oxasulfuron and cloransulam-methyl alone and tank
mixed with clethodim and quizalofop.

 

 

 

 

 

21 DAT‘I 1995 1996
Dry weight Control Control
Treatment" Rate reductionc 56 DAT 56 DAT
g ai/ha %
Clethodim 140 87 a 99 a 99 a
Quizalofop 50 ---------- 96 a
Oxasulfiiron 79 46 b 13 c 25 c
Oxasulfuron 79 79 a 50 b 63 b
+ clethodim + 140
Oxasulfiiron 79 ---------- 86 a
+ quizalofop + 50
Cloransulam 17.5 0 c 7 d 11 c
Cloransulam 17.5 84 a 96 a 99 a
+ clethodim +140
Cloransulam 17.5 ---------- 96 a

+ quizalofop + 50

aAbbreviations: Cloransulam, cloransulam-methyl; DAT, days after treatment.

l’All treatments included 0.25% (v/v) nonionic surfactant and 2.6% (v/v) urea ammonium
nitrate. Means followed by a common letter are not significantly different with an LSD at
the 5% level.

cData was combined over 1995 and 1996.

 

86

Table 4. Weed control by oxasulfuron and cloransulam-methyl in greenhouse
experiments 28 days afier treatment.

 

 

Treatment‘ Rate ABUTHb AMBEL CYPES CHEAL
g ai/ha -------------- % Dry weight reduction --------------
Oxasulfuron 20 98 69 c 21 de 37 c
40 98 81 b 10 e 59 b
79 98 93 a 38 cd 60 b
Cloransulam 4.4 97 84 b 18 e --~-
8.8 98 94 a 19 e ----
17.5 98 96 a 50 be ----
Chlorimuron 13.5 ---- ---- 94 a ----
Thifensulfuron 2.2 ---- ---- ---- 82 a

 

aAll treatments included 0.25% (v/v) nonionic surfactant and 2.5% (v/v) urea ammonium
nitrate. Means followed by a common letter are not significantly different with an LSD at
the 5% level.

bAbbreviations: ABUTH, velvetleaf; AMARE, redroot pigweed; AMBEL, common
ragweed; CHEAL, common lambsquarters; Cloransulam, cloransulam-methyl; CYPES,
yellow nutsedge; DAT, days after treatment.

87

Table 5. Giant foxtail control by oxasulfuron alone and tank mixed with graminicides 21
days after treatment.

 

 

 

 

None Oxasulfuron
Treatmenta Rate 0 79
g ai/ha ---------- % Dry weight reductionb ----------
None 0 0 j 26 i
Quizalofop 25 87 ab 82 d-g *(90)
50 89 a 84 c-e *(92)
100 89 a 86 b-d "(92)
Sethoxydim 107 86 a-c 79 gh I"(90)
213 86 a-c 79 e-h *(90)
426 88 ab 83 d-f *(91)
Fluazifop 93 8S b-d 79 h *(89)
186 89 a 80 f-h *(92)
358 88 ab 80 f-h *(91)
Clethodim 71 87 a-c 78 h *(90)
140 88 ab 77 h *(91)
280 88 ab 80 f-h *(91)

 

'All treatments included 0.25% (v/v) nonionic surfactant and 2.5% (v/v) urea ammonium
nitrate. Means followed by a common letter are not significantly different with an LSD at
the 5% level.

b Comparisons between columns are valid. Values in parentheses are the calculated
expected dry weight reduction for the herbicide tank mixture. A single asterisk indicates
a significant antagonistic tank mixture, while two asterisks indicate actual control that
was not significantly different from the graminicide applied alone.

88

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CHAPTER 3

WEED CONTROL IN SOYBEAN [Glycine max (1..) Merr]
WITH IMAZAMOX AND IMAZETHAPYRl

Kelly A. Nelson, Karen A. Renner, and Donald Penner2
Abstract. Field and greenhouse experiments were conducted in 1995 and 1996 to
determine soybean response and weed control from imazamox and imazethapyr applied
alone and with selected tank mixtures applied postemergence. Imazamox injured
soybean more than imazethapyr in the greenhouse, but not in the field. Applying a
methylated seed oil with imazamox or imazethapyr increased soybean injury and weed
control compared to the use of a nonionic surfactant. Imazethapyr provided greater
yellow nutsedge control than imazamox, while imazamox provided greater control of
common lambsquarters than imazethapyr in the field and greenhouse. Common
lambsquarters control by imazamox was reduced when lactofen was tank mixed with
imazamox in the field. Tank mixing thifensulfuron with imazethapyr increased common
lambsquarters control; however, soybean response increased when thifensulfuron was
tank mixed with imazamox or imazethapyr. Wild mustard, velvetleaf, redroot pigweed,

and common chickweed were controlled by imazamox and imazethapyr in the field.

 

1Received for publication and in revised form

 

2Grad. Res. Asst., Prof., and Prof., Mich. State Univ., East Lansing, MI 48824-1325.

89

90

Common ragweed dry weight was reduced 61 to 64% from imazamox at 35 g ai/ha and
imazethapyr at 70 g ai/ha in the field; however, imazamox provided greater common
ragweed control than imazethapyr in the greenhouse. Tank mixtures of lactofen with
imazamox or imazethapyr increased common ragweed control and resulted in greater
soybean seed yield than when imazamox and imazethapyr were applied alone; however,
tank mixtures of lactofen, acifluorfen, or fomesafen with imazamox and imazethapyr
reduced giant foxtail control in the greenhouse and field. Giant foxtail control was
antagonized more when diphenyl ether herbicides were tank mixed with imazethapyr than
imazamox. Imazamox reduced giant foxtail dry weight more than imazethapyr in
greenhouse studies, and adding urea ammonium nitrate with nonionic surfactant to
imazamox or imazethapyr increased giant foxtail control and reduced tillering. Both
herbicides antagonized giant foxtail control by clethodim in the greenhouse and in the
field in 1995, but imazethapyr was more antagonistic than imazamox in the greenhouse.
Nomenclature: Acifluorfen, 5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic
acid; chlorimuron, 2-[[[[(4-chloro-6-methoxy-2-
pyrimidinyl)amino]carbonyl]amino]su1fonyl]benzoic acid; clethodim, (E,E)—(_t)-2-[1-
[[(3-chloro-2-propenyl)oxy]imino]propyl]-5-[2-(ethy1thio)propyl]-3-hydroxy—2-
cyclohexen-l-one; fomesafen, 5-[2-chloro-4—(trifluoromethyl)phenoxy]-N-
(methylsulfonyl)-2-nitrobenzamide; imazamox, 2-(4—isopropyl-4-methyl-5-oxo-2-
imidazolin-2-yl)-5-(methoxymethyl) nicotinic acid; imazethapyr, 2-[4,5-dihydro-4-
methyl-4-(1-methylethyl)-5-oxo-1H—imidazol-Z-yl]-5-ethyl-3-pyridinecarboxylic acid;

lactofen, (i)-2-ethoxy-1-methyl-2-oxoethyl 5-[2-chloro-4-(trifluoromethyl)phenoxy]2-

91
nitrobenzoate; thifensulfuron, 3-[[[[(4-methoxy-6-methyl-1,3,5-tn'azin-2-
y1)arnino]carbonyl]amino]sulfonyl]-2-thiophenecarboxylic acid; common chickweed,
Stellaria media (L.) Vill. #3 STEME; common lambsquarters, Chenopodium album L. #
CHEAL; common ragweed, Ambrosia artemisiifolia L. # AMBEL; eastern black
nightshade, Solanum ptycanthum Dun. # SOLPT; giant foxtail, Setariafaberi Herrm. #
SETFA; wild mustard, Sinapis arvensis L. # SINAR; redroot pigweed, Amaranthus
retroflexus L. # AMARE; velvetleaf, Abutilon theophrasti Medik. # ABUTH; yellow
nutsedge, Cyperus esculentus L. # CYPES; soybean, Glycine max (L.) Merr.

Additional index words: Adjuvant, antagonism, herbicide interaction, tank mixture.

 

3Letters following this symbol are WSSA-approved computer code fi'om Composite List
of Weeds, Revised 1989. Available from WSSA, 1508 West University Ave.,
Champaign, IL 61821-3133.

92

INTRODUCTION

Imazamox and imazethapyr are imidazolinone herbicides used for postemergence
grass and broadleaf weed control in soybeans. These herbicides inhibit acetolactate
synthase (ALS)‘ which is essential for leucine, valine, and isoleucine synthesis
(Anonymous; Anonymous 1995). The replacement of the ethyl substituent on the
pyridine carboxylic acid ring of imazethapyr with a methoxymethyl characterizes the
only chemical structure difference between imazamox and imazethapyr. The
recommended use rate for imazamox is 35 to 45 g ai/ha compared to 70 g ai/ha for
imazethapyr (Anonymous; Anonymous 1995).

There are numerous postemergence herbicide tank mixtures available to soybean
producers. Tank mixture decisions depend primarily on weed pressure, weed species and
size, resistance management strategies, rotational crop restrictions, interactions between
herbicides, and cost effectiveness. Interactions between herbicides in a tank mixture have
been classified as additive, synergistic, or antagonistic (Hatzios and Penner 1985; Putnam
and Penner 1974). Thifensulfinon has been tank mixed with several herbicides to
enhance common lambsquarters control (Green 1991; Monks et al. 1993), while the
diphenyl ether herbicides have been tankrnixed with similar herbicides to control
common ragweed (Leif and Taylor 1993; Monks et al. 1993; Wilcut 1991) and ALS-

resistant weeds (Gaeddert et al. 1996) in soybeans. However, tank mixtures may increase

 

‘Abbreviations: ALS, acetolactate synthase (E.C. 4.1.3.18); MSO, methylated seed oil;
NIS, nonionic surfactant; and UAN, 28% urea ammonium nitrate; DAT, days afier
treatment; WAT, weeks after treatment.

93

soybean injury (Simpson and Stoller 1996) or antagonize weed control by one or more of
the herbicides in the mixture.

Adjuvants have been utilized in herbicide tank mixtures to reduce antagonism
(Gerwick et al. 1990; Hart et al. 1992; Wanamarta et al. 1993) and increase weed control
(Gednalski et a1. 1995; Gronwald et al. 1993; Nalewaja et al. 1990). Adjuvants enhance
herbicide absorption by increasing spray retention and foliar penetration (Gronwald et al.
1993; Penner 1989). Foy and Witt (1993) reported that soybean injury increased when
adjuvants were applied with imazethapyr. Hager and Renner (1994) increased common
ragweed control in the greenhouse by adding 28% urea ammonium nitrate (U AN)4 to
imazethapyr plus a nonionic surfactant (NIS)‘ or petroleum oil adjuvant. Kochia [Kochia
scoparia (L.) Schrad.] control by imazethapyr (Nalewaja et al. 1990); giant foxtail control
by primisulfuron (Hart et al. 1992); and weed control by imazethapyr (Ogg et al. 1988) or
imazamox (Gednalski et a1. 1995) was increased with a methylated seed oil (MSO)4
compared to a NIS. Thompson et al. (1996) reported increased l“C-imazethapyr
absorption by leafy spurge (Euphorbia esula L.) when a methylated seed oil was applied
instead of a nonionic surfactant with or without UAN. Hart et al. (1992) enhanced l“C-
primisulfirron absorption by giant foxtail and had less tank mixture antagonism when a
MSO was included.

Several researchers have reported graminicide antagonism by the ALS-inhibiting
herbicides. Imazethapyr antagonized giant foxtail control by sethoxydim (2-[1-
ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-l —one) (Cantwell ct

al. 1989), broadleaf signalgrass [Brachiaria platyphylla (Griseb.) Nash] control by

94
fluazifop ((:)-2-[4-[[5-(trifluoromethyl)-2-pyridinyl]oxy]phenoxy]propanoic acid) and
sethoxydim, and large crabgrass [Digitaria sanguinalis (L.) Scop.] and fall panicum
(Panicum dichotomiflorum Michx.) control by fluazifop, clethodim, quizalofop ((:)-2-[4-
[(6-chloro-2-quinoxa1inyl)oxy]phenoxy]propanoic acid), and sethoxydim (Myers and
Coble 1992). Grass antagonism by the ALS-inhibitors was greatest when plants were
water stressed and not actively growing (Croon and Merkle 1988; Myers and Coble 1992;
Vidrine 1989).

Giant foxtail control by imazethapyr has been antagonized by acifluorfen
(Cantwell et al. 1989). Similarly, the diphenyl ether herbicides have antagonized grass
control by the acetyl CoA carboxylase (ACCase) inhibitors by reducing absorption
(Godley and Kitchen 1986; Holshouser and Coble 1990; Minton et al. 1989; Vidrine
1989; Whitwell et a1. 1985).

Sodium cations have reduced herbicide absorption similar to bentazon (3-(1-
methylethyl)-(1H)-2,1,3-benzothiadiazin-4(3H)—one 2,2-dioxide) tank mixtures (Bauer et
al. 1995; Wanamarta et al. 1989). Cantwell et a1. (1989) reported that bentazon and
acifluorfen tank mixed with imazethapyr reduced giant foxtail and smooth pigweed
(Amaranthus hybridus L.) control. Since acifluorfen and fomesafen are formulated as
sodium salts similar to bentazon, the presence of sodium cations could explain the giant
foxtail antagonism reported by Cantwell et al. (1989).

Herbicide antagonism is detrimental to effective and efficient postemergence
weed management. Imazamox offers an additional option for postemergence grass and

broadleaf weed control for producers whose choices are limited by rotational crop

95

restrictions (Lueschen et al. 1997). Imazamox has broadspectrum activity that may not
require a herbicide tank mixture. The objectives of this research were: to evaluate
postemergence applications of imazamox and imazethapyr alone and with selected tank
mixtures for soybean injury and weed control; to determine soybean and weed sensitivity
to imazamox and imazethapyr; to determine the influence of adjuvants on soybean injury
and weed control; and to evaluate giant foxtail antagonism with imazamox and

imazethapyr tank mixtures.

96

MATERIALS AND METHODS
W. A field experiment was conducted in 1995 and 1996 at the
Michigan State University Research Farm at East Lansing, MI on a Capac sandy clay
loam (fine-loamy, mixed mesic Aeric Ochraqualfs) soil with 2.7% organic matter in 1995
and 2.4% organic matter in 1996. The soil pH was 6.6 in 1995 and7.9 in 1996. The
1995 site was fall chisel plowed as well as cultimulched and field cultivated in the spring,
while the 1996 site was disked in the fall and cultimulched twice in the spring.

‘Conrad’ soybeans were planted in 76 cm rows at 341,000 seeds/ha in 1995 and
395,000 seeds/ha in 1996 with a John Deere 7200 Max-Emerge” 25 planter. The plots
were 3 m wide and 9.1 m long in 1995, and 3 m wide and 12.2 m long in 1996.

Herbicide treatments included an untreated control, imazamox at 35 g ai/ha, and
imazethapyr at 70 g ai/ha. Each of these treatments were applied alone and also in a tank
mixed with clethodim at 140 g ai/ha, lactofen at 70 g ai/ha, thifensulfuron at 2.2 g ai/ha,
and lactofen plus thifensulfuron (Table 2). Acifluorfen at 280 g ai/ha and fomesafen at
140 g ai/ha were included in the factorial design in 1996 to compare weed control when
imazamox and imazethapyr were tank mixed with each of the three diphenyl ether
herbicides (Table 8). All treatments were applied with 2.6% (v/v) UAN and 0.25% (v/v)

NIS.6

 

‘Deere and Co., 501 River Drive, Moline, IL 61265-1100.

6Nonionic surfactant was Activator-90, a mixture of alkyl polyoxyethylene ether and free
fatty acids, Loveland Industries Inc., P.O. Box 1289, Greeley, CO 80632.

97

Herbicides were applied at a standard postemergence timing on June 9, 1995 and
June 12, 1996 (29 and 26 days after planting) with a tractor mounted compressed air
sprayer traveling at 6.3 km/h and delivering 178 L/ha at 207 kPa of pressure. Treatments
were applied with 8003 flat fan nozzles7 on a 51 cm spacing at 48 cm above the weed and
crop canopy. At the time of herbicide application, the air temperature was 14 C with 80%
relative humidity in 1995, and 27 C with 70% relative humidity in 1996. Rainfall 10
days prior to herbicide application was 0.5 cm in 1995 and 3.6 cm in 1996, while rainfall
from 1 to 10 days after herbicide application totaled 0.5 cm in 1995 and 9.8 cm in 1996.
Soybeans were at the V1 to V2 stage of development in 1995 and 1996. Weed heights
and leaf number at the time of application are presented in Table 1.

Soybean injury was evaluated at 3, 7, l4, and 21 days after treatment (DAT)“.
Soybean injury evaluation was based on chlorosis, necrosis, and stunting. Injury values
are presented for 7 and 14 DAT. Weed control was evaluated 7, 14, 21, 28, and 56 DAT.
Evaluations taken at 21 , 28, and 56 DAT were most representative of herbicide
effectiveness. Weed control was visually evaluated and measured as a percent reduction
in dry weight. Visual ratings were based upon a scale of 0 (no effect) to 100 (complete
weed or crop death) percent. Percent dry weight reduction was determined 21 DAT by
harvesting the above ground plant matter of three common ragweed and three common

lambsquarters plants that were marked in each plot with plastic garden stakes8 prior to the

 

7Teej et flat fan tips. Spraying Systems Co., North Ave. and Schmale Road, Wheaton, IL
60188.

“Pylon Plastics, Inc. 2111 Ogden Ave., PO. Box 505, Lisle, IL 60532.

98

herbicide application. Percent dry weight reduction was calculated as 100*[1-(p1ant dry
weight/untreated plant dry weight)]. Yellow nutsedge control was evaluated 8 weeks
afier treatment (W AT)4 in 1995 and again 56 WAT in July of 1996. Other weeds were
controlled in 1996 with postemergence applications of glyphosate (N-
(phosphonomethyl)glycine) at 840 g/ha plus 2100 g/L ammonium sulfate and 0.5% (v/v)
NIS on May 2, 1996, glyphosate at 840 g/ha plus pyrazon (5-amino-4-chloro-2-phenyl-
3(2H)-pyridazinone) at 3360 g/ha and diethatyl (N-(chloroacetyl)-N-(2,6-
diethylphenyl)glycine) at 3360 g/ha on May 20, 1996, clopyralid (3,6-dichloro-2-
pyridinecarboxylic acid) at 420 g/ha on June 3, 1996, and quizalofop ((1)-2-[4-[(6-chloro-
2-quinoxalinyl)oxy]phenoxy]propanoic acid) at 49 g/ha plus crop oil concentrate’ at 1.0%
(v/v). Since no two-way interaction was observed and the effect of tank mix partners was
not significant for yellow nutsedge control data, the data was averaged over these factors
for the evaluation of imazamox, imazethapyr, and tank mix partners applied alone. The
center two soybean rows were harvested in each plot with a Massey 1010 small plot
combine and moisture adjusted to 13%.
W. Yellow nutsedge tubers, common
ragweed, common lambsquarters, velvetleaf, and giant foxtail seeds were planted in
BACCTO” potting soil in 946 ml plastic pots. Environmental conditions were

maintained at 27 i 5 C. Weeds were grown in a 16 h photoperiod of natural and

 

9Crop oil concentrate was Herbirnax, 83% petroleum oil and 17% surfactant, Loveland
Industries Inc., PO. Box 1289, Greeley, CO 80632.

loKincaid Equipment Manufacturing. PO. Box 400, Haven, KS 47543.

“Baccto is a product of Michigan Peat Co. Houston, TX 7 7098.

99

supplemental sodium vapor lighting that provided a photosynthetic photon flux density'2
of 120 aE/mz/s. Midday photosynthetic photon flux density was approximately 1000
,uE/mz/s. Plants were fertilized with 0.1 g of water soluble fertilizer (20% N, 20% P202,
20% K20) and watered as needed.

Herbicide applications were made with a continuous belt-link sprayer traveling at
1.5 km/hr and equipped with an 8001 even flat fan nozzle7 calibrated to deliver 234 L/ha
at 193 kPa of pressure. All treatments were replicated four times and repeated in time.
The soil was moist at the time of herbicide application and soil temperature was 26 j; 1 C.
Percent dry weight reduction of the oven dried top plant growth was calculated as
previously described. Expected control of each herbicide tank mixture was calculated by
the method described by Colby (1967). An LSD at the 5% level was used to compare the
observed and expected values to determine significant differences.
Weed species sensitivity and adjuvant study. Common ragweed, common
lambsquarters, and velvetleaf seeds were purchased from V & J Seed Farms13 and thinned
to one plant per pot prior to herbicide application. Yellow nutsedge tubers were
purchased from Valley Seed Service" and selected for plants with two shoots that arose
fiom one tuber prior to herbicide application. At the time of application, common
ragweed was 4 cm tall with four to six leaves, common lambsquarters were 4 cm tall with

four to six leaves, velvetleaf was 7 cm tall with three leaves, and yellow nutsedge was 10

 

”LI-COR, 4421 Superior Street, Lincoln, NE 68504.
13PO. Box 82, Woodstock, IL 60098.

”PO. Box 9335, Fresno, CA 93791.

100

cm tall with five leaves. Air temperature was 28 i 1 C and relative humidity was low (36
to 40%) at the time of herbicide application. All herbicide applications were made in the
evening from 2:00 pm. to dusk. Plant top growth was harvested 28 DAT. Herbicide
treatments consisted of NIS or MSOls with imazamox or imazethapyr, each of which
were applied at 18, 35, and 70 g/ha (Table 9). All treatments included 2.5% (v/v) UAN
and either 1.0% (v/v) M80 or 0.25% (v/v) NIS.

Influence of adjuvants on giant foxtail control. Giant foxtail seeds were purchased
from V & J Seed Farms and after emergence plants were thinned to ten per pot prior to
herbicide application. Imazamox and imazethapyr were applied at 35 g/ha, and evaluated
alone and with NIS at 0.25% (v/v), UAN at 2.5% (v/v), or NIS plus UAN. Treatments
were applied to giant foxtail that were 10 cm tall with four to five leaves in the evening
with a photon flux density of 150 aE/mzls and moderate relative humidity (60%).

In addition to dry weight reduction, tiller presence was determined. Tiller
presence was a measure of giant foxtail regrth since dry weight reduction did not
completely quantify the results. Tiller presence was evaluated as a 1 if more than one-
half of the plants had tillered, and 0 if less than one-half of the plants had tillered. This
was converted to a percentage of tillered plants (100 = tillering and regrowth, while 0 =
no tillering or complete plant death).

Giant foxtail antagonism studies. Giant foxtail seeds were purchased fi‘om V & J Seed

Farms and after emergence plants were thinned to ten per pot prior to herbicide

 

”Methylated seed oil was Sun-It II”, a modified vegetable oil containing surfactant, from
Agsco, Inc., P.O. Box 13458, Grand Forks, ND 58208-3458.

101

application. Clethodim antagonism by imazamox and imazethapyr, imazamox and
imazethapyr antagonism by the diphenyl ether herbicides, and the effects of adjuvants or
sodium acetate on giant foxtail antagonism were evaluated.

Clethodim antagonism by imazamox and imazethapyr. Giant foxtail was 5 to 7 cm tall
with four leaves, photosynthetic photon flux density was 300 to 320 aE/mzls, and relative
humidity was low (39 to 55%) at the time of herbicide application. Herbicide
applications included clethodim at 0, 18, and 36 g/ha; imazamox and imazethapyr at 9,
18, 35, and 70 g/ha; and an untreated control. All treatments included 0.25% (v/v) NIS
and 2.5% (v/v) UAN.

Imazamox and imazethapyr antagonism by the diphenyl ether herbicides. Giant foxtail
was 5 to 7 cm tall with four leaves, photosynthetic photo flux density was 175 to 450
aE/mzls, and relative humidity was low (40 to 52%) at the time of herbicide application.
Herbicide treatments included fomesafen, acifluorfen, and lactofen each at 140 g/ha
which were applied with imazamox at 35 and 70 g/ha, imazethapyr at 35 and 70 g/ha, and
an untreated control. All treatments included 2.5% (v/v) UAN and 0.25% (v/v) NIS. The
data presented are the means of three experiments with three replications in each.
Adjuvant effects on diphenyl ether antagonism. NIS at 0.25% (v/v) and UAN at 2.5%
(v/v) were evaluated for their effect on diphenyl ether antagonism of imazamox and
imazethapyr. Giant foxtail size and environmental conditions were previously described
in the methods section of the influence of adjuvants on giant foxtail control study.
Herbicide treatments included NIS, imazethapyr at 35 g/ha, imazamox at 35 gm,

imazethapyr plus NIS and UAN, imazamox plus NIS and UAN, and NIS plus UAN

102

which were applied with an untreated control, lactofen at 140 g/ha, acifluorfen at 280
g/ha, and fomesafen at 280 g/ha.

Sodium acetate effects on giant foxtail control with imazethapyr and imazamox. Giant
foxtail size and environmental conditions were as previously described for the adjuvant
evaluation. Herbicide treatments included an untreated control, imazamox at 35 g/ha, and
imazethapyr at 35 gm which were combined with NIS at 0.25% (v/v), lactofen at 140
g/ha, acifluorfen at 280 g/ha, fomesafen at 280 g/ha, and sodium acetate at 19 mM. A
sodium acetate concentration was calculated to provide an adequate number of sodium
cations that would compete with ammonium in the imazamox or imazethapyr
formulation. Means were separated with an LSD at the one percent level for control,
tiller presence, and dry weight reduction.

Soybean injury study. ‘Conrad’ soybeans were planted and thinned to one plant per pot
prior to herbicide application. Soybeans were in the V1 to V2 stage of development 17
days after planting, air temperature was 22 i 1 C, photosynthetic photon flux density was
100-350 uE/mz/s, and the relative humidity was low (32 to 52%) at the time of herbicide
application. The average leaf area and height for each run was 16 cm2 and 8 cm, 32 cm2
and 10 cm, and 61 cm2 and 10 cm, respectively. The differences between leaf areas were
related to the time of the year. Herbicide treatments consisted of imazamox and
imazethapyr at O, 35, 70, 140, and 280 g/ha applied with 2.5% (v/v) UAN and NIS at

0.25% (v/v) or MSO at 1.0% (v/v).

103

Soybean injury was evaluated 7 DAT, while leaf area‘6 was measured 21 DAT.
The percent reduction in leaf area was calculated as 100*[1-(soybean leaf area/untreated
soybean leaf area)]. Plants were harvested 21 DAT and the percent reduction in dry
weight was calculated as previously described.

Data was combined over time since no interactions over time were detected.
Soybean injury, leaf area reduction, and dry weight reduction were subjected to
regression analysis. Regression curves for a logistic dose response were determined by
TableCurve 2D17 software. Averages were plotted and the LSD at the five percent level
of probability was utilized to determine significant differences between treatments.
W. All of the research was conducted as factorial experiments arranged
in randomized complete block designs with four replications. An analysis of variance
was conducted and experiments were combined overtime. The percent data was
transformed to the arc sine for an analysis of variance and means were separated with an
LSD at the 5% level. Original means are reported. In field experiments, giant foxtail
control 56 DAT, common lambsquarters control 28 DAT, and soybean yield for 1995 and

1996 are presented separately due to interactions between years.

 

”LI-COR, P.O. Box 4425, Lincoln, NE 68504.

l7TableCurve 2D v. 3.1. Jandel Scientific, 2591 Kemer Boulevard, San Rafael, CA
94901.

104

RESULTS AND DISCUSSION
Field experiments. Soybean injury from imazamox and imazethapyr was 16 and 14% 7
DAT, respectively (Table 2). Tank mixtures with lactofen were the most injurious to
soybeans. Tank mixtures of imazamox or imazethapyr plus thifensulfuron injured
soybeans more than imazamox, imazethapyr, or thifensulfirron alone 7 DAT. Simpson
and Stoller (1996) reported increased injury to sulfonylurea tolerant soybeans when
imazethapyr was tank mixed with thifensulfuron because imazethapyr inhibited the ALS
enzyme. Soybean injury 14 DAT was 13% for imazamox and 11% for imazethapyr, but
by 21 DAT all treatments resulted in less than 10% injury to soybean (data not
presented).

Imazamox reduced common ragweed dry weight by 64% and imazethapyr
reduced common ragweed by 61% 21 DAT (Table 3). Common ragweed regrowth fi'om
the axillary and lateral buds was prominent by 21 to 28 DAT. Similar observations have
been reported by Ballard et al. (1996). There was no difference in common ragweed dry
weight reduction, control at 28 DAT, and control at 56 DAT between imazamox and
imazethapyr alone and when tank mixed with other herbicides. In other research,
imazamox at 35 and 45 g/ha provided greater common ragweed control than imazethapyr
in the presence of a MSO in wide row soybean (Nelson and Renner 1996). Lactofen
alone and lactofen tank mixtures with imazamox or imazethapyr effectively controlled
common ragweed based on dry weight reduction and visual control 28 and 56 DAT.
Tank mixtures of imazamox and imazethapyr with thifensulfuron provided greater control

of common ragweed than thifensulfuron alone 28 and 56 DAT.

105

Giant foxtail control by imazamox was 92% and by imazethapyr was 88% 28
DAT (Table 4). Tank mixtures with clethodim did not reduce control compared to
imazamox or imazethapyr alone 28 DAT; however by 56 DAT, giant foxtail control by
clethodim was reduced by imazamox and imazethapyr in 1995, but not in 1996. This
difference was probably due to adequate rainfall and actively growing giant foxtail prior
to and after the herbicide application in 1996. Myers and Coble (1992) reported severe
grass antagonism under water stressed conditions when imazethapyr was tank mixed with
several graminicides. Tank mixtures with lactofen reduced giant foxtail control
compared to either imazamox or imazethapyr 28 and 56 DAT. Lactofen reduced giant
foxtail control by imazethapyr more than by imazamox in 1996. Giant foxtail plants
treated with these tank mixtures initiated several new shoots or tillers by 14 to 21 DAT.
Giant foxtail control was 30% greater 56 DAT in 1996 compared to 1995 for both
imazamox and imazethapyr.

Imazamox reduced common lambsquarters dry weight by 80% and imazethapyr
reduced common lambsquarters dry weight by 55% 21 DAT (Table 5). Tank mixtures of
imazethapyr with lactofen plus thifensulfuron, but not thifensulfuron alone, reduced
common lambsquarters dry weight more than imazethapyr alone. By 28 DAT, common
lambsquarters control was 81% in 1995 and 1996 from imazamox, and 67 to 68% from
imazethapyr. Imazamox and imazethapyr enhanced common lambsquarters control when
tank mixed with lactofen compared to lactofen alone in 1995 and 1996. Thifensulfuron
tank mixed with imazethapyr increased common lambsquarters control compared to

imazethapyr alone.

106

Imazethapyr provided greater yellow nutsedge control than imazamox 8 WAT,
and this difference was still evident 56 WAT (Table 6). Grichar et al. (1992) reported the
suppression of yellow nutsedge by imazethapyr applied postemergence up to 142 DAT.
Wild mustard in 1995 and 1996, velvetleaf in 1995, redroot pigweed in 1996, and
common chickweed in 1996 were controlled with all treatments that included imazamox
or imazethapyr (data not presented).

All herbicide treated soybeans yielded less than the handweeded control in 1995
(Table 7). Soybeans treated with tank mixtures of imazamox or imazethapyr plus
clethodim, thifensulfuron, or lactofen plus thifensulfuron yielded more than plots treated
with clethodim, thifensulfuron, or lactofen plus thifensulfuron alone. Yield reduction
was primarily attributed to lactofen reducing giant foxtail control by imazamox and
imazethapyr in 1995. Common ragweed was not controlled by imazamox and
imazethapyr treatments that did not include lactofen in 1995 and 1996 and this resulted in
reduced soybean yields. Plots treated with tank mixtures of imazamox or imazethapyr
plus lactofen or thifensulfuron yielded the same as the handweeded control in 1996.
Soybeans treated with tank mixtures of clethodim, lactofen, thifensulfuron, and lactofen
plus thifensulfuron with imazamox or imazethapyr yielded more than soybeans treated
with clethodim, lactofen, thifensulfirron, and lactofen plus thifensulfuron alone. Giant
foxtail density was 140 plants/m2 in 1995 and 70 plants/m2 in 1996. Knake and Slife
(1962) reported soybean yields decreased as giant foxtail density increased. Greater giant
foxtail population densities could cause antagonistic interactions to have a greater effect

on yields. This could explain reduced yields in plots treated with lactofen plus

107

imazethapyr or imazamox compared to the handweeded control in 1995 that were not
seen in 1996. Antagonistic tank mixtures may not affect soybean yield at low weed
population densities, but weed seed production and maximum soybean productivity may
not be achieved.

Tank mixing imazamox or imazethapyr with any diphenyl ether did not cause
more injury than the diphenyl ether herbicide applied alone 7 and 14 DAT (Table 8).
Common ragweed was controlled and giant foxtail control reduced when imazamox and
imazethapyr were tank mixed with any diphenyl ether herbicide 28 and 56 DAT.
Common lambsquarters control by imazamox was reduced by lactofen, while acifluorfen
increased common lambsquarters control when tank mixed with imazamox or
imazethapyr. All tank mixtures with acifluorfen and lactofen resulted in yields greater
than imazamox or imazethapyr applied alone even though giant foxtail and common
lambsquarters control was reduced by lactofen.
Weed species sensitivity and adjuvant study. Imazamox at 35 g/ha plus a MSO
controlled common ragweed, while at 18 g/ha imazamox controlled common
lambsquarters and velvetleaf 28 DAT (Table 9). Imazethapyr at 70 g/ha plus MSO
reduced common ragweed dry weight similar to imazamox at 35 g/ha plus NIS, but
common ragweed regrowth from the lateral and axillary buds was observed for all
treatments except imazamox at 70 g/ha plus MSO. Ballard et a1. (1995) reported
metabolism and reduced basipetal translocation were the primary mechanisms for
reduced common ragweed control by imazethapyr. Common ragweed control differences

between imazamox and imazethapyr could be related to common ragweed

108

metabolism differences. Imazethapyr at 35 g/ha controlled velvetleaf. Seventy g/ha of
imazethapyr reduced common lambsquarters dry weight by 60% regardless of adjuvant.
Yellow nutsedge was suppressed by 70 g/ha of imazamox and imazethapyr, but
chlorimuron at 13 g ai/ha reduced yellow nutsedge dry weight by 94% (data not
presented). In contrast, other researchers have reported similar yellow nutsedge control
by imazethapyr and chlorimuron in the greenhouse (Ackley et al. 1996; Derr and Wilcut
1993)

Influence of adjuvants on giant foxtail control. Imazamox at 35 g/ha plus UAN and
NIS stopped giant foxtail tillering and reduced dry weight by 93% (Table 10). Tiller
number increased by 50% when UAN was not added to imazamox plus NIS.
Imazethapyr plus UAN and NIS reduced giant foxtail dry weight by 91%, yet 88% of the
plants tillered. Hager and Renner (1994) enhanced common ragweed control by adding
UAN to imazethapyr plus NIS. Thompson et al. (1996) reported enhanced l“C-
irnazethapyr absorption by leafy spurge when either NIS or NIS plus UAN were present.
Clethodim antagonism by imazamox and imazethapyr. Giant foxtail dry weight was
reduced by 91% from imazamox at 9 g/ha and 93% from imazethapyr at 35 g/ha 21 DAT
(Table 11). Imazethapyr antagonized giant foxtail control by clethodim in every
treatment combination. Antagonism was significant with imazamox plus clethodim tank
mixtures, but when the observed value controlled giant foxtail then a calculated value
may not be of great use unless the weed species is not controlled. Giant foxtail control by

clethodim tank mixtures was similar to the control observed by imazamox and

109

imazethapyr applied alone. Cantwell et a1. (1989) reported reduced giant foxtail control
as the imazethapyr rate decreased when imazethapyr was tank mixed with sethoxydim.
Imazamox and imazethapyr antagonism by the diphenyl ether herbicides.
Imazamox at 35 g/ha and imazethapyr at 70 g/ha controlled giant foxtail with no tillering
(Table 12). Tank mixtures of lactofen, acifluorfen, or fomesafen with imazamox or
imazethapyr at 35 or 70 g/ha reduced visual control and increased giant foxtail tillering.
The reason for increased tillering in the presence of the diphenyl ether herbicides could
be a result of reduced translocation. Chao et a1. (1994) reported a reduction in main shoot
fi'esh weight of wild oat (Avenafatua L.) when treated with imazamethabenz ((:)-2-[4,5-
dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H—imidazol-2-yl]-4(and 5)-methylbenzoic
acid (3:2)) because of limited translocation to the tiller buds as a result of apical
dominance (Chao et al. 1993, Little and Shaner 1991). This could be the reason for
increased tillering in the presence of the diphenyl ether herbicides. Giant foxtail
antagonism was similar for all diphenyl ether herbicides, even though acifluorfen alone
reduced giant foxtail dry weight by 84% while lactofen and fomesafen reduced giant
foxtail dry weight by 45 and 41%, respectively (data not shown). Lactofen and
fomesafen have some activity on grass species, but to a much lesser extent than
acifluorfen (Holshouser and Coble 1990; Minton et al. 1989).

Adjuvant effects on diphenyl ether antagonism. Imazamox plus NIS and UAN
reduced giant foxtail dry weight by 93% and stopped giant foxtail tillering (Table 13).
Not adding UAN or adding a diphenyl ether herbicide to the spray solution did not

increase giant foxtail dry weight, but increased the presence of tillers. Imazethapyr plus

110

NIS and UAN reduced giant foxtail dry weight by 91%, but 88% of the plants still
tillered. Not using UAN or adding a diphenyl ether herbicide increased giant foxtail dry
weight and this was considered antagonistic according to Colby’s test. Similar dry
weight reductions were observed when imazethapyr was tank mixed with lactofen,
acifluorfen, or fomesafen compared to imazethapyr alone. This was unlike the previous
study and was probably due to the smaller size of giant foxtail in this experiment and the
lower diphenyl ether application rates which resulted in less antagonism (Table 12).
Murphy and Briske (1992) reviewed numerous factors that influence tiller initiation by
grass species. The biomass differences could be related to the physiological development
differences of giant foxtail at the time of application.

Sodium acetate effects on giant foxtail control with imazethapyr and imazamox.
The addition of sodium acetate to imazamox or imazethapyr plus NIS did not reduce
giant foxtail control or increase tillering compared to imazamox or imazethapyr plus NIS
(Table 14). Thus, the formation of the sodium salts of the imidazolinone herbicides did
not explain the antagonism. The antagonism may be due to the difference in the timing
of expression of the action of the two classes of herbicides. Godley and Kitchen (1986)
as well as Westberg and Coble (1992) have proposed that the diphenyl ethers cause
widespread cell membrane disruption of the epidermal cells and reduce normal
absorption and translocation of systemic herbicides which results in antagonism. This
may occur when imazethapyr and imazamox are tank mixed with these herbicides on

giant foxtail. Since the primary sink of the imidazolinone herbicides is the main culm of

111

grass species (Chao et al. 1993, Little and Shaner 1991), this would allow the release of
lateral buds or tillers from apical dominance and allow regrowth.

Soybean injury study. Imazamox injured soybeans more than imazethapyr, regardless
of adjuvant 7 DAT (Figure la). Imazamox at 280 g/ha plus MSO injured soybeans more
than with NIS. Imazamox reduced soybean dry weight (Figure 1b) and leaf area (Figure
1c) more than imazethapyr 21 DAT. High rates of the imidazolinone herbicides were
used to identify potential injury differences when imazamox or imazethapyr were
combined with these adjuvants. At the recommended field rates of imazamox (35 g/ha)
and imazethapyr (70 g/ha), there was no difference in soybean injury, dry weight
reduction, or leaf area reduction between imazamox and imazethapyr. In other field
studies, increasing the imazamox rate to 45 g/ha with a methylated seed oil injured
soybeans greater than the imazamox at 35 g/ha and imazethapyr at 70 gm (Nelson and
Renner 1996).

This research has shown the need for tank mixtures with imazethapyr for the
control of common lambsquarters and common ragweed. Tank mixtures with imazamox
at 35 gm plus NIS and UAN were needed for common ragweed control. Application
rates of 45 g/ha may increase common ragweed control (Nelson and Renner 1996) with a
MSO. However, increased soybean injury and reduced weed control depended upon the
tank mixture and environmental conditions at the time of application. Lactofen,
acifluorfen, and fomesafen enhanced common ragweed control, but reduced giant foxtail

control by imazamox and imazethapyr. Imazethapyr and imazamox reduced giant foxtail

112

control by clethodim in the field, but imazethapyr was more antagonistic in the
greenhouse.

Adj uvants are an important tool to improve weed control and reduce herbicide
antagonism. Giant foxtail control was increased by the addition of UAN and NIS to
imazamox and imazethapyr. A MSO increased broadleaf weed control by imazamox and
imazethapyr compared to NIS in the greenhouse. Thompson et al. (1996) reported greater
absorption of l“C-imazethapyr by leafy spurge with MSO compared to NIS when UAN
was present. This could explain differences in weed control and soybean injury between
MSO and NIS treatments with imazamox and imazethapyr. Sodium cations present in
the diphenyl ether herbicide formulations do not appear to influence giant foxtail
antagonism since sodium acetate did not affect giant foxtail control by imazamox or
imazethapyr.

Imazamox with a MSO caused more soybean injury but provided greater weed
control than imazethapyr (Nelson and Renner 1996). Future research evaluating soybean
response to imazamox at 45 g/ha is needed (Nelson and Renner 1996). Absorption,
translocation, and metabolism studies of imazamox and imazethapyr on tolerant and
susceptible species could explain differences in weed control and lead to a greater

understanding of imidazolinone chemistry.

113

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117

Table I . Weed height and leaf number at the time of herbicide application for field
experiments conducted in 1995 and 1996.

 

 

 

1995 1996
Weed Species Height Leaf Height Leaf
cm number cm number
Common ragweed 3-4 4-8 3-4 4-8
Giant foxtail 5-8 3-4 5-8 3-4
Common lambsquarters 3-4 4-6 3-4 4-6
Wild mustard 3-8 4-6 3-8 4-6
Yellow nutsedge 5-15 5-7 --- ---
Velvetleaf 3 1-3 m ---
Redroot pigweed --- --- 1-15 4-7

Common chickweed --- --- 3-8 10-18

 

118

Table 2. Soybean injury 7 and 14 days after treatment by imazamox and imazethapyr
alone and when tank mixed with other herbicides in 1995 and 1996.

 

 

 

 

Soybean injury
Treatment‘ Rate 7 DAT 14 DAT
g ai/ha %

Imazamox 35 16 d 13 c-e
Imazethapyr 70 14 d 11 e
Imazamox + clethodim 35 + 140 15 d 13 c-e
Imazethapyr + clethodim 70 + 140 14 d 10 e
Clethodim 140 O f 1 f
Imazamox + lactofen 35 + 70 29 ab 16 b-d
Imazethapyr + lactofen 70 + 70 29 ab 18 ab
Lactofen 70 27 b 15 b—d
Imazamox + thifensulfuron 35 + 2.2 19 c 12 e
Imazethapyr + thifensulfuron 70 + 2.2 17 cd 12 e
Thifensulfuron 2.2 9 e 11 e
Imazamox + lactofen + thifensulfuron 35 + 70 + 2.2 29 ab 20 a
Imazethapyr + lactofen + thifensulfuron 70 + 70 + 2.2 31 a 19 a
Lactofen + thifensulfuron 70 + 2.2 29 ab 16 be

 

'All treatments included 0.25% (v/v) nonionic surfactant and 2.6% (WV) 28% urea
ammonium nitrate. Means followed by a common letter are not significantly different

with an LSD at the 5% level.

119

Table 3. Common ragweed control by imazamox and imazethapyr alone and when tank
mixed with other herbicides.‘

 

 

 

 

 

Common ragweed”

Dry weight reduction Control Control

Treatmentc Rate 21 DAT 28 DAT 56 DAT
g ai/ha %

Imazamox 35 64 be 79 de 39 cd
Imazethapyr 70 61 be 76 e 35 d
Imazamox 35 61 bc 79 de 39 cd
+ clethodim + 140
Imazethapyr 70 64 be 76 de 38 cd
+ clethodim + 140
Clethodim 140 0 d 0 g 0
Imazamox 35 100 a 95 be 94 b
+1actofen + 70
Imazethapyr 70 99 a 97 a 96 a
+ lactofen + 70
Lactofen 70 100 a 95 a-c 97 a
Imazamox 35 69 b 80 d 43 c
+ thifensulfuron + 2.2
Imazethapyr 70 61 bc 76 de 42 c
+ thifensulfuron + 2.2
Thifensulfuron 2.2 42 c 40 f 18 e
Imazamox 35 99 a 96 a-c 93 b
+ lactofen + 70
+ thifensulfuron + 2.2
Imazethapyr 70 100 a 97 ab 97 a
+ lactofen + 70
+ thifensulfuron + 2.2
Lactofen 70 100 a 94 c 95 ab
+ thifensulfuron + 2.2

 

‘All data was combined over years. Means followed by a common letter are not

significantly different with an LSD at the 5% level.

bAbreviation: DAT, days after treatment.
cAll treatments included 0.25% (v/v) nonionic surfactant and 2.6% (v/v) 28% urea

ammonium nitrate.

120

Table 4. Giant foxtail control by imazamox and imazethapyr alone and when tank mixed
with other herbicides.‘

 

 

 

 

 

 

 

Giant foxtailb
Control 28 DAT Control 56 DAT
Treatmentc Rate 1995 1996
g ai/ha %
Imazamox 35 92 b 69 b 99 a
Imazethapyr 70 88 cd 69 b 99 a
Imazamox 35 94 be 73 b 99 a
+ clethodim + 140
Imazethapyr 70 87 de 74 b 99 a
+ clethodim + 140
Clethodim 140 98 a 96 a 99 a
Imazamox 35 79 f 45 d 80 c
+ lactofen + 70
Imazethapyr 70 59 g 50 d 74 d
+ lactofen + 70
Lactofen 70 11 h 5 f 5 f
Imazamox 35 90 cd 66 be 99 a
+ thifensulfuron + 2.2
Imazethapyr 70 83 cf 67 bc 99 a
+ thifensulfuron + 2.2
Thifensulfuron 2.2 4 hi 8 f 23 e
Imazamox 35 82 f 45 d 83 be
+ lactofen + 70
+ thifensulfuron + 2.2
Imazethapyr 7O 78 f 55 cd 86 b
+ lactofen + 70
+ thifensulfuron + 2.2
Lactofen 70 6 hi 23 e 5 f

+ thifensulfuron + 2.2

 

“Data was combined over years except when interactions between years were observed.
Means followed by a common letter are not significantly different with an LSD at the 5%
level.

bAbreviation: DAT, days after treatment.

°All treatments included 0.25% (v/v) nonionic surfactant and 2.6% (v/v) 28% urea
ammonium nitrate.

121

Table 5. Common lambsquarters control by imazamox and imazethapyr alone and when
tank mixed with other herbicides.‘

 

 

 

 

Common lambsquarters“
Dry weight reduction Control 28 DAT Control 28 DAT
Treatmentc Rate 21 DAT 1995 1996
g ai/ha %

Imazamox 35 80 a-e 81 c-e 81 c-e
Imazethapyr 70 55 e 68 f 67 cf
Imazamox 35 77 a-e 82 c-e 79 d-f
+ clethodim + 140

Imazethapyr 70 54 de 64 f 68 cf
+ clethodim + 140

Clethodim 140 0 f 0 g 0 h
Imazamox 35 87 ab 69 cf 64 f
+ lactofen + 70

Imazethapyr 7O 73 b-e 74 d-f 76 d-f
+ lactofen + 70

Lactofen 70 61 c-e 18 g 33 g
Imazamox 35 84 a-c 90 a-c 87 b-d
+ thifensulfuron + 2.2

Imazethapyr 70 79 a-e 87 a-c 92 a-c
+ thifensulfuron + 2.2

Thifensulfinon 2.2 84 a-e 83 b-d 97 a
Imazamox 35 92 a 92 ab 92 a-c
+ lactofen + 70

+ thifensulfuron + 2.2

Imazethapyr 70 92 ab 92 a-c 94 ab
+1actofen + 70

+ thifensulfuron + 2.2

Lactofen 70 84 a-d 94 a 65 cf

+ thifensulfuron + 2.2

 

‘Data was combined over years except when interactions between years were observed.
Means followed by a common letter are not significantly different with an LSD at the 5%
level.

bAbreviation: DAT, days after treatment.

6All treatments included 0.25% (v/v) nonionic surfactant and 2.6% (v/v) 28% urea
ammonium nitrate.

122

Table 6. Yellow nutsedge control by imazamox and imazethapyr 8 and 56 weeks after
treatment.‘

 

 

 

 

 

Yellow nutsedgeb
Treatmentc Rate 8 WAT 56 WAT
g ai/ha %
Imazamox 35 5 b 15 b
Imazethapyr 7O 42 a 73 a
Alone 1 b 10 b

 

‘Data was combined over all tank mixtures. Means followed by a common letter are not
significantly different with an LSD at the 5% level.

t’Abreviation: WAT, weeks after treatment.

cAll treatments included 0.25% (v/v) nonionic surfactant and 2.6% (v/v) 28% urea
ammonirun nitrate.

123

Table 7. Soybean yield in plots treated with imazamox or imazethapyr alone and when
tank mixed with other herbicides in 1995 and 1996.

 

 

 

Soybean yield

Treatment‘ Rate 1995 1996

g ai/ha ----------- kg/ha -----------
Imazamox 35 1820 c-f 1950 b
Imazethapyr 70 2290 b-d 2150 b
Imazamox + clethodim 35 + 140 2290 b—d 2220 b
Imazethapyr + clethodim 70 + 140 2560 bc 2220 b
Clethodim 140 1410 e-g 130 c
Imazamox + lactofen 35 + 70 2090 b-e 2890 a
Imazethapyr + lactofen 70 + 70 1680 d-f 2890 a
Lactofen 70 1140 fg 400 c
Imazamox + thifensulfinon 35 + 2.2 2890 b 2620 ab
Imazethapyr + thifensulfuron 70 + 2.2 2420 b-d 2560 ab
Thifensulfuron 2.2 870 g 130 c
Imazamox + lactofen + thifensulfuron 35 + 70 + 2.2 2290 b-d 3160 a
Imazethapyr + lactofen + thifensulfuron 70 + 70 + 2.2 1950 c-e 2960 a
Lactofen + thifensulfuron 70 + 2.2 1140 fg 200 c
Untreated 0 740 g 70 c
Handweeded 0 3970 a, 2890 a

 

‘All treatments included 0.25% (v/v) nonionic surfactant and 2.6% (WV) 28% urea
ammonium nitrate. Means followed by a common letter are not significantly different
with an LSD at the 5% level.

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126

Table 10. The effect of 28% urea ammonium nitrate or nonionic surfactant on giant
foxtail control by imazamox and imazethapyr 21 days after treatment.

 

 

 

 

Treatment‘ Tiller presenceb Dry weight reduction
%
Imazamox 100 a 71 d
Imazamox + UAN 88 a 80 c
Imazamox + NIS 50 b 89 b
Imazamox+UAN+NlS O c 93 a
Imazethapyr 100 a 35 f
Imazethapyr + UAN 100 a 52 e
Imazethapyr + NIS 100 a 71 d
Imazethapyr + UAN + NIS 88 a 91 b

 

“All treatments included imazamox or imazethapyr at 35 g ai/ha, 28% urea ammonium
nitrate at 2.5% (v/v), and nonionic surfactant at 0.25% (v/v). Means followed by a
common letter are not significantly different with an LSD at the 5% level.

bRatings evaluated tiller presence by over half of the plants in a pot as 1 and less than half
was 0 (100 = tillering and regrowth, while 0 = no tillering or complete plant death).

127

Table 11. Giant foxtail control by imazamox and imazethapyr tank mixed with clethodim
21 days after treatment.‘

 

 

 

 

Clethodim
Treatment" Rate 0 18 36
g ai/ha --------------- % Dry weight reductionc
None 0 O k 90 cf 93 b-f
Imazamox 9 91 d-f 90" f (99) 90'" f (99)
Imazamox 18 94 a-d 93“ a-d (99) 93" a-d (99)
Imazamox 35 95 a 95 a (99) 95 ab (99)
Imazamox 7O 95 a 95 a (99) 95 a (99)
Imazethapyr 9 62 i 63* i (96) 54* j (97)
Imazethapyr 18 83 g 85* g (98) 79* h (99)
Imazethapyr 35 93 a-d 92" c-f (99) 93**a-e (99)
Imazethapyr 70 94 a-c 92" c-f (99) 94**a-c (99)

 

‘Comparisons between columns are valid. Means followed by a common letter are not
significantly different with an LSD at the 5% level.

t’All treatments included 0.25% (v/v) nonionic surfactant and 2.5% (v/v) 28% urea
ammonium nitrate.

cValues in parentheses are the expected dry weight reduction for the herbicide
combination using Colby’s method. A single asterisk denotes a significant antagonistic
response and a double asterisk indicates a calculated antagonistic response, but when the
observed value controlled giant foxtail then the calculated value may not be of great use.

128

Table 12. Giant foxtail control by imazamox and imazethapyr tank mixed with the
diphenyl ethers 21 days after treatment.

 

 

Treatment‘ Rate Tiller presence” Control Dry weight reductionc
g ai/ha %
Imazamox 35 0 d 94 b 96 ab
Imazamox + lactofen 35 + 140 100 a 88 cd 95 " a-c (98)
Imazamox + acifluorfen 35 + 140 78 b 86 de 95 "' b-d (99)
Imazamox + fomesafen 35 + 140 100 a 85 de 95 * a-c (98)
Imazamox 70 0 d 99 a 97 a
Imazamox +1actofen 70 +140 33 c 93 b 96 ab (98)
Imazamox + acifluorfen 70 + 140 11 d 90 c 96" ab (99)
Imazamox + fomesafen 70 + 140 33 c 90 c 96 ab (98)
Imazethapyr 35 78 b 85 de 93 cd
Imazethapyr + lactofen 35 + 140 100 a 70 g 81* f (96)
Imazethapyr + acifluorfen 35 +140 100 a 65 h 75* g (99)
Imazethapyr+ fomesafen 35 + 140 100 a 71 g 85* f (96)
Imazethapyr 70 11 d 93 b 96 ab
Imazethapyr+ lactofen 70 +140 100 a 83 cf 92" de (97)
Imazethapyr + acifluorfen 70 +140 100 a 80 f 90* e (99)
Imazethapyr + fomesafen 70 + 140 100 a 83 cf 93" cd (98)

 

 

 

‘All treatments included imazamox or imazethapyr at 35 or 70 g ai/ha; lactofen,
acifluorfen, and fomesafen at 140 g ai/ha; UAN at 2.5% (v/v); and NIS at 0.25% (v/v).
Means followed by a common letter are not significantly different with an LSD at the 5%
level.

”Ratings evaluated tiller presence by over half of the plants in a pot as 1 and less than half
was 0 (100 = tillering and regrowth, while 0 = no tillering or complete plant death).
cValues in parentheses are the expected dry weight reduction for the herbicide
combination using Colby’s method. A single asterisk denotes a significant antagonistic
response while a double asterisk indicates an antagonistic response, but when regrowth
represented by tiller presence indicated control then this value may not be of great use.

129

Table 13. The effects of adj uvants on giant foxtail control by imazamox and imazethapyr
tank mixed with the diphenyl ethers 21 days after treatment.

 

 

 

 

Treatment' Tiller presence” Dry weight reductionc
%
Imazamox + NIS 50 b 89 bc
Imazamox + lactofen + NIS 100 a 87 cd (92)
Imazamox + acifluorfen + NIS 88 a 90* be (97)
Imazamox + fomesafen + NIS 100 a 89 be (92)
Irnazamox+NIS+UAN 0 c 93 a
Imazamox + lactofen + NIS + UAN 63 b 94 a (96)
Imazamox + acifluorfen + NIS + UAN 88 a 92 ab (99)
Imazamox + fomesafen + NIS + UAN 100 a 94 a (97)
Imazethapyr + NIS 100 a 71 e-g
Imazethapyr + lactofen + NIS 100 a 73* cf (79)
Imazethapyr + acifluorfen + NIS 100 a 69* fg (93)
Imazethapyr + fomesafen + NIS 88 a 66* gh (80)
Imazethapyr + NIS + UAN 88 a 91 a-c
Imazethapyr + lactofen + NIS + UAN 100 a 89* bc (95)
Imazethapyr + acifluorfen + NIS + UAN 100 a 85* d (97)
Imazethapyr + fomesafen + NIS +UAN 100 a 89 bc (89)
NIS 100 a 0 k
Lactofen + NIS 100 a 29 j
Acifluorfen + NIS 100 a 75 e
Fomesafen + NIS 100 a 30 j
NIS + UAN 100 a 0 k
Lactofen + NIS + UAN 100 a 41 i
Acifluorfen + NIS + UAN 100 a 90 a-c
Fomesafen + NIS + UAN 100 a 62 h

 

3All treatments included imazamox or imazethapyr at 35 g ai/ha, lactofen 140 g ai/ha,
acifluorfen 280 g ai/ha, fomesafen 280 g ai/ha, urea ammonium nitrate at 2.5% (v/v), and
nonionic surfactant at 0.25% (v/v). Means followed by a common letter are not
significantly different with an LSD at the 5% level.

”Ratings evaluated tiller presence by over half of the plants in a pot as 1 and less than half
was 0 (100 = tillering and regrowth, while 0 = no tillering or complete plant death).
cValues in parentheses are the expected dry weight reduction for the herbicide
combination using Colby’s method. An asterisk denotes a significant antagonistic
response.

130

Table 14. The effect of sodium acetate on giant foxtail control with imazamox and
imazethapyr.

 

 

 

 

Dry weight

Treatment‘ Tiller presence” Control reductionc
%

Imazamox+NIS 50 b 81 ab 89 a
Imazamox + lactofen + NIS 100 a 77 bc 87 a (92)
Imazamox + acifluorfen + NIS 88 a 78 a-c 90* a (97)
Imazamox + fomesafen + NIS 100 a 74 c 89 a (92)
Imazamox + NIS + sodium acetate 38 b 83 a 91 a (90)
Imazethapyr + NIS 100 a 60 d 71 bc
Imazethapyr + lactofen + NIS 100 a 55 de 73* be (79)
Imazethapyr + acifluorfen + NIS 100 a 58 de 69* bc (93)
Imazethapyr + fomesafen + NIS 88 a 52 e 66* c (80)
Imazethapyr + NIS + sodium acetate 100 a 59 d 71 be (73)
NIS 100 a 0 g 0 e
Lactofen + NIS 100 a 22 f 29 d
Acifluorfen + NIS 100 a 56 de 75 b
Fomesafen + NIS 100 a 15 f 30 d
NIS + sodium acetate 100 a 0 g 8 e

 

*All treatments included imazamox or imazethapyr at 35 g ai/ha, lactofen at 140 g ai/ha,
acifluorfen at 280 g ai/ha, fomesafen at 280 g ai/ha, and nonionic surfactant at 0.25%
(v/v). Sodium acetate was applied at 19 mM. Means followed by a common letter are
not significantly different with an LSD at the 1% level.

”Ratings evaluated tiller presence by over half of the plants in a pot as 1 and less than half
was 0 (100 = tillering and regrowth, while 0 = no tillering or complete plant death).
cValues in parentheses are the expected dry weight reduction for the herbicide
combination using Colby’s method. An asterisk denotes a significant antagonistic
response.

131

 

 

 

 

 

7O
. Imammox + MSO
y = 0.02 +£11.2/(1 + (x/115.9)~1'07)
r = 0.84
60 '7 . lmazamox+NlS - 5
y = 0.11 +254.8/(1 + (x1922) ' )
r = 0.91
O Imazethapyr + M80
095
50 _ y = 0.37 + 67.4/(1 + (x/300.3) )
2 O
r = 0.77
O Imazethapyr + NIS 11
y = 0.78 + 30.9/(1 + (x/92.5) ' )
40 _ f2 = 0.78 . .
§ 0
b
.2.
C
- A
30 — '
»
’ o
6
20 -— Q
10 — --
O o
LSD = 6
(950.05)
_L
O
0 I I I I
0 35 70 140 280

Rate (9 ai/ha)

Figure Ia. Soybean injury by imazamox and imazethapyr applied with a nonionic
surfactant or a methylated seed oil 7 DAT. All treatments included 2.5% (v/v) 28% urea

ammonium nitrate and methylated seed oil at 1.0% (v/v) or nonionic surfactant at 0.25%
(v/v).

70

132

 

60—

.h 01
8 o o
l I I

Dry weight reduction (%)

N
O
I

10—

 

. Imazamox + MSO ,1 35
y = -0.55 + 74.1/(1 + (x/116-9) ' l

r2 = 0.79

O Imazamox + NIS -1

y = 0.17 +261.7/(1+(xl91.6) ' )
r = 0.69

O lmazethapyr+ MSO _1 17

 

 

y = 0.04 + 57.8/(1 + (xl196.3) ) 9
r2 = 0.56
O Imazethapyr + NIS
y = 0.42 + 30.4/(1 + (x/78.2)'1'86) .
r2 = 0.34
9
O
O
/‘ 5
/3
'1/ -
' I I I '
35 70 140 280
Rate (9 ai/ha)

Figure I b. Soybean dry weight reduction by imazamox and imazethapyr applied with a

nonionic surfactant or a methylated seed oil 21 DAT. All treatments included 2.5% (v/v)
28% urea ammonium nitrate and methylated seed oil at 1.0% (v/v) or nonionic surfactant
at 0.25% (v/v).

 

133

70

 

. Imazamox + MSO 1 08
= -1.10 +293.5/(1 + (x/248.1) . )
r = 0.68
60 O Imazamox + NIS
fl -1.75
y = 0.20 + 56.4](1 + (x/85.0) )

   
 

r =0.60
O Imazethapyr + MSO
50 _ _ -O.81
y - 0.77 +153.1/(1 + (x/2301) )
r =0.39
,3 O Imazethapyr + NIS _3 75 .
a" y = -0.03 + 21.0/(1 + (0133.4) ‘ )
5 40 -‘ r =0.23
3
3
U
9 o
8
a 30 —
“11':
3 O
20 — 9
7
10 -1

 

 

 

__L
\I/ I I
O 35 70 140 280
Rate (9 ailha)

 

Figure 1c. Soybean leaf area reduction by imazamox and imazethapyr applied with a
nonionic surfactant or methylated seed oil 21 DAT. All treatments included 2.5% (v/v)

28% urea ammonium nitrate and methylated seed oil at 1.0% (v/v) or nonionic surfactant
at 0.25% (v/v).

CHAPTER 4
WEED CONTROL IN WIDE AND NARROW ROW
SOYBEAN [Glycine max (L.) Merr] WITH IMAZAMOX,
IMAZETHAPYR, AND OXASULFURONl
Kelly A. Nelson and Karen A. Renner2
Abstract. Field experiments were conducted at East Lansing and Clarksville, MI to
evaluate the efficacy of imazamox, imazethapyr, and oxasulfuron applied postemergence
in wide (76 cm) and narrow (19 cm) row soybean. Soybean injury from all herbicides
was minimal 14 days after treatment except for oxasulfuron plus quizalofop which
injured soybean 30% at the Clarksville location. Imazamox, imazethapyr, and
oxasulfuron controlled velvetleaf, wild mustard, and common chickweed. Imazamox and
imazethapyr also controlled eastern black nightshade. Adding CGA-248757 to
oxasulfuron reduced common ragweed control, but increased redroot pigweed control
compared to oxasulfuron alone. Imazamox at 35 and 45 g ai/ha provided greater
common ragweed and common lambsquarters control than imazethapyr at 70 g ai/ha 28
days after treatment. Common ragweed, common lambsquarters, and redroot pigweed

control was enhanced in narrow compared to wide row soybean 56 days after treatment.

Total weed biomass and soybean yield in wide row soybean treated with imazamox at

 

‘Received for publication and in revised form

 

 

2Grad. Res. Asst. and Prof., Mich. State Univ., East Lansing, MI 48824-1325.

134

135

45 g/ha was similar to that of the handweeded control. In narrow row soybean, total
weed biomass and soybean yield was similar to the handweeded control for imazamox at
both the 35 and 45 g/ha rate as well as imazethapyr at 70 g/ha. These postemergence
herbicide treatments resulted in less weed biomass and greater soybean yield in narrow
compared to wide row soybean. Nomenclature: CGA-248757, (proposed fluthiacet-
methyl) methyl [[2-chloro-4-fluoro-5-[(tetrahydro-3-oxo-1H,3H-[1,3,4]thiadiazolo[3,4-
a]pyridazin-l-ylidene)amino]phenyl]thio]acetate; imazamox, 2-(4-isopropyl-4-methyl-5-
oxo-2-imidazolin-2-yl)-5-(methoxymethyl) nicotinic acid; imazethapyr, 2-[4,5-dihydro—4-
methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-5-ethyl-3-pyridinecarboxylic acid;
oxasulfuron (CGA-27747 6), 2-[[[[(4,6-dimethyl-2-pyrimidinyl)—
amino]carbonyl]amino]sulfonyl]benzoic acid, 3-oxetanyl ester; quizalofop, (:)-2-[4-[(6-
chloro-2-quinoxalinyl)oxy]phenoxy]propanoic acid; common chickweed, Stellaria media
(L.) Vill. #3 STEME; common lambsquarters, Chenopodium album L. # CHEAL;
common ragweed, Ambrosia artemisiifolia L. # AMBEL; eastern black nightshade,
Solanum ptycanthum Dun. # SOLPT; giant foxtail, Setariafaberi Herrm. # SETFA; wild
mustard, Sinapis arvensis L. # SINAR; redroot pigweed, Amaranthus retroflexus L. #
AMARE; velvetleaf, Abutilon theophrasti Medik. # ABUTH; soybean, Glycine max (L.)
Merr.

Additional index words: Row spacing, postemergence.

 

3Letters following this symbol are WSSA-approved computer code from Composite List
of Weeds, Revised 1989. Available from WSSA, 1508 West University Ave.,
Champaign, IL 61821-3133.

136

INTRODUCTION

It has been well documented that soybean planted in narrow rows reduce weed
yield and have potentially greater soybean yield than soybean planted in wide rows
(Burnside and Collville 1964; Burnside and Moomaw 1977; McWorter and Sciumbato
1988; Mickelson and Renner 1997; Patterson et al. 1988; Peters et al. 1965; Wax and
Pendleton 1968). Faster canopy closure in narrow row soybean can increase weed
suppression by herbicides. Soybean planted in narrow rows (25 cm) reached 95% solar
interception 11 to 17 days before wide row (102 cm) soybean depending upon soybean
density (Shibles and Weber 1966). Canopy closure has been reported from 35 to 50 and
56 to 90 days after planting for soybean in 19 to 25 cm and 76 cm rows, respectively
(Burnside and Colville 1964; Mickelson and Renner 1997; Wax and Pendleton 1968).
An earlier canopy closure can enhance weed control and broaden the use of a herbicide or
allow a herbicide to be applied at a reduced rate.

Burnside and Moomaw (1977) showed that reduced rates of preemergence
herbicides provided adequate weed control in narrow row soybean. Chloramben (3-
amino-2,5-dichlorobenzoic acid) provided greater control of weeds in narrow than wide
row soybean (Peters et al. 1965). Postemergence herbicide treatments in narrow row
soybean had 30% less weed biomass than the same postemergence treatments in wide
rows, and narrow row soybean were more profitable for most of the postemergence
herbicide programs evaluated (Mickelson and Renner 1997).

Imazamox and imazethapyr are imidazolinone herbicides for broadspectrum

postemergence weed control in soybean (Anonymous; Anonymous 1995; Ballard and

137

Hellmer 1995). A methylated seed oil adjuvant improved control of several weed species
by imazethapyr and imazamox (Gednalski et al. 1995; Nalewaja et al. 1990; Thompson et
al. 1996). Oxasulfuron is a sulfonylurea herbicide for broadleaf weed control and grass
suppression (Kidder and Johnson 1997; Kidder and Porpiglia 1996; Ritter and Menbere
1997). Quizalofop has been tank mixed with oxasulfiiron for giant foxtail control and
and CGA-248757 has been tank mixed with oxasulfuron for enhanced broadleaf weed
control (Bauer et al. 1995; Bauer et al. 1996; Kidder and Porpiglia 1996; Nelson and
Renner 1997). The objectives of this research were to evaluate soybean injury, weed
control, and soybean yield following postemergence applications of imazamox,

imazethapyr, and oxasulfuron in wide and narrow row soybean.

138

MATERIALS AND METHODS

Field experiments were conducted in 1996 at the Michigan State University
Research Farm at East Lansing, MI and the Michigan State University Experiment
Station at Clarksville, MI. The soil was a Capac clay loam (fine-loamy, mixed mesic
Aerie Ochraqualfs) with a pH of 7.0 and 3.5% organic matter at the East Lansing
location. This site was disked in the fall and cultimulched twice in the spring. The
Clarksville soil was a Lapeer loam (coarse-loamy, mixed, mesic Mollic Haplaquepts)
with a the soil pH of 6.8 and 1.2% organic matter. The site was field cultivated once in
the spring.

‘Conrad’ soybean were planted in 76 cm rows at 358,000 seeds/ha with a John
Deere 7200 Max-Emerge”?4 planter and at 469,000 seeds/ha in 19 cm wide rows with a
Great Plains Solid Stand 105 grain drill at East Lansing. The plots were 3 m wide and
12.2 m long. At Clarksville, soybean were planted in 76 cm rows at 371,000 seeds/ha
with a John Deere 7200“ planter, while 19 cm wide row soybean were planted at 508,000
seeds/ha with an International 5100” grain drill. At this site, plots were 12.2 m long and
wide row plots were 4.6 m wide, while the narrow row plots were 2.3 m wide.

This research was arranged as a split plot design with four replications. The main
plots were row spacing and subplots were herbicide treatments. Herbicide treatments

included imazamox at 35 and 45 g ai/ha, imazethapyr at 70 g ai/ha, oxasulfuron at 79 g

 

“Deere and Co., 501 River Drive, Moline, IL 61265-1100.
5Great Plains Manufacturing Inc., PO. Box 218, Assaria, KS 67416.

”Concord, 3000 7‘” Ave. North, Fargo, ND 58102.

139

ai/ha tank mixed with quizalofop at 69 g ai/ha, and oxasulfuron at 65 g/ha tank mixed
with CGA-2487 57 at 4 g ai/ha and quizalofop at 69 g/ha. Imazamox and imazethapyr
treatments included methylated seed oil7 (MSO)8 at 1.0% (v/v), while oxasulfuron tank
mixtures included nonionic surfactant9 (NIS)8 at 0.25% (v/v). All herbicide treatments
were applied with 2.6% (v/v) 28% urea ammonium nitrate (UAN)3.

Herbicide treatments were applied at a standard postemergence timing on June 23,
1996 at East Lansing and June 20, 1996 at Clarksville (25 and 26 days after planting)
with a tractor mounted compressed air sprayer traveling at 6.3 km/h and delivering 17 8
L/ha at 207 kPa of pressure. At the time of application, soybean were at the V1 to V2
stage of development at both locations. Weed heights, leaf number, and weed density are
presented in Table l. Herbicides were applied with 8003 flat fan nozzleslo on a 51 cm
spacing at 48 cm above the weed and crop canopy. At East Lansing, the air temperature
was 21 C with 67% relative humidity and 15% cloud cover at 9:00 am, and at
Clarksville the air temperature was 27 C with 71% relative humidity and 80% cloud
cover at 6:00 pm. at the time of application. Rainfall ten days prior to herbicide

application was 9.8 cm at East Lansing and 11.5 cm at Clarksville, while rainfall from 1

 

7Methylated seed oil was Sun-It II”, a modified vegetable oil containing surfactant, from
Agsco, Inc., PO. Box 13458, Grand Forks, ND 58208-3458.

8Abbreviations: MSO, methylated seed oil; NIS, nonionic surfactant; UAN, 28% urea
ammonium nitrate; DAT, days after treatment.

9Nonionic surfactant was Activator-90, a mixture of alkyl polyoxyethylene ether and free
fatty acids, Loveland Industries Inc., PO. Box 1289, Greeley, CO 80632.

lo'I‘eej et flat fan tips. Spraying Systems Co., North Ave. and Schmale Road, Wheaton, IL
60188.

140

to 10 days after herbicide application totaled 0.7 cm at East Lansing and 1.0 cm at
Clarksville.

Soybean injury was evaluated 3, 7, 14, and 21 days afier treatment (DAT)3.
Soybean injury data is presented for 7 and 14 DAT. Weed control was evaluated 7, 14,
21, 28, and 56 DAT. Evaluations at 28 and 56 DAT were the most representative of
weed control. Visual evaluations were based upon a scale of 0 (no effect) to 100
(complete weed or crop death) percent. Three days prior to herbicide application, 30 by
76 cm long quadrats were placed in every plot, and soybean and weeds were counted.
The weeds and soybean in these quadrats were harvested, separated, and oven dried 56
DAT. Total weed and soybean biomass reduction was calculated as: total weed biomass
reduction = 100*(1-(total weed biomass/untreated wide row total weed biomass) and
soybean biomass reduction = 100*(1-(total soybean biomass/handweeded wide row
soybean biomass). Total weed biomass percent was the quotient of the weed biomass and
total plant biomass multiplied by 100. Two rows of the wide row soybean and a 1.5 m
width of the narrow row soybean were harvested with a Massey 10“ small plot combine
and moisture adjusted to 13%.

The percent data was transformed to the arc sine for an analysis of variance and
means were separated with an LSD at the 5% level, but reported as the original means.
Due to interactions between location and herbicide treatments for soybean injury and
weed control 28 DAT, data are presented for the East Lansing and Clarksville locations

separately, but were combined over row spacing. Redroot pigweed control is presented

 

“Kincaid Equipment Manufacturing, PO. Box 400, Haven, KS 47543.

141

for location, row spacing, and herbicide treatment since there were interactions between
these factors 28 and 56 DAT. By 56 DAT, data were combined over locations, and
interactions between row spacing and herbicide treatments are presented for all responses
except soybean biomass reduction, where only the herbicide treatment effects were

significant.

142

RESULTS AND DISCUSSION

Imazamox at 45 g/ha caused more soybean injury than the other herbicide
treatments at East Lansing; however, oxasulfuron tank mixtures were the most injurious
at Clarksville 7 DAT (Table 2). Injury symptoms on soybean treated with oxasulfuron
tank mixed with CGA-248757 plus quizalofop were primarily leaf necrosis, while
oxasulfuron tank mixed with quizalofop caused more stunting and reddening of soybean
leaf veins. By 14 DAT, there was no difference in soybean response to herbicide
treatments at East Lansing. However at Clarksville, soybean treated with imazamox at 45
g/ha were injured more than soybean treated with imazamox at 35 g/ha, while
oxasulfuron tank mixed with quizalofop severely injured soybean 14 DAT (Table 2).
This injury was still greater than 10% 21 DAT (data not shown). Malefyt and
Quakenbush (1991) reported that tolerant species like soybean metabolize imidazolinone
herbicides slower at lower temperatures. This could explain more injury from the
imidazolinones at the East Lansing site. Other environmental factors may have caused
increased soybean injury from oxasulfuron at the Clarksville site.

Common ragweed control by all treatments was reduced at East Lansing
compared to the Clarksville location because the population density of common ragweed
was greater at East Lansing and some larger common ragweed (6 to 8 cm) were not
controlled by oxasulfuron or 35 g/ha of imazamox (personal observation). Imazamox at
45 g/ha provided the best common ragweed control at East Lansing, while at Clarksville

both imazamox rates provided similar common ragweed control (Table 3). Common

143

ragweed control was reduced at East Lansing and Clarksville when CGA-248757 was
tank mixed with oxasulfuron.

Imazamox at 45 g/ha controlled common lambsquarters at East Lansing, while
both 35 and 45 g/ha of imazamox controlled common lambsquarters at Clarksville (Table
3). Imazamox provided greater common lambsquarters control than imazethapyr at both
locations. The population density of common lambsquarters was greater at Clarksville.
Larger (4 to 8 cm) common lambsquarters were not controlled by oxasulfuron tank
mixtures (personal observation).

Imazamox and imazethapyr controlled giant foxtail 28 DAT at both locations
(Table 3). Oxasulfuron plus quizalofop tank mixtures did not adequately control giant
foxtail at East Lansing possibly due to larger giant foxtail (8 to 13 cm) and higher giant
foxtail population density. Eastern black nightshade was controlled by imazamox and
imazethapyr, but not by oxasulfuron tank mixtures at East Lansing (Table 3). All
treatments controlled velvetleaf, wild mustard, and common chickweed (data not shown).

Redroot pigweed was controlled by imazamox and imazethapyr regardless of row
spacing or location (Table 4). Redroot pigweed control from oxasulfuron plus quizalofop
increased 24% at the East Lansing site in narrow compared to wide row soybean 28 DAT.
Redroot pigweed control was enhanced at both sites when CGA-248757 was tank mixed
with oxasulfuron in wide row soybean. By 56 DAT, oxasulfuron plus CGA-248757
provided greater control of redroot pigweed than oxasulfuron alone in both row spacings

at Clarksville and in the wide row spacing at East Lansing. Oxasulfuron provided less

144

redroot pigweed control at East Lansing because of a greater pigweed population density
and a few larger pigweed (10 cm) that were not controlled (personal observation).

By 56 DAT, common ragweed control was greater in narrow than in wide row
soybean for all herbicide treatments (Table 5). In narrow rows, common ragweed control
was similar for both rates of imazamox, imazethapyr, and oxasulfiiron plus quizalofop;
however, imazamox and oxasulfuron plus quizalofop provided the greatest common
ragweed control in wide rows. Common lambsquarters were controlled by all treatments
in narrow row soybean and control was increased in narrow compared to wide row
soybean for all treatments except imazamox at 35 g/ha.

Both imazamox rates reduced total weed biomass similar to that of the
handweeded control in wide and narrow row soybean (Table 5). Imazamox at 45 g/ha
reduced total weed biomass more than imazethapyr and oxasulfuron tank mixtures in
wide row soybean. Adding CGA-248757 to oxasulfuron reduced weed biomass in
narrow row soybean comparable to oxasulfuron alone, but not in wide rows.

There was no interaction between row spacing and herbicide treatment for
soybean biomass reduction 56 DAT; therefore, the effect of herbicide treatment is
presented (Table 5). Soybean biomass was reduced 15 to 21% by the weeds present in
the oxasulfuron and imazethapyr treatments. Soybean biomass was similar to the hand
weeded control where 45 g/ha of imazamox was applied. Soybean biomass was reduced
18% more in wide than in narrow row soybean when data was combined over all

herbicide treatments (data not shown).

145

Soybean yielded more in narrow compared to wide rows in all herbicide
treatments except the handweeded control and imazamox at 45 g/ha (Table 5). All
treatments in wide row soybean yielded less than the handweeded control except
imazamox at 45 g/ha. Increased soybean yield in the herbicide treated plots of narrow
row soybean was due to weed control differences, since the soybean yield of the
handweeded control did not differ between narrow and wide row soybean.

Total weed biomass was evaluated as a percentage of the total plant biomass
harvested from each plot 56 DAT (Figure 1). Both imazamox treatments in wide and
narrow row soybean had a very low total weed biomass percent that was similar to the
handweeded control. The weed biomass was greater in wide than narrow row soybean
for imazethapyr, oxasulfuron tank mixtures, and the untreated control. Common ragweed
contributed to the total weed biomass in all treatments, giant foxtail and common
lambsquarters contributed in the imazethapyr treatments, and common lambsquarters and
redroot pigweed contributed in the oxasulfuron treatments.

Our research indicated that weed competition was reduced and soybean yield
increased in narrow compared to the wide row soybean when a total postemergence weed
control strategy was used. Canopy closure dates were approximately 45 days after
planting for narrow row soybean and 80 days after planting for wide row soybean at both
sites. The advantage of narrow row spacing for increased weed control was not evident
28 DAT (56 days after planting) except for redroot pigweed control by oxasulfuron tank
mixtures. Between 28 and 56 DAT, soybean competition was greater in narrow than in

wide soybean due to canopy closure in narrow but not wide row soybean during this

146

time period. Earlier canopy closure in narrow row soybean is an important factor that
allow soybean a competitive advantage to weeds that are marginally suppressed by a
postemergence herbicide treatment.

Imazamox at 45 g/ha provided broad spectrum postemergence weed control in
wide and narrow row soybean. Imazamox provided increased common ragweed and
common lambsquarters control compared to imazethapyr 28 and 56 DAT in wide row
soybean. Imazamox has controlled common lambsquarters greater than imazethapyr in
other research (Nelson and Renner 1995). Narrow row soybean treated with imazamox at
35 g/ha or imazethapyr controlled a broad spectrum of broadleaf weeds as well as giant
foxtail, and soybean yields were equal to the handweeded control. Ballard and Hellmer
(1995) reported a tendency for increased common ragweed and common lambsquarters
control by imazamox at 45 compared to the 35 g/ha in wide row soybean. Other research
has found no difference in common ragweed control with imazamox at 35 g/ha and
imazethapyr at 70 g/ha in wide row soybean (Nelson and Renner 1995).

Utilization of narrow row soybean can prove beneficial in years when weed sizes
and environmental conditions are not as favorable for postemergence weed control.
Postemergence broadleaf weed control with oxasulfuron tank mixtures had variable weed
control that appeared extremely sensitive to the weed size at the time of application.
Additional cost effective weed management strategies are needed to enhance redroot
pigweed, eastern black nightshade, and common ragweed control. Other researchers have
evaluated early postemergence applications of oxasulfuron and preemergence acetanilide

followed by oxasulfuron postemergence as methods to broaden the spectrum of weeds

147

controlled with oxasulfuron in the weed management system (Bauer et al. 1996, Kidder

and Johnson 1997).

148

LITERATURE CITED

Anonymous. Pursuit”. Technical information bulletin. American Cyanamid Company,
Princeton, NJ. 18 p.

Anonymous. 1995. Raptor”. Technical information bulletin. American Cyanamid
Company, Princeton, NJ. 8 p.

Ballard, T. O. and M. Hellmer. 1995. AC 299,263: efficacy in soybeans as influenced by
postemergence timing. Proc. North Cent. Weed Sci. Soc. 50:132-133.

Bauer, T. A., S. W. Pruss, T. R. Dill, and D. Kidder. 1996. Sequential soybean herbicide
programs including CGA-277476. Proc. North Cent. Weed Sci. Soc. 51:131-132.

“:1
' I

Bauer, T., T. Threewitt, L. Stahlberg, A. Moses, K. Jones, D. Kidder, and P. Porpiglia.
1995. Weed management programs in soybeans with CGA-277476. Proc. North Cent. "
Weed Sci. Soc. 50:140.

Burnside, O. C. and W. L. Colville. 1964. Soybean and weed yields as affected by
irrigation, row spacing, tillage, and amiben. Weeds 12:109-112.

Burnside, O. C. and R. S. Moomaw. 197 7. Control of weeds in narrow-row soybeans.
Agron. J. 69:793-796.

Gednalski, J. V., W. G. Johnson, and G. W. Alley. 1995. Influence of various adjuvants
on postemergence weed control in soybeans with AC 299,263. Proc. North Cent. Weed
Sci. Soc. Am. 50:133.

Kidder, D. and M. D. Johnson. 1997. Grass control in soybeans with CGA-277476;
alone, in sequential, and in tanlonix programs. Abstr. Weed Sci. Soc. Am. 37:1.

Kidder, D. W. and P. J. Porpiglia. 1996. CGA-277476 and CGA-248757 for broad
spectrum, postemergence weed control in soybeans. Abstr. Weed Sci. Soc. Am. 36:12.

Malefyt, T. and L. Quakenbush. 1991. Influence of environmental factors on the
biological activity of the imidazolinone herbicides. Pages 103-127 in Shaner, D. L. and
S. L. O’Connor eds. The Imidazolinone Herbicides. CRC Press, Boca Raton, FL.

McWorter, C. G. and G. L. Sciumbato. 1988. Effects of row spacing, benomyl, and
duration of sicklepod (Cassia obtusifolia) interference on soybean (Glycine max) yields.
Weed Sci. 36:254-259.

Mickelson, J. A. and K. A. Renner. 1997. Weed control using reduced rates of
postemergence herbicides in narrow and wide row soybeans. J. Prod. Agric. In press.

149

Nalewaja, J. D., J. Palczynski, and F. Manthey. 1990. Imazethapyr efficacy with
adjuvants and environments. Weed Technol. 4:765-770.

Nelson, K. A. and K. A. Renner. 1995. A comparison of imazethapyr and AC 299,263
for postemergence weed control in soybeans. Proc. North Cent. Weed Sci. Soc. 50:129-
130.

Nelson, K. A. and K. A. Renner. 1997. Postemergence weed control in soybean with
CGA-277476 and cloransulam-methyl. Abstr. Weed Sci. Soc. Am. 37:1.

Patterson, M. G., R. H. Walker, D. L. Colvin, G. Wehtje, and J. A. McGuire. 1988.
Comparison of soybean (Glycine max)--weed interference from large and small plots.
Weed Sci. 36:836—839.

Peters, E. J., M. R. Gebhardt, and J. F. Stritzke. 1965. Interrelations of row spacings,
cultivations and herbicides for weed control in soybeans. Weeds 13:285-289.

Ritter, R. L. and H. Menbere. 1997. Use of CGA-277476 in soybeans. Abstr. Weed Sci.
Soc. Am. 37:1.

Shibles, R. M. and C. R. Weber. 1966. Interception of solar radiation and dry matter
production by various soybean planting patterns. Crop Sci. 6:55-59.

Thompson, W. M., S. J. Nissen, and R. A. Masters. 1996. Adjuvant effects on
imazethapyr, 2,4-D and picloram absorption by leafy spurge (Euphorbia esula). Weed
Sci. 44:469-475.

Wax, L. M. and J. W. Pendleton. 1968. Effect of row spacing on weed control in
soybeans. Weed Sci. 16:462-465.

150

Table I . Weed height, leaf number, and population density at the time of herbicide
application for field experiments conducted at East Lansing and Clarksville, MI.

 

East Lansing Clarksville
Weed species Height Leaf Density Height Leaf Density

 

 

 

cm number plants/m2 cm number plants/m2

Giant foxtail 3-10 3-5 300 3-8 3-4 75

Redroot pigweed 3-10 3-7 400 1-8 3-8 290
Common ragweed 3-9 4-8 75 3-6 4-6 20

Common 34 4 30 3-8 4-6 65

lambsquarters

Velvetleaf 3-5 2 10 3-5 2 10

Wild mustard 3-15 4—6 10 --- --- ---

Eastern black 0.5-1 1 65 --- --- ---

nightshade

Common --- --- --- 3-15 6-30 40

chickweed

 

151

Table 2. Soybean injury by imazamox, imazethapyr, and oxasulfuron tank mixtures 7
and 14 days after treatment at East Lansing and Clarksville, MI.‘

 

 

 

 

 

 

 

Soybean injury
7 DAT 14 DAT
East East
Treatment” Rate Lansing Clarksville Lansing Clarksville
Imazamox 35 17b 11bc 4ab 4d
Imazamox 45 22 a 14 b 6 a 9 b
Imazethapyr 70 19 b 12 bc 6 a 7 b-d
Oxasulfuron 79 17 b 19 a 4 ab 30 a
+ quizalofop + 69
Oxasulfuron 65
+ CGA-248757 + 4 18 b 22 a 4 ab 8 be
+ quizalofop + 69

 

aTreatment means were combined over row spacing. Means followed by a common letter
are not significantly different with an LSD at the 5% level.

”All treatments included 2.6% (v/v) 28% urea ammonium nitrate. Imazamox and
imazethapyr treatments included 1.0% (v/v) methylated seed oil and oxasulfirron tank
mixtures included 0.25% (v/v) nonionic surfactant.

152

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153

Table 4. Redroot pi gweed control 28 and 56 days after treatment at East Lansing and
Clarksville, MI in wide and narrow row soybean.

 

 

 

 

 

 

 

 

Control 28 DAT Control 56 DAT

East Lansing Clarksville” East Lansing Clarksville

Treatment‘ Rate Wide Narrow Wide Narrow Wide Narrow
g ai/ha %

Imazamox 35 99 a 99 a 99 a 99 a 99 a 99 a 99 a
Imazamox 45 99 a 98 a 99 a 92 b 99 a 99 a 99 a
Imazethapyr 70 99 a 98 a 99 a 99 a 99 a 97 ab 99 a
Oxasulfuron 79 60 d 84 b 60 c 18 d 45 c 36 e 77 c
+ quizalofop + 69
Oxasulfuron 65
+ CGA-248757 + 4 75 c 71 cd 68 b 53 c 50 c 60 d 92 b

+ quizalofop

+69

 

aAll treatments included 2.6% (v/v) 28% urea ammonium nitrate. Imazamox and
imazethapyr treatments included 1.0% (v/v) methylated seed oil and oxasulfuron tank

mixtures included 0.25% (v/v) nonionic surfactant. Comparisons within East Lansing or
Clarksville columns are valid.
”Treatment means were combined over row spacing. Means followed by a common letter
are not significantly different with an LSD at the 5% level.

154

 
 

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Figure 1. The percent of total weed biomass 56 days after treatment for wide and narrow
row herbicide treatments. Wide row soybean treatments have vertical lines through the
soybean portion of the vertical bars. White portions of the histogram bars indicate the
percent soybean biomass and textured portions indicate the percent weed biomass.
Numbers above the textured lines are the sum of the weed biomass percentages.
Miscellaneous (Misc) weeds included common chickweed, wild mustard, and eastern
black nightshade.

SUMMARY

Imazamox, imazethapyr, oxasulfuron, and cloransulam-methyl injured soybean 7
days after treatment, but soybean recovered by 21 days after treatment. Imazamox,
imazethapyr, oxasulfuron, and cloransulam-methyl controlled velvetleaf, wild mustard,
and common chickweed. Imazamox and imazethapyr controlled redroot pigweed and
eastern black nightshade. Imazamox controlled common lambsquarters, while
cloransulam-methyl controlled common ragweed. Our research indicated the need for
tank mixtures with oxasulfuron and cloransulam-methyl for grass and enhanced broadleaf
weed control. Grass antagonism was species and herbicide dependent with oxasulfuron.
Tank mixtures enhanced, but were not always needed for common ragweed control with
imazamox, while tank mixtures with imazethapyr were needed for common
lambsquarters and common ragweed control in wide row soybean. Imazamox at 35 and
45 g/ha controlled a broadspectrum of broadleaf weeds and giant foxtail in narrow row
soybean. The diphenyl ether herbicides enhanced common ragweed control, but
increased tillering and antagonized giant foxtail control by imazamox and imazethapyr.
Soybean yield in wide rows was greater when a diphenyl ether was tank mixed with
imazamox or imazethapyr, while yields were similar in narrow row soybeans without a
tank mixture. Sodium cations similar to the ones present in the diphenyl ether herbicide

formulations did not reduce giant foxtail control by imazamox or imazethapyr.

156

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