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'This is to certify that the
thesis entitled

WEED CONTROL IN SOYBEANS USING REDUCED
RATES OF POSTEMERGENCE HERBICIDES

presented by

James Alan Mickelson

has been accepted towards fulﬁllment
of the requirements for

M. S . degree in C19]; and $9121 Sciences

 

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Major professor

Date LI'IS'7é

0-7639 MS U is an Afﬁrmative Action/Equal Opportunity Institution

 

 

LIBRARY

Michigan State
University

 

 

 

PLACE IN RETURN 30X to remove thle checkout tram your record.
TO AVOID FINES return on or betore date due.

DATE DUE DATE DUE DATE DUE

   

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

MSU leAnAtﬂmetiveAction/Emel Opportunity lnetituion
Wm1

WEED CONTROL IN SOYBEAN S USING REDUCED
RATES OF POSTEMERGENCE HERBICIDES

By

J ames Alan Mickelson

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

MASTER OF SCIENCE

Department of Crop and Soil Sciences

1996

ABSTRACT

WEED CONTROL IN SOYBEAN S USING REDUCED
RATES OF POSTEMERGEN CE HERBICIDES

By

James Alan Mickelson

Weed control programs that use reduced rates of postemergence herbicides offer
the potential to decrease producer input costs while maintaining weed control and
soybean yield. Experiments were conducted in the greenhouse and ﬁeld to examine
using reduced rates of postemergence soybean herbicides. In the greenhouse, clethodirn
was more effective in controlling giant foxtail and bamyardgrass at reduced rates than
quizalofop or ﬂuazifop-P&fenoxaprop. Fomesafen and lactofen were the most effective
herbicides for control of common ragweed at reduced rates. Chlorimuron,
thifensulfuron, and fomesafen were the most effective herbicides for control of
velvetleaf at reduced rates. In the ﬁeld, weed control and soybean yield were greater in
narrow row soybeans (19 cm) than wide rows (76 cm). All four herbicide tank
mixtures applied at reduced rates in a split application (one quarter of a full rate at
early postemergence followed by one quarter of a full rate at standard postemergence
timing) resulted in weed control and soybean yield equal to that of full rate treatments.
Three of four tank mixtures in 1994 and two of four in 1995, applied at one half the
full rate at a standard postemergence timing, resulted in weed control and soybean

yield equal to that of full rate treatments.

ACKNOWLEDGEMENTS

I would like to extend a sincere thank you to Dr. Karen Renner, my major
advisor, for her excellent guidance and support throughout my graduate career at
Michigan State University. I would also like to extend my appreciation to Dr. Jim Kells
and Dr. Bernie Zandstra, members of my advisory committee, for their valuable
contributions towards my degree.

I would like to thank Jason Fausey for his help and friendship during our
simultaneous journey through graduate school, as well as for having someone to share
the rent with. Special thanks to all of the people who have contributed in many ways to
my research and education: Gary Powell, Andy Chomas, Julie Lich, Kelly Nelson, Bob
Starke, Karen Novosel, Cory Ransom, Brent Tharp, Eric Spandl, Paul Knoerr, Jim
Sherman, Mike Particka, Marla Van Wormer, and Joe Simmons. My experience at
Michigan State has been especially enjoyable because of the great friendships and
camaraderie that these people have provided me. Lastly, I would like to thank my
family and friends for their continued support and encouragement throughout my

graduate career.

iii

TABLE OF CONTENTS

Chapter 1. Review of Literature

Introduction .......................................... 1
Postemergence Herbicide Use in Soybeans ....................... 2
Soybean Row Spacing .................................... 9
Use of Reduced Rates of Soil Applied Herbicides ................. 14
Use of Reduced Rates of Postemergence Herbicides ................ 17
Economics of Reduced Rate Herbicide Programs .................. 22
Literature Cited ....................................... 24

Chapter 2. Rate Response of Giant Foxtail, Barnyardgrass, Redroot Pigweed,
Common Ragweed, and Velvetleaf to Postemergence Herbicides.

Abstract ............................................ 32
Introduction ......................................... 33
Materials and Methods .................................. 34

Results and Discussion

Giant Foxtail .................................... 37
Barnyardgrass ................................... 37
Redroot Pigweed ................................. 38
Common Ragweed ................................ 38
Velvetleaf ...................................... 38
Literature Cited ....................................... 40

iv

TABLE OF CONTENTS (cont.)

Chapter 3. Weed Control Using Reduced Rates of Postemergence Herbicides in
Narrow and Wide Row Soybeans.
Abstract ............................................ 46
Introduction ......................................... 47
Materials and Methods .................................. 49

Results and Discussion

Annual Grass .................................... 54
Common Lambsquarters ............................. 55
Redroot Pigweed ................................. 57
Common Ragweed ................................ 58
Velvetleaf ...................................... 59
Weed Biomass ................................... 6O
Soybean Injury ................................... 61
Sclerotinia Stem Rot ............................... 61
Soybean Yield ................................... 62
Economic Analysis ................................ 63
Literature Cited ....................................... 67

Chapter 3.

Table 1.

Table 2.

Table 3.

Table 4.

Table 5.

Table 6.

Table 7.

Table 8.

Table 9.

Table 10.

LIST OF TABLES

Weed Control Using Reduced Rates of Postemergence Herbicides in
Narrow and Wide Row Soybeans.
Height of weed species at the time of herbicide application ...... 71

Inﬂuence of herbicide tank mixture and soybean row spacing on annual
grass control in 1994 .............................. 72

Control of annual grass in soybeans with reduced rates of
postemergence herbicides in 1994 and 1995 ................ 73

Inﬂuence of herbicide tank mixture and soybean row spacing on
common lambsquarters control in 1994 and 1995 ............ 74

Inﬂuence of application rate and timing, and soybean row spacing on
common lambsquarters control in 1994 and 1995 ............ 75

Control of common lambsquarters in soybeans with reduced rates of
postemergence herbicides in 1994 and 1995 ................ 76

Control of redroot pigweed in soybeans with reduced rates of
postemergence herbicides in 1994 ...................... 77

Inﬂuence of herbicide tank mixture and soybean row spacing on redroot
pigweed control in 1995 ............................ 78

Control of redroot pigweed in soybeans with reduced rates of
postemergence herbicides in 1995 ...................... 79

Control of common ragweed in soybeans with reduced rates of
postemergence herbicides in 1994 ...................... 80

vi

LIST OF TABLES (cont.)

Table 11.

Table 12.

Table 13.

Table 14.

Table 15.

Table 16.

Table 17.

Control of common ragweed in soybeans with reduced rates of
postemergence herbicides in 1995 ...................... 81

Inﬂuence of herbicide tank mixture and soybean row spacing on
velvetleaf control in 1995 ........................... 82

Control of velvetleaf in soybeans with reduced rates of postemergence

herbicides in 1995 ................................ 83
Inﬂuence of various rates of postemergence herbicides on weed biomass
in 1994 and 1995 ................................. 84
Injury to soybean from postemergence herbicides in 1994 and

1995 ......................................... 85
Inﬂuence of various rates of postemergence herbicides on soybean yield
in 1994 and 1995 ................................. 86
Gross margin of soybean treatments in 1994 and 1995 ......... 87

vii

Chapter 2.

Figure 1.

Figure 2.

Figure 3.

Figure 4.

LIST OF FIGURES

Rate Response of Giant Foxtail, Barnyardgrass, Redroot Pigweed,
Common Ragweed, and Velvetleaf to Postemergence Herbicides.

Reduction in giant foxtail dry weight 14 days after postemergence
herbicide application .............................. 42

Reduction in barnyardgrass dry weight 14 days after postemergence
herbicide application .............................. 43

Reduction in common ragweed dry weight 21 days after postemergence
herbicide application .............................. 44

Reduction in velvetleaf dry weight 14 days after postemergence
herbicide application .............................. 45

viii

REVIEW OF LITERATURE

Introduction

Weed competition can greatly reduce soybean (Glycine max (L.) Merr.) yield (68).
Coble et al. (15) reported that as little as four common ragweed (Ambrosia
artemisiifolia L.) plants per 10 m of row reduced soybean yield 8%. Vail and Oliver
(78) reported soybean yield loss of up to 78% from bamyardgrass (Echinochloa crus-
galli (L.) Beauv.) densities of 500 plants per meter of row. Numerous studies have
shown soybean yield reductions of 27 to 37% from giant foxtail (Setaria faberi
Herrm.) densities of 17 to 177 plants per meter of row (32, 43, 72). To attain
maximum yields weeds must be controlled. Cultural, mechanical, and chemical means
used to control weeds can make up a signiﬁcant portion of the input costs associated
with producing soybeans. Low proﬁt margins associated with producing soybeans have
prompted growers to look for ways to decrease their input costs, while maintaining
high yields in order to maximize proﬁtability. Weed control equal to using full rates
has been achieved using 25 and 50% of the recommended rates of postemergence
herbicides (19, 22, 73). Weed control programs that use reduced rates of
postemergence herbicides offer the potential to decrease producer input costs while
maintaining weed control and soybean yield. This results in increased proﬁt margins
and reduced risk of environmental damage such as herbicide runoff into surface water
or leaching into ground water. However, growers must be willing to spend more time

and management efforts towards weed control and be willing to accept the risk of using

below label rates.
Postemergence Herbicide Use in Soybeans

The use of postemergence herbicides has become a common and effective strategy
to control weeds in Michigan soybean ﬁelds. Postemergence herbicide use gained
popularity in the 1980's, as more postemergence herbicides became available (14, 39,
46, 51). Postemergence herbicides give growers more options for controlling weeds in
soybeans and provide growers with a way to control weeds if soil applied herbicides
fail.

Bentazon (3-(1-methylethyl)-(lI-I)—2,l,3-benzothiadiazin-4(3H)-one 2,2—dioxide), is
a postemergence contact herbicide used in several crops, including soybeans. Bentazon
has activity on many annual broadleaf weeds including velvetleaf (Abutilon theophrasti
Medic.) , common ragweed, common lambsquarters (Chenopodium album L.), and
common cocklebur (Xanthium strumarium L.) (51,71). Bentazon is a benzothiadiazole
herbicide. It inhibits photosynthesis in susceptible species by binding at the reducing
side of Photosystem 11 between the primary electron acceptor Q and plastiquinone (74).

Aciﬂuorfen (5-[2—chloro-4-(triﬂuoromethyl)phenoxy]-2-nitrobenzoic acid),
fomesafen (5-[2-chloro-4-(triﬂuoromethyl)phenoxy]-N—(methylsulfonyl)-2-
nitrobenzamide), and lactofen ((j;)-2-ethoxy-l-methyl-2-oxoethyl 5-[2-chloro-4—
(triﬂuoromethyl)phenoxy]-2—nitrobenzoate), are members of the diphcnylether herbicide
class. These contact herbicides have activity on many annual broadleaf weeds including
common ragweed, redroot pigweed (Amaranthus retroﬂexus L.), and eastern black

nightshade (2, 39, 49, 63, 77). Diphenylether herbicides act by inhibiting the enzyme

3

protoporphyrinogen oxidase (Protox). This leads to the attack and oxidization of lipids
and proteins resulting in the disruption of plant cell membranes (92).

Chlorimuron (ethyl 2-[[[[(4-chloro-6-methoxy-2-
pyrimidinyl)amino]carbonyl]amino]sulfonyl]benzoate) and thifensulfuron (methyl 3-
[[[[(4-methoxy-6-methyl-l ,3-5-triazin—2-yl)amino]carbonyllamino]sulfonyl]-2-
thiophenecarboxylate) are sulfonylurea herbicides. Chlorimuron controls many annual
broadleaf weeds in soybeans including common ragweed, redroot pigweed, velvetleaf,
and common cocklebur (28). Thifensulfuron controls annual weeds such as common
lambsquarters, pigweed species, and velvetleaf (28). These systemic herbicides inhibit
acetolactate synthase (ALS), an enzyme necessary for the synthesis of the essential
branched chain amino acids, isoleucine, leucine, and valine (92).

Clethodirn ((E,E)-( j;)-2-[l-[[(3-chloro-2-propenyl)oxy]iminolpropyll-S-[Z-
(ethylthio)propyl]-3-hydroxy-2-cyclohexen—l-one), is a member of the
cyclohexanedione family. Fenoxaprop ((i)-2-[4-[(6-chloro-2-
benzoxazolyl)oxy]phenoxylpropanoicacid), ﬂuazifop-P ((R)-2-[4-[[5-(triﬂuoromethyl)-
2-pyridinyl]oxy]phenoxy]propanoic acid), and quizalofop-P ((R)-2-[4-[(6-chloro-2-
quinoxalinyl)oxy]phenoxy]propanoic acid) are members of the aryloxyphenoxy
propionate family. Herbicides in these two families are systemic herbicides which
control most annual and many perennial grasses in soybeans including giant foxtail and
bamyardgrass (7, 45, 70). They act by inhibiting acetyl-CoA carboxylase (ACCase), an
essential enzyme in the synthesis of fatty acids, thus stopping the production of lipids in

the plant (92).

4

The efﬁcacy of postemergence herbicides is affected by many factors including
environmental conditions. Soil moisture, air temperature, relative humidity and light
intensity are some of the environmental conditions which can vary greatly and
inﬂuence the efﬁcacy of postemergence herbicides (31).

Conditions of moisture stress or high light intensity can cause thickening of the
cuticle and epicuticular waxes on plant leaves (31 , 66) which may result in reduced
herbicide absorption. Boydston (6) reported reduced control of green foxtail (Setan'a
viridis (L.) Beauv.) with fenoxaprop, ﬂuazifop-P, and sethoxydirn (2-[-
(ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen-1-one) when plants
were subjected to low soil moisture conditions 10 days before and 7 days after
application. Water and light levels may also affect translocation of herbicides. Kells et
al. (41) reported greater control of quackgrass with ﬂuazifop when plants were grown
under moist conditions compared to plants grown under moisture stress conditions.
Radioautographs of treated plants suggest less distribution of the “C-ﬂuazifop in the
moisture stressed plants. The effect of light on control of quackgrass with ﬂuazifop was
also investigated. Greater translocation of “C-ﬂuazifop occurred in plants grown in full
light compared to plants grown in shade.

High temperatures at application can increase the activity of some postemergence
herbicides. Lee and Oliver (46) investigated weed control with aciﬂuorfen when weeds
were grown under high temperature (27 to 35 C) compared to low temperature
conditions (18 to 27 C) in the growth chamber. High temperatures increased pitted

morningglory (Ipomoea lacunosa L.) control by 18-52% and velvetleaf and common

5

cocklebur control by 10% , compared to the low temperature conditions. Kells et al.
(41) observed greater control of quackgrass (Elytrigia repens (L.) Beauv.) by 0.28
kg/ha of ﬂuazifop when plants were grown at 30 C compared to 20 C. Absorption of
14C-ﬂuazifop was 71% at 30 C and 45% at 20 C.

Other research indicates the effect of temperature on weed control with sulfonylurea
herbicides appears to be opposite. Control of velvetleaf with thifensulfuron was greater
when plants were grown under cool (20 C) rather than warm conditions (30 C) (93).
Similarly, sicklepod (Senna obtusifolia L.) was more susceptible to Chlorimuron when
exposed to cooler rather than warmer temperatures (24).

Application of postemergence herbicides in high relative humidity conditions can
increase efﬁcacy. Ritter and Coble (62) investigated the effects of temperature and
relative humidity on the control of common ragweed and common cocklebur with
aciﬂuorfen. Control of common ragweed with aciﬂuorfen was greater at 85 %
compared to 50% relative humidity conditions and at temperatures of 32 C compared to
26 C. With common cocklebur, temperature was less critical than relative humidity.
Absorption of l“C-aciﬂuorfen in treated plants was 42% greater at 85% relative
humidity than at 50% relative humidity. Translocation of l“C—aciﬂuorfen in treated
plants was also greater under high temperature and high relative humidity conditions.
In a ﬁeld study comparing weed size at application, Lee and Oliver (46) suggested that
higher relative humidity conditions (88%) explained greater control of velvetleaf at the
2 leaf stage with aciﬂuorfen than at the 1 leaf stage (54% relative humidity).

Reduced control of prickly sida (Sida spinosa L.), pitted morningglory, entireleaf

6

morningglory (Ipomoea hederacea var. integriuscula Gray), and common cocklebur
occurred when aciﬂuorfen, fomesafen, or aciﬂuorfen + bentazon was applied at 50%
compared to 85 % relative humidity (89).

Efﬁcacy of postemergence herbicides is also affected by stage of growth or weed
size at application. Generally, smaller weeds are more susceptible to herbicides than
larger weeds. Aciﬂuorfen provided greater than 90% control of 4 to 6 leaf common
ragweed, while 8 to 10 leaf plants resulted in less than 75% control (64). Lee and
Oliver (46) investigated several rates of aciﬂuorfen for control of weeds at the 1, 2, 4,
and 8 leaf stages. Control of common cocklebur, entireleaf morningglory, and
velvetleaf decreased as leaf stage increased from 1 to 2 to 4 to 8. Zhao et al. (93) found
that velvetleaf susceptibility to thifensulfuron decreased as size of treated plants
increased from 1 to 3 to 5 leaf. Derr et al. (21) reported excellent control of giant
foxtail and large crabgrass (Digitaria sanguinalis (L.) Scop.) when ﬂuazifop was
applied at the pretillering stage (4 leaf, 10 cm tall). Control was reduced when
ﬂuazifop was applied at the tillering (7 leaf, 28 cm) and late tillering (9 leaf, 41 cm)
stages in greenhouse and ﬁeld experiments. Similarly, control with ﬂuazifop on green
foxtail, large crabgrass, yellow foxtail (Setaria glauca (L.) Beauv.), giant foxtail and
Japanese millet (Echinochloa crus-galli var. frumentacea (Roxb.) W.F. Wright) was
reduced as growth stage increased from 3 to 5 to 7 leaf (69). Kells et al. (41) observed
signiﬁcantly greater control of quackgrass with 0.56 kg ai/ha of ﬂuazifop when
treatments were applied to 2 to 3 leaf plants compared to 5 to 6 leaf plants.

Radioautographs of treated plants suggest greater distribution of the 14C-ﬂuazifop in 2

 

7

to 3 leaf stage plants compared to Sta 6 leaf plants.

King and Oliver (42) investigated the effect of timing of application on weed
control with aciﬂourfen, bentazon, Chlorimuron, and imazaquin (2-[4,5-dihydro-4-
methyl—4-(l-methylethyl)-5-oxo-1H-imidazol-2-yl]-3-quinolinecarboxylic acid) on eight
broadleaf weed species. Based on their data they developed linear or polynomial
regression equations which predicted control of the weed species dependent on
herbicide rate and weed age. These equations illustrate the common trend that as weeds 44'
increase in size or age, susceptibility to a postemergence herbicide application is LA
reduced. Thus the smaller the weed or the earlier an application, the lower the rate of
herbicide necessary to provide control.

Control of a larger number of weed species can be achieved using tank mixtures of
postemergence herbicides (55). Tank mixing reduces the number of application trips
through the ﬁeld, however, antagonistic interactions between two herbicides can occur,
resulting in reduced control in some cases (34, 71, 80, 86, 88).

Aciﬂuorfen antagonized the control of jimsonweed (Datum stramonium L.) by
bentazon and bentazon antagonized the control of redroot pigweed by aciﬂuorfen when
tank mixed (71). Radio labeled herbicide experiments revealed that the uptake of these
herbicides was reduced in the presence of the other. Westburg and Coble (87) observed
reduced control of common cocklebur and sicklepod when aciﬂuorfen was tank mixed
with Chlorimuron compared to chlorirnuron alone. Absorption and translocation of 1“C—
chlorimuron by common cocklebur was reduced when Chlorimuron was tank mixed

with aciﬂuorfen (88).

8

Environmental conditions can inﬂuence the severity of antagonistic tank mixtures.
Wesley and Shaw (85) reported no antagonism when Chlorimuron was tank mixed with
a diphcnylether herbicide in 1989 with optimal growing conditions at the time of
application. However they did observe antagonistic responses when aciﬂuorfen or
fomesafen was tank mixed with Chlorimuron in 1990 when temperatures at the time of

application were cool. Weed size did not inﬂuence the antagonistic effects. Reduced

“I

control was experienced when these tank mixtures were applied to either 3 leaf or 8
leaf common cocklebur plants. In contrast to Westburg and Coble (88), the addition of
aciﬂuorfen or lactofen to 1“C—chlorimuron increased absorption and translocation in
common cocklebur (67). However, in this study herbicide was applied only to a
localized area on the leaf, not to the entire plant. Thus absorption and translocation of
Chlorimuron is probably reduced when aciﬂuorfen application causes rapid foliar injury
to the entire plant.

Numerous studies have shown reduced control of several grass species when
sethoxydim is tank mixed with bentazon (35, 53, 54, 60, 61). Absorption of 1“C-
sethoxydim was greatly reduced when tank mixed with bentazon (38, 61, 81).
Wanamarta et al. (81) provided evidence for explaining the antagonism. Na” ions from
the sodium salt of bentazon exchanged with the H+ of the hydroxyl group of
sethoxydirn when tank mixed, and the sodium salt of sethoxydim was formed. The
absorption of the sodium salt of sethoxydirn is reduced because it is more polar than
sethoxydim (81).

Antagonism with other grass and broadleaf herbicides has also been reported.

9

Fluazifop can be antagonized by tank mixtures with aciﬂuorfen, chlorirnuron,
fomesafen, and lactofen (18, 27, 54). Clethodim, quizalofop, and fenoxaprop can also
be antagonized by broadleaf herbicides (54, 80). Of the three diphcnylether herbicides,
fomesafen appears to be least likely to cause an antagonistic response with a grass
herbicide (38, 53, 80).

The antagonistic response seen between a graminicide and an ALS inhibiting

‘Ei

a“-
O

herbicide is not due to reduced absorption as is the case with bentazon and sethoxydim.

‘ :_- :0
.I' huh

Chlorimuron reduced control of sethoxydim on large crabgrass when tank mixed,
however Chlorimuron did not reduce the uptake of l4C-sethoxydim (38).

The antagonistic response observed when broadleaf herbicides are tank mixed with
graminicides can sometimes be overcome by increasing the rate of the graminicide
(60). The antagonistic interaction between grass and broadleaf herbicides is species
dependent (34) and can be inﬂuenced by environmental conditions (38). Antagonism
from tank mixing may not reduce control if conditions for plant growth are optimal
(27).

Soybean Row Spacing

One factor to consider in a soybean production system which can inﬂuence yield is
soybean row spacing. Wiggans (90) reported in 1938 that soybean seed yields increased
as the spacing between rows was decreased. He remarked, “The nearer the
arrangement of plants on a given area approaches a uniform distribution, the greater
will be the yield.” Many researchers since then have similarly concluded that higher

soybean yields can be attained by planting soybeans in narrow row spacings (15 to 50

10

cm) (16, 17, 25, 48, 84), although this is not always the case (12, 48, 56). The
increase in yield from narrow row spacings is dependent on many factors. Increases in
yields from narrow row soybeans compared to wide rows are most likely to occur in
the northern growing regions of the US. (17), with early maturing varieties (16, 17,
25), under optimal growing conditions including high moisture levels (75), or with late
planted soybeans (4).

Decreasing the row spacing and spacing of plants within the row can result in
increased plant height which can increase lodging of soybeans. This can counteract the
yield advantage of narrow rows. Higher populations in narrow rows have resulted in
increased lodging and resulted in no difference in soybean yield between row spacings
(17, 25, 48). Heatherly (37) reported no yield advantage to planting soybeans in
narrow rows under either irrigated or nonirrigated treatments at normal or late planting
dates and no lodging.

Narrow row soybeans have the potential to yield higher than wide row soybeans
mainly because of increased light interception by the soybean canopy (4, 5, 76). With
the same leaf area index in narrow and wide rows, there was greater light interception
in narrow rows because of a more uniform leaf distribution, which resulted in higher
yield (76).

Prior to the 1980's most soybeans were planted in wide rows (76 to 104 cm) to
allow cultivation of weeds. There was no practical advantage of planting soybeans in
narrow rows because sufﬁcient weed control could not be obtained without cultivation.

The narrow choice of herbicides usually did not control all of the weed species present

ﬂ_‘h# ‘-
.
k r i... _

Ir.

11

so one or two cultivations were commonly necessary to control weeds which were not
susceptible to a herbicide. However, if weeds can be controlled early in the growing
season greater season long weed control can be achieved in narrow rows due to faster
canopy closure which can suppress late emerging weeds (19, 57, 82). Wax et al. (83)
found that when triﬂuralin (2,6-dinitro-N,N-dipr0pyl-4-(triﬂuoromethyl)benzenamine)
was applied alone annual grasses and pigweeds were controlled, but velvetleaf,
jimsonweed and ivyleaf morningglory (Ipomea hederacea (L.) Jacq.) were not. In this
situation, soybeans which were planted in 76 cm rows and were cultivated outyielded
soybeans in 18 cm rows by up to 50%. However if triﬂuralin was applied with
metribuzin (4-amino-6-( l ,1-dimethylethyl)—3-(methylthio)-l ,2 ,4—triazin-5 (4H)-one) or
followed by an application of bentazon to control broadleaf weeds, narrow row
soybeans outyielded wide row soybeans by 9%. Similarly, Burnside and Moomaw (11)
proved that weeds could adequately be controlled in narrow row soybeans. A variety of
treatments using preemergence or preemergence plus postemergence herbicides applied
to 38 cm row soybeans provided control of grass and broadleaf weeds similar to the
handweeded plots.

Wax and Pendleton (82) compared soybeans planted in 25, 50, 75, and 100 cm row
spacings either cultivated or treated with triﬂuralin. They reported increased grass and
broadleaf weed biomass as row spacing increased. Complete soybean canopy was
formed at 35, 50, 65, and 80 days after planting in 25, 50, 75, and 100 cm row
spacings, respectively. They noted narrow rows provided earlier shading which

resulted in better weed control, however, early season weed control is still very

12

important with narrow rows. Soybean yields increased by 10, 18, and 20% in 75, 50,
and 25 cm rows, respectively, compared to 100 cm rows.

Peters et al. (57) noted that in soybeans treated with chloramben (3-amino—2,5-
dichlorobenzoic acid), weed yields decreased and soybean yields increased as row
width decreased. They observed that the soybean canopy of narrow rows provided an
earlier and more complete canopy than wide rows, which resulted in a greater shading

and weed suppression.

51’“ ii

Burnside and Colville (10) compared soybean row spacings of 25, 50, 75, and 100
cm, and found the ground was completely shaded in 36, 47, 58, and 67 days,
respectively. They concluded that chemical weed control only needs to be effective for
about a month in 25 cm row spacings. Weed control from chloramben treatments was
more effective as row spacing was decreased. Chloramben rates could be reduced by
50 to 75 % for equivalent weed control in 25 cm rows compared to 100 cm rows.
Soybean yields increased as row spacing decreased with soybeans in 25 cm rows
yielding 39% higher than soybeans in 100 cm rows.

Patterson et al. (56) compared 15, 30, 45, and 90 cm row spacings grown in
competition with common cocklebur and sicklepod under irrigated and nonirrigated
conditions. In competition with sicklepod, soybean biomass decreased 14 kg/ha, seed
yield decreased 12 kg/ha, and sicklepod biomass increased 12 kg/ha, for each cm
increase in soybean row spacing. In competition with common cocklebur soybean
biomass decreased 10 kg/ha, seed yield decreased 16 kg/ha, and common cocklebur

biomass increased 10 kg/ha for each cm increase in soybean row spacing. Without

l3

weed interference soybean biomass and seed yield were similar between row spacings
and under irrigated and nonirrigated conditions.

McWhorter and Sciumbato (52) indicated the competitive advantage of narrow row
soybeans is not a factor in the early growth stages but becomes apparent later. They
reported no difference in the wet weight of sicklepod plants early in the season,
however, weight of sicklepod plants were reduced when measured at 72 through 120
days after emergence.

Légére and Shreiber (47) investigated the effects of row spacing on redroot pigweed
interference in soybeans. They reported soybean canopy closure was faster in 25 cm
rows than in 76 cm rows. Redroot pigweed contributed 24% of the total biomass in
narrow rows compared to 43 % in wide rows. Total biomass produced by both species
in a mixed stand was similar to the biomass produced by soybeans alone in a weed free
stand. Narrow row soybeans produced higher yields under weedy and weed free
conditions. In dry growing seasons narrow row soybeans proved advantageous over
wide rows, especially under weedy conditions.

The data presented by these researchers suggests increased competitiveness of
soybeans when grown in narrow rows. Therefore planting soybeans in narrow rows is
an option which could increase weed control in a reduced rate postemergence weed
control program. A faster canopy enclosure can allow soybeans to outcompete weeds
which may be injured but not completely killed from a reduced rate herbicide

application or which emerge after herbicide application.

;F'_" ‘:-]

14

Use of Reduced Rates of Soil Applied Herbicides

Research on the use of reduced rates (less than the recommended rate on the
pesticide label) of soil applied herbicides has been conducted in response to growers
seeking ways to maximize the proﬁtability of producing soybeans. In the 1970's
Burnside and Moomaw (11) applied full and 50% rates of alachlor (2-chloro-N-(2,6-
diethylphenyl)—N—(methoxymethyl)acetamide), chloramben, or ﬂuorodifen (p-
nitrophenyl a,a,a-triﬂuro-2-nitro-p-tolyl ether) with a full rate of a broadleaf herbicide
to narrow row (38 cm) soybeans. Weed control and soybean yields were similar to full
rate treatments.

Banding herbicides over the crop row followed by cultivation can effectively
control weeds while reducing herbicide inputs. Preemergence applications of alachlor
plus chloramben applied in a 20 cm band over the crop row followed by one or two
cultivations resulted in weed control and soybean yield equal to that of a full rate
applied broadcast (3).

Adequate rainfall is necessary for soil applied herbicides to be effective, especially
at reduced rates. Imazaquin applied preplant incorporated or preemergence at 50% of
the recommended rate controlled common cocklebur if adequate rainfall was received
(30). Up to 88% control of common cocklebur was achieved at locations where
adequate rainfall was received, but as low as 8% control resulted when no rainfall was
received. Reduced rates of postemergence applied imazaquin was not as dependent on
rainfall in these studies. Control with 50% of the recommended rate applied

postemergence ranged from 86% to 97 % and control with postemergence applied

15

imazaquin at the lowest rate, 1/6 of the recommended rate, ranged from 70 to 95% at
the ﬁve locations.

Jones et al. (40) investigated the use of reduced rates of imazaquin (2/3x, 1/3x) plus
full rates of alachlor, applied preplant incorporated (PPI) to narrow and wide row
soybeans. Acceptable control of common lambsquarters and Pennsylvania smartweed
(Polygonum pensylvam'cum L.) was obtained with 2/3x or 1/3x rates of imazaquin at 5
of 6 locations. At the sixth location, where velvetleaf and ivyleaf morningglory were
present, reduced rates of imazaquin did not provide adequate control of these less
susceptible species. Planting soybeans in narrow rows resulted in 19% less broadleaf
weed biomass and 4% higher soybean yields compared to wide rows.

Buhler et al. (9) investigated the use of reduced rates of broadcast or band applied
alachlor plus metribuzin in combination with cultivation. Full or 1/2x rates applied in a
band plus one cultivation controlled pigweed species and giant foxtail both years and
common lambsquarters one of two years. A second cultivation increased control. A
1/2x rate applied broadcast plus a cultivation also controlled pigweed species and giant
foxtail both years and common lambsquarters one of two years. The reduced rate
treatments decreased soybean injury compared to full rate treatments applied broadcast.
Yield of soybeans treated with 1/2x rate banded plus 1 cultivation was equal to the
yield of the handweeded soybeans one of two years. With two cultivations soybean
yields were equal both years. A full rate of the herbicides banded plus one or two
cultivations resulted in soybean yields equal to yield of the handweeded treatment both

years. One-half of the recommended rate applied broadcast produced soybeans which

 

l6

yielded equal to the handweeded treatment one of two years and, with cultivation, was
equal both years. A second cultivation often increased weed control but usually did not
increase soybean yields.

The University of Wisconsin (23), the University of Missouri (20), and Iowa State
University (33) have published extension bulletins which outline guidelines for using
reduced rates of herbicides based on their research. Some of the reasons for using
reduced rates of soil applied herbicides include: 1) decreased production costs, 2)
reduced negative environmental effects from herbicide use, 3) response to government
regulations on herbicide use, and 4) lessened risk of herbicide carryover in soil. Soil
applied herbicide rates can be reduced by three ways: reducing the rate of a broadcast
application, applying a full rate in a band over the row, or by applying a reduced rate
in a band. Doll et al. (23) extensively investigated the use of reduced rates of soil
applied herbicides in corn and soybeans. Excellent weed control was obtained in one
soybean and fourteen corn studies with all three of these methods when followed by a

cultivation. They stressed that cultivation must be timely and rotary hoeing may be

necessary if no rainfall is received within 7 to 10 days after planting. Reduced rates are

not recommended for ﬁelds with difﬁcult to control grasses such as shattercane
(Sorghum bicolor (L.) Moench.), wild proso-millet (Panicum miliaceum L.), woolly
cupgrass (Eriochloa villosa (Thunb.) Kunth.), or quackgrass, and may not be effective
on cloddy soils, high organic matter soils, or in a no-tillage system. It is recommended
that growers ﬁrst try reduced rates on a small scale basis.

DeFelice and Kendig (20) found that 50% of the recommended rates of soil applied

III—“’7!

17

herbicides did not provide weed control equal to using full rates. They concluded that a
cultivation and/or a postemergence herbicide application was almost always necessary
for adequate weed control. Cultivation has been an instrumental tool in weed control
for decades, however, DeFelice and Kendig warn that there are signiﬁcant drawbacks
with cultivation. One concern which should be considered is the increased risk of soil
erosion. Additionally, timeliness, favorable soil conditions, properly adjusted
equipment, energy, and labor are all necessary for cultivation to be effective. I

Researchers at Iowa State University conducted sixty-four on—farm research trials LE
comparing full rate banded applications followed by cultivation to full rate broadcast
applications of soil applied herbicides (33). At 92% of the sites weed control with
banded applications equaled that of full rate broadcast applications, and 99% achieved
soybean yields equal to yield of soybeans applied with full broadcast rates.
Use of Reduced Rates of Postemergence Herbicides

Numerous studies have examined various aspects of using reduced rates of
postemergence herbicides in soybeans (l, 9, 13, 19, 22, 26, 29, 42, 58, 59, 65, 73).
These studies have investigated many of the factors which can inﬂuence the efﬁcacy of
reduced rate postemergence weed control progams. Such factors include: rate of
herbicide, timing of application, choice of herbicide or herbicide tank mixtures,
soybean row spacing, cultivation, and tillage systems.

DeFelice et al. (19) investigated the use of 1/2x and 1/4x rates of bentazon,
aciﬂuorfen, or Chlorimuron tank mixed with a 1/2x rate of sethoxydim. Treatments

were applied to narrow row soybeans at two locations and to wide rows at a third

18

location. Single appplications of the reduced rates were applied 7 days after planting
(DAP) to weeds less than 2 cm tall and full rates were applied 14 DAP to weeds 5 cm
tall or less. The sequential application of l/4x of the full rate of the broadleaf herbicide
tank mixed with a 1/2x rate of sethoxydim applied at 7 and 21 DAP gave the most
consistent control of grasses and broadleaves, providing control of giant foxtail,
velvetleaf, and common cocklebur equal to that of full rate treatments. In most cases
single applications of reduced rates did not provide control equal to full rate treatments
and soybean yields were not equal to sequential or full rate treatments. One location in
1987 experienced hot and dry conditions at the time of application. Because of these
conditions none of the reduced rate or full rate treatments adequately controlled
velvetleaf or common cocklebur, illustrating that reduced rates of postemergence
herbicides should not be applied when plants are moisture stressed. Antagonism of
sethoxydim was observed when tank mixed with bentazon. Control of crabgrass with
the 1/2x bentazon + l/2x sethoxydim treatment was less than the l/4x bentazon + l/2x
sethoxydim treatment. Decreased weed control including late season germination and
weed growth and lower soybean yields were observed at the wide row soybean site.
Steckel et al. (73) investigated the effect of a cultivation in combination with using
reduced rates of postemergence herbicides. Twenty-ﬁve percent of the recommended
rates of bentazon, Chlorimuron, and imazaquin, and 50% of the recommended rates of
irnazethapyr (2-[4,5-dihydro-4-methyl-4-(l-methylethyl)-5-oxo-lH-imidazol-Z-yll-S-
ethyl-3-pyridinecarboxylic acid) were applied at 10 DAP, sequential 25% use rates

were applied 10 and 21 DAP, and full rates were applied 21 DAP. All treatments were

 

19

cultivated. Sequential applications of 25 % of the recommended rate of these herbicides
controlled common cocklebur and velvetleaf as well as the full rate applications. Single
applications of 25 % of the recommended rate did not control these weeds as well as
full or sequential rates. Soybean yield with sequential or single reduced rate treatments
were equal to yield of soybeans treated with full rates of the herbicides. Even though
weed control with single reduced rate treatments was lower, yield was not reduced.

Devlin et al. (22) reported 1/2x rates of bentazon, aciﬂuorfen, a tank mixture of

bentazon + aciﬂuorfen, or Chlorimuron applied 14 DAP controlled common cocklebur,
common sunﬂower (Helianthus annuus L.), smooth pigweed (Amaranthus hybridus
L.), and velvetleaf as well as full rates of these herbicides applied 28 DAP, except at
one location. At this location cool damp weather conditions delayed soybean and weed
emergence and as a result many weeds germinated after the 14 DAP application.
Applications of 1/4x rates of these herbicides at 14 DAP were less consistent in
controlling weeds. Cultivation at 28 DAP appeared to improve weed control but was
not statistically signiﬁcant when averaged over all treatments. Cultivation improved
weed control with 1/4x treatments to equal to that of full rate treatments with no
cultivation. Soybean yield of l/2x and 1/4x treatments was similar to soybean yield in
the full rate treatments.

Prostko and Meade (58) found that applying a sequential 1/4x rate treatment of
fomesafen + sethoxydim, Chlorimuron + sethoxydim, or bentazon + aciﬂuorfen +
sethoxydim to wide row soybeans controlled common ragweed and common

lambsquarters as well as full rate applications. Single applications of 1/2x rates of these

20

herbicides applied at an early timing generally did not control broadleaf weeds as well
as using full rates. Control of stinkgrass (Eragrostis cilianensis (All.) Lutati.) with
l/2x or 1/4x sequential applications was equal or better than a single application at a
full rate. Yield of soybean treated with 1/4x sequential applications was equal to yield
of soybean treated with full rates. Yield of soybean treated with l/2x rates also was not
different than yield from full rate treatments even though weed control was not equal.

Rathjen and Martin (59) compared weed control with reduced rates of
postemergence herbicides applied to soybeans planted in 25 cm or 76 cm row spacings.
The 76 cm rows were cultivated. Imazethapyr, Chlorimuron, and bentazon +
aciﬂuorfen applied at 1/2x rates 20 DAP were compared to full rates of these
herbicides applied 25 DAP for control of velvetleaf and common sunﬂower. Rotary
hoeing did not increase weed control in either row spacing. In general the reduced rate
treatments provided weed control similar to the standard rates. Weed control in narrow
rows was similar to weed control in wide rows with a cultivation.

Results of research conducted in no-till soybeans with the use of reduced rates of
postemergence herbicides is similar to the research conducted in conventionally tilled
soybeans. Single and sequential applications of reduced rates of postemergence
herbicides can adequately control weeds, however, sequential applications are more
consistent in providing weed control (36, 65). The reduction in annual weed pressure in
long term no-till ﬁelds, may provide greater opportunity for the use of reduced rates of
postemergence herbicides.

Recommendations for applying reduced rates of postemergence herbicides in

E—‘l

21

soybeans have been published by the University of Missouri (20) and by Iowa State
University (33). DeFelice and Kendig (20) recommend early applications for broadleaf
weed control. Herbicide rates of 1/4x should be applied 8 to 10 DAP, which usually
coincides with 4 to 6 days after weed emergence (DAWE) and weeds 0.6 to 1.3 cm in
height. Growers should make a second application of 1/4x rate or cultivate if necessary
due to late weed germination. Rates of 1/2x should be applied 11 to 16 DAP (7 to 12
DAWE or 1.3 to 2.5 cm weeds). A second herbicide application or cultivation is less
likely to be necessary with 1/2x rates. Reduced rates of postemergence grass herbicides
are not recommended by the University of Missouri.

DeFelice and Kendig (20) commented that results of research indicating that less
than label rates can successfully control weeds, does not imply that the labeled rates are
too high. Herbicide rates recommended on product labels are set high enough to ensure
control of a range of weeds that vary in susceptibility and are applied under a range of
environmental conditions. Herbicide manufacturers can not afford to risk labeling a
herbicide rate which will not consistently provide effective weed control over a wide
range of conditions.

One concern with using reduced rates, is that the manufacturer is not liable for
herbicide performance if applied at less than the labeled rate, therefore the grower
assumes all risk. However, DeFelice and Kendig (20) stated there are ways to apply
less herbicide and still comply with the manufacturers label. Many weeds are present
only in patches in a ﬁeld. By scouting ﬁelds for these patches, application can be made

at a labeled rate to only these areas rather than the entire ﬁeld. Also, some herbicide

22

labels recommend different application rates depending on the species and weed size. If
indicated on the label, some herbicides can be applied at a lower rate if it is a highly
susceptible species or small in size.

Economics of Reduced Rate Herbicide Programs

The critical factor determining whether or not reduced rate herbicide programs are
effective is the resulting gross margin. Some researchers have judged the proﬁtability
of reduced rate herbicide programs by comparing the gross margins of the treatments.
Gross margins for each treatment can be estimated by subtracting the variable input
costs (input costs which vary between treatments) from the gross income (yield
multiplied by the assumed market price). Others have economically justiﬁed reduced
rate treatments simply by comparing input costs if there are no differences in soybean
yields between full and reduced rates.

Bicki et al. (3) reported that using reduced rate band applications plus cultivation
can produce weed control and soybean yield equal to using full rate broadcast
applications. Under this system herbicide use was decreased by 73% and weed control
costs were reduced by 51 and 29% with one or two cultivations. Other research has
shown that the highest input treatments do not always result in the highest gross margin
(8, 44). Woods (91) compared preemergence and postemergence herbicide treatments
applied at full and reduced rates in a band with cultivation. In one of two years, higher
gross margins were achieved with banded applications of preemergence and
postemergence herbicide treatments compared to full rates applied broadcast. Vencill et

al. (79) conducted experiments to compare the gross margins from soybean systems

23

using different weed control methods. Treatments included broadcast preemergence,
banded preemergence, postemergence, and postemergence directed herbicide
applications, and cultivation. Dry weather in 1990 resulted in low soybean yields and
low gross margins. Under these conditions low input treatments gave the highest gross
margins but all treatments were unacceptable. Better growing conditions in 1991
resulted in higher yields and higher gross margins. The best weed control treatments
resulted in the highest yielding soybeans and also the highest gross margins.
Metribuzin treatments applied broadcast preemergence resulted in gross margins that
were higher than banded metribuzin treatments. In 1992 gross margins of banded or
broadcast applied metribuzin treatments were not different. Thus, reducing herbicide

input costs in this study did not increase gross margins.

10.

11.

12.

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McWhorter, C. G. 1981. Effect of temperature and relative humidity on

51.

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28

translocation of l4C-metriﬂufen in johnsongrass (Sorghum halepense) and
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29

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87.

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31

(Xanthium strumarium) control. Weed Technol. 3:364-368.

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24.

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activity of thifensulfuron. Weed Sci. 38:553-557.

RATE RESPONSE OF GIANT F OXT AIL, BARNYARDGRASS,
REDROOT PIGWEED, COMMON RAGWEED, AND VELVETLEAF
TO POSTEMERGENCE HERBICIDES

Abstract. Greenhouse experiments were conducted to compare the efﬁcacy of
postemergence soybean herbicides applied at reduced rates to ﬁve weed species in the
greenhouse. The results of these experiments indicate that the ranking for control of
giant foxtail based on the relative reduced rates applied was: clethodim > quizalofop

> ﬂuazifop-P&fenoxaprop; and the ranking for barnyardgrass was: clethodim >
ﬂuazifop-P&fenoxaprop > quizalofop. Thifensulfuron, fomesafen, lactofen, and
aciﬂuorfen controlled redroot pigweed similarly. Chlorimuron was less effective. The
relative ranking for control of common ragweed based on the rates applied was:
fomesafen = lactofen > aciﬂuorfen > bentazon = Chlorimuron > thifensulfuron; and
for velvetleaf was: fomesafen = Chlorimuron = thifensulfuron > lactofen > bentazon

> aciﬂuorfen.

32

33

INTRODUCTION

The use of postemergence herbicides is a common and effective strategy to
control weeds in Michigan soybean ﬁelds. The efﬁcacy of a postemergence herbicide
application is highly inﬂuenced by the application rate. Rates recommended on
herbicide labels must consistently control a range of weed species that vary in
susceptibility and size at application, and which are applied under an array of
environmental conditions. Herbicide manufacturers are hesitant to label a herbicide at a
rate which will control only a few weed species or a rate that may lack consistency
over a wide range of conditions.

The application rate of a postemergence herbicide necessary to kill a susceptible
plant species is dependent on many factors including weed size and environmental
conditions at the time of application. Smaller weeds are generally more susceptible to a
postemergence herbicide application than large weeds (3, 7) and therefore less
herbicide may be necessary to control small weeds (9). Plant moisture stress due to hot
dry weather can cause thickening of the epicuticular waxes on plant leaves resulting in
reduced herbicide absorption (1, 6). Greater weed control can be achieved with
postemergence herbicides when environmental conditions at application are ideal.
Adequate soil moisture, high relative humidity, high air temperature, and high light
intensity can increase the absorption and translocation of herbicides resulting in greater
weed control (8, 7, 12). Therefore, when postemergence herbicides are applied to

small weeds under ideal environmental conditions, herbicide rates could potentially be

34

reduced.

Weed control equal to using full rates has been achieved using 25 and 50% of
the recommended rates of postemergence herbicides (2, 4, 13). However, when
applying reduced rates of postemergence herbicides, it is important to carefully
consider the choice of herbicide or herbicide tank mixture. In some cases, this choice
can make the difference between adequate weed control and weed control failure when
using reduced rates (11). The susceptibility of a given weed species to two herbicides
can vary greatly. Even if both herbicides will control the weed at a labeled rate, one
herbicide may be more active. At a reduced rate this more active herbicide may
provide good control while the less active herbicide will fail.

The objective of these experiments was to compare the efﬁcacy of
postemergence soybean herbicides applied at reduced rates on ﬁve weed species in the
greenhouse to determine if there were differences in the relative effectiveness of these

herbicides as application rates were reduced.

MATERIALS AND METHODS

Giant foxtail and barnyardgrass seeds were planted into potting soil (60%
Sphagnum peat, 20% perlite, and 20% rock wool) in 945 ml plastic pots in the
greenhouse. Prior to herbicide application, pots were fertilized with 0.1 g of soluble
fertilizer (20% N, 20% P205, 20% K20) and thinned to 10 plants per pot.

Environmental conditions were maintained at 27 C j; 5 C with a 16 hour photoperiod

35

of natural lighting supplemented with sodium vapor lighting giving a cloudless midday
photosynthetic photon flux density of 500 uE/mz/s.

Herbicides were applied to three leaf (7 to 10 cm) giant foxtail and
barnyardgrass plants using a moving nozzle sprayer equipped with an 8002B even ﬂat
fan nozzle, traveling 4.8 kin/hr, and delivering 234 um at a pressure of 193 kPa.

The experiments were designed as randomized complete blocks with a factorial
arrangement of treatments and repeated in time. Clethodim, the formulated premix of
ﬂuazifop-P&fenoxaprop, and quizalofop were applied to giant foxtail and
barnyardgrass plants at ﬁve rates. These herbicides have similar labeled ﬁeld use rates
of 584 ml/ha of formulated product for control of barnyardgrass. Preliminary
experiments showed 100% control of barnyardgrass and giant foxtail with application
rates of as little as 146 ml/ha (1/4x). Therefore, rates used for these experiments
started at one eighth of the labeled ﬁeld use rate (1/8x) and subsequent rates decreased
in half (1/ 16x, 1/32x, 1/64x, l/ 128x). The 1/8x rates were: clethodim at 17 g ai/ha, the
premix of ﬂuazifop-P&fenoxaprop at 23 g/ha (17 g + 6 g), and quizalofop at 8 g/ha.
All treatments included nonionic surfactant at 0.125 % v/v and 28% urea ammonium
nitrate at 2.3 L/ha.

Redroot pigweed, common ragweed, and velvetleaf seeds were planted into
Bacctol greenhouse potting soil in 945 ml plastic pots. Prior to herbicide application,
pots were fertilized with 0.1 g of soluble fertilizer (20% N, 20% P205, 20% K20) and

thinned to one plant per pot. Enviromnental conditions were maintained at 29 C j; 5 C

 

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

36

with a 16 hour photoperiod of natural lighting supplemented with metal halide lighting
giving a midday photosynthetic photon ﬂux density of 1000 uE/mzls.

Herbicides were applied to 6 leaf redroot pigweed, 3 leaf velvetleaf, and 6 leaf
common ragweed using a continuous link belt sprayer equipped with an 8001B even
ﬂat fan nozzle, traveling 1.5 km/hr, and delivering 234 um at a pressure of 221 kPa.

The experiments were designed as randomized complete blocks with a factorial
arrangement of treatments and repeated in time. Herbicide rates used in these
greenhouse studies were based on recommended ﬁeld use rates. lactofen, fomesafen,
aciﬂuorfen, thifensulfuron, and chlorirnuron were applied at ﬁve rates (1/4x, 1/8x,
1/16x, 1/32x, 1/64x) to redroot pigweed. These herbicides were also applied at ﬁve
rates (1/2x, 1/4x, 1/8x, 1/16x, 1/32x) and bentazon at four rates (1/2x, l/4x, 1/8x,
1/16x) to common ragweed and velvetleaf. Aciﬂuorfen and bentazon, which are
commonly applied as a formulated premix in the ﬁeld, were applied separately in the
greenhouse to examine the effects of each herbicide. Therefore the rates of aciﬂuorfen
and bentazon are not considered true full rates, because the recommended ﬁeld rate of
either herbicide would be higher when applied alone. This should be considered when
comparing aciﬂuorfen or bentazon to other herbicides. The 1/2x rates of the herbicides
used in these studies were: lactofen at 53 g/ha + nonionic surfactant (NIS) at 0.125%
v/v, fomesafen at 140 g/ha + NIS at 0.125% v/v + 28% urea ammonium nitrate
(UAN) at 2.3 L/ha, aciﬂuorfen at 93 g/ha + crop oil concentrate (COC) at 2.3 L/ha,
bentazon at 421 g/ha + COC at 2.3 L/ha, thifensulfuron at 2.2 g/ha + NIS at 0.125%

v/v + UAN at 4.6 L/ha, and chlorirnuron at 2.9 g/ha + NIS at 0.125% v/v +UAN at

37

4.6 L/ha. The adjuvants included were always applied at the full rate listed regardless
of the herbicide rate.

Injury to each species was evaluated visually at 7 and 14 days after treatment
(DAT). Plant dry weight was measured by harvesting, oven drying and weighing all
above ground portions of giant foxtail, barnyardgrass, redroot pigweed, and velvetleaf
at 14 DAT. Common ragweed growth in the greenhouse was slower than other species.
Dry weight measurement was delayed to 21 DAT in order to attain sufﬁcient regrowth
similar to the other broadleaf experiments.

Data were combined over experiment repetitions and analysis of variance
revealed signiﬁcant herbicide by rate interactions. Means were separated using
Duncan’s Multiple Range Test. Results and discussion are based on dry weight data
which was converted to a percent reduction in dry weight, based on the untreated
control in each replication of each experiment for each weed species using the equation

100 - ((plant dry weight/untreated plant dry weight) X 100).

RESULTS AND DISCUSSION

Giant foxtail. All three herbicides provided excellent control (91 to 92% dry weight
reduction) of giant foxtail when applied at the l/8x rate (Figure 1). However, as the
rate was further reduced, clethodim provided greater control than quizalofop, and
ﬂuazifop-P&fenoxaprop provided the least control.

Barnyardgrass. Control of barnyardgrass at the 1/8x rate was excellent with clethodim

38

and ﬂuazifop-P&fenoxaprop (93 and 95 % dry weight reduction), while control with
quizalofop was less (83%) (Figure 2). As rates were further reduced, clethodim
provided the greatest control, followed by ﬂuazifop-P&fenoxaprop. QuizalofOp
provided the least control.
Redroot pigweed. There was no difference in redroot pigweed dry weight reduction
between lactofen, fomesafen, aciﬂuorfen, or thifensulfuron at any rate (data not
presented). Chlorimuron provided similar control to other herbicides at the 1/4x rate,
however control with Chlorimuron was less than other herbicides at rates lower than
1/4x. Green (5) reported that redroot pigweed was less susceptible to Chlorimuron than
thifensulfuron. The amount of active ingredient necessary for Chlorimuron to provide
90% control of redroot pigweed was 4 times that of thifensulfuron.
Common ragweed. All herbicides applied at 1/2x provided greater than 80% reduction
in common ragweed dry weight (Figure 3). At rates less than l/2x, fomesafen and
lactofen provided the greatest control. Aciﬂuorfen provided greater control than
bentazon or Chlorimuron, and thifensulfuron provided the least control of common
ragweed.
Velvetleaf. All herbicides applied at 1/2x resulted in 86% or greater reduction in
velvetleaf dry weight, except for aciﬂuorfen (42%) (Figure 4). At rates less than 1/2x,
fomesafen, thifensulfuron, and Chlorimuron provided the greatest control of velvetleaf,
followed by lactofen. Bentazon and aciﬂourfen provided the least control.
Comparisons between herbicides based on rate responses are dependent on the

rate chosen as a “full rate”. In most cases a herbicide label does not list a single rate at

39

which it is to be applied. The labeled recommended application rate can be dependent
on weed species, crop species, weed size, environmental conditions, soil
characteristics, location, tank mixtures, adjuvants, or governmental regulations.
Therefore, choosing the “full rate” of a particular herbicide is a subjective decision and
can inﬂuence the results. However, at the rates applied in these studies, results indicate
there were differences between the effectiveness of these herbicides as rates were
reduced. Clethodirn appeared to be the best choice for control of giant foxtail or
barnyardgrass when applied at reduced rates. Fomesafen, lactofen, aciﬂuorfen, and
thifensulfuron were equally effective in controlling redroot pigweed at reduced rates.
Fomesafen and lactofen were the most effective herbicides for common ragweed
control, while fomesafen, thifensulfuron, and Chlorimuron were the most effective for
control of velvetleaf in the greenhouse.

Results from these experiments suggest which herbicides may be more likely to
provide adequate weed control in the ﬁeld when applied at reduced rates. However,
plants can respond differently to postemergence herbicides in the greenhouse compared
to the ﬁeld (10), due to differences in the environment or due to root uptake in the
ﬁeld. It is also likely that differences in plant response between greenhouse and ﬁeld
environments will vary with herbicide class. Field research is needed to conﬁrm which

herbicides are most effective when applied at reduced rates.

10.

ll.

12.

LITERATURE CITED

Boydston, R. A. 1990. Soil and water content affects the activity of four
herbicides on green foxtail. Weed Sci. 38:578—582.

DeFelice, M. S., W. B. Brown, R. J. Alderich, B. D. Sims, D. T. Judy, and
D. R. Guethle. 1989. Weed control in soybeans (Glycine max) with reduced
rates of postemergence herbicides. Weed Sci. 37:365-374.

Derr, J. F., T. J. Monaco, and T. J. Sheets. 1985. Response of three annual
grasses to ﬂuazifop. Weed Sci. 33:693-697.

Devlin, D. L., J. H. Long, and L. D. Maddux. 1991. Using reduced rates of
postemergence herbicides in soybeans (Glycine max). Weed Technol. 5:834-
840.

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

Hammerton, J. L. 1967. Environmental factors and susceptibility to herbicides.
Weeds. 15:330-336.

Lee, S. D. and L. R. Oliver. 1982. Efﬁcacy of aciﬂuorfen on broadleaf weeds.
Times and methods for application. Weed Sci. 30: 520-526.

Kells, J. J ., W. F. Meggitt, and D. Penner. 1984. Absorption, translocation,
and activity of ﬂuazifop-butyl as inﬂuenced by plant growth stage and
environment. Weed Sci. 32:143-149.

King, C. A. and L. R. Oliver. 1992. Application rate and timing of aciﬂuorfen,
bentazon, chlorirnuron, and imazaquin. Weed Technol. 6:526-534.

Lich, J. M. 1995. Interaction of glyphosate with postemergence soybean
herbicides. M. S. Thesis. Michigan State University. p 80-81.

Mickelson, J. A. and K. A. Renner. 1996. Reduced rates of postemergence
herbicides for drilled and row soybeans. Abstr. Weed Sci. Soc. Amer. 36:11.

Ritter, R. L. and H. D. Coble. 1981. Inﬂuence of temperature and relative
humidity on the activity of aciﬂuorfen. Weed Sci. 29:480-485.

40

41

13. Steckel, L. E., M. S. Defelice, and B. D. Sims. 1990. Integrating reduced rates
of postemergence herbicides and cultivation for broadleaf weed control in
soybeans (Glycine max). Weed Sci. 38:541-545.

42

-0. clethodim -o- ﬂuazifop-P&fenoxaprop ﬁ— quizalofop

 

 

 

 

 

 

100
ns
- 142/70
80 j///
-§
4 6°‘
4::
g 40
4s 20 .
Giant Foxtail
0 ﬂ* ; 4 ﬂ ; ;
0 0.04 0.06 0.08 0.1 0.12

Herbicide Rate (x of recommended ﬁeld rate)

Figure 1. Reduction in giant foxtail dry weight 14 days after postemergence herbicide
application. Means denoted by the same letter are not signiﬁcantly different according to
Duncan’s Multiple Range Test. Mean comparisons are valid only within a herbicide rate.

43

o 4 clethodim o ﬂuazifop-P&fenoxaprop + quizalofop

 

100

4:7 ii”

80-

% Growth Reduction
8

Barnyardgrass

 

 

 

 

0 0.02 0.04 0.06 0.08 0.1 0.12

Herbicide Rate (x of recommended ﬁeld rate)

Figure 2. Reduction in barnyardgrass dry weight 14 days after postemergence
herbicide application. Means denoted by the same letter are not signiﬁcantly different
according to Duncan’s Multiple Range Test. Mean comparisons are valid only within a

herbicide rate.

44

 

4+ lactofen -o- fomesafen _.- aciﬂuorfen
100 + berrazon 4.— thifemulfuron -o- chlon'mmon
3 _ns‘
a g ‘ fjg_*#_ _._ -
a/‘oZ-g “we“: : LT“. - - — -;,,,4
m‘ / b I b ’E’t‘i’ A” --/ ,n
a a DC Hr r f, / 7,,
.S / .. J a /z:' ., // /
g 60- a /fb ,_,..;;-7’/‘ Cd ,—-
B c/ /,/ g ,/
M 1’,’ {CI// d
4 4; /~--::-.—.
('2 bc% '
e 20‘ c c
Common Ragweed
O r r r I 1 I ' 1 r

 

 

 

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

Herbicide Rate (x of recommended ﬁeld rate)

Figure 3. Reduction in common ragweed dry weight 21 days after postemergence
herbicide application. Means denoted by the same letter are not signiﬁcantly different
according to Duncan’s Multiple Range Test. Mean comparisons are valid only within a

herbicide rate.

45

 

1(X)

 

% Growth Reduction

 

Velvetleaf

 

 

r I l I I T

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

Herbicide Rate (x of recommended ﬁeld rate)

Figure 4. Reduction in velvetleaf dry weight 14 days after postemergence herbicide
application. Means denoted by the same letter are not signiﬁcantly different according
to Duncan’s Multiple Range Test. Mean comparisons are valid only within a herbicide

rate .

WEED CONTROL USING REDUCED RATES OF
POSTEMERGENCE HERBICIDES IN NARROW
AND WIDE ROW SOYBEANS

Abstract. Field studies were conducted in 1994 and 1995 to examine the effects of
soybean row spacing and application rate and timing of four postemergence herbicide
tank mixtures on weed control and soybean yield. Weed control and soybean yield
were greater in narrow rows (19 cm) than wide rows (76 cm). Herbicide tank mixtures
applied at one quarter of the full recommended rate at an early postemergence timing
followed by a second application of one quarter of the full rate applied at a standard
postemergence timing (1/4x E Post + 1/4x Post) resulted in weed control and soybean
yield equal to that of herbicide tank mixtures applied at the full recommended rate at a
standard postemergence timing ( 1x Post). Three of four tank mixtures in 1994 and two
of four in 1995, applied at one half of the full rate applied at a standard postemergence
timing (1/2x Post) resulted in weed control and soybean yield equal to that of 1x Post
applications. All tank mixtures applied at one half of the full rate applied at an early
postemergence timing (1/2x E Post) resulted in poor weed control and low soybean
yield. In most cases it was more proﬁtable to plant soybeans in narrow rows than wide
rows regardless of application rate or timing, based on economic gross margin
calculations. Gross margins of tank mixtures applied at 1/4x E Post + 1/4x Post were
similar to or greater than the gross margin of the same tank mixture applied at the full
rate in thirteen of sixteen cases. Gross margins of tank mixtures applied at 1/2x Post
were similar to or greater than the gross margin of the same tank mixture applied at the

full rate in eight of sixteen cases.

46

47

INTRODUCTION

Weed competition can greatly reduce soybean (Glycine max (L.) Merr.) yield
(8, 15, 19, 28, 30, 34). To attain maximum yields weeds must be controlled. Cultural,
mechanical, and chemical means used to control weeds can make up a signiﬁcant
portion of the input costs associated with producing soybeans. Low proﬁt margins
associated with producing soybeans have prompted growers to look for ways to
decrease their input costs, while maintaining high yields in order to maximize
proﬁtability. The use of herbicides as the principle method of weed control has
contributed to increased input costs associated with producing soybeans. It has also
resulted in increased concerns of the environmental impacts of herbicide use. Weed
control programs that use reduced rates of postemergence herbicides can potentially
lower producer input costs while maintaining adequate weed control. However,
growers must be willing to spend more time and management efforts towards weed
control and be willing to accept the risk of using below label rates.

The efﬁcacy of postemergence herbicides is affected by many factors including
environmental conditions and weed size. Soil moisture, air temperature, relative
humidity and light intensity are some of the environmental conditions which can vary
greatly and inﬂuence the efﬁcacy of postemergence herbicides (6, 14, 18, 20, 26, 41).
Poor weed control may result if reduced rates are applied under non-ideal growing
conditions. DeFelice et al. (10) reported unacceptable weed control with full and

reduced rates of postemergence herbicides when conditions at the time of herbicide

48

application were hot and dry.

Tank mixing postemergence herbicides allows growers to minimize the number
of trips through the ﬁeld, however, antagonistic interactions between two herbicides
can occur, resulting in reduced weed control in some cases (16, 29, 35, 40). Few
studies have evaluated weed response to tank mixtures of herbicides applied at reduced
rates.

Weed control equal to using full rates has been achieved using 25 and 50% of
the recommended rates of postemergence herbicides in conventionally tilled soybeans
(10, 11, 24, 25, 31). Only three of the ﬁfteen locations were planted to soybeans in
narrow row spacings. Most research has been conducted in wide rows, often in
combination with cultivation. Planting soybeans in narrow rows can result in greater
season long weed control than wide row soybeans due to a faster closure of the soybean
canopy which results in greater shading and weed suppression (7, 21, 23, 36). Also,
planting soybeans in narrow rows can result in higher soybean yields due to increased
light interception by the soybean canopy (4, 5, 33). Therefore, planting soybeans in
narrow rows is an option which could increase weed control in a reduced rate
postemergence herbicide program and maintain or increase soybean yields.

Information on reduced rate herbicide programs for soybean growers in the
northern production region of the US. is lacking. This research was initiated to assess
the potential of reduced rates of postemergence herbicides for weed control in this
region. Two aspects studied in this research have not been previously reported;

applying reduced rates of herbicide tank mixtures containing a grass and two broadleaf

49

herbicides in order to control a broad spectrum of weed species, and the efﬁcacy of
reduced rate treatments is directly compared between narrow and wide row soybeans.
The objectives of this research were to: 1) evaluate differences in weed control
between narrow and wide row soybeans using full and reduced postemergence
herbicide rates, 2) determine the effects of row spacing on soybean yield, and 3)
determine if planting soybeans in narrow rows can increase the effectiveness of reduced

rate postemergence herbicide programs.

MATERIALS AND METHODS

Field experiments were conducted in 1994 and 1995 at Michigan State
University, in East Lansing, MI. The soil was a Capac sandy clay loam (ﬁne-loamy,
mixed, mesic Aerie Ochraqualfs) with 3.1% organic matter and pH of 7.1 in 1994. In
1995, the soil was a Capac loam with 3.2% organic matter and pH of 6.6. The 1994
site was chisel plowed the previous fall, and disked and ﬁeld cultivated in the spring.
The 1995 site was moldboard plowed, disked, cultirnulched, and ﬁeld cultivated in the
spring.

‘Conrad’ soybeans were planted on May 13, 1994 and May 11, 1995. Soybeans
were planted in 19 cm row spacings with a Great Plains Solid Stand 101 grain drill at a

seeding rate of 470,000 seeds/ha, and in 76 cm row spacings with a John Deere 7200

 

‘Great Plains Manufacturing Inc., PO. Box 218, Assaria, KS 67416.

50

Max-Emerge'2 planter2 at a seeding rate of 358,000 seeds/ha. Preemergence herbicides
were applied with a tractor mounted compressed air plot sprayer using 8003 ﬂat fan
nozzles calibrated to deliver 206 L/ha at a pressure of 207 kPa. Postemergence
herbicides were applied using 8002 ﬂat fan nozzles calibrated to deliver 140 L/ha at a
pressure of 207 kPa.

Plots were 3 m wide by 12 m long in 1994 and 3 m wide by 10.7 m long in
1995. The experimental design was a split plot with four replications. Main plots were
narrow or wide rows. Subplots were a factorial arrangement of four herbicide tank
mixtures applied at four application rate/ timing combinations. Additional treatments
included a weedy control, weedfree control, standard preemergence herbicide
treatment, and mechanical control only. The preemergence treatment contained
metolachlor applied at 2240 g ai/ha + linuron at 560 g ai/ha + metribuzin at 280 g
ai/ha. The mechanical treatment consisted of two rotary hoes in narrow row soybeans,
and two rotary hoes followed by two cultivations in wide rows.

Weeds were counted the day before early postemergence herbicide application.
Weed densities were: 125 and 527 giant foxtail plants/m2, 274 and 40 common
lambsquarters plants/m2, 152 and 17 redroot pigweed plants/m2, 182 and 7 common
ragweed plants/m2, and 0 and 17 velvetleaf plants/m2 in 1994 and 1995, respectively.
SOYHERB (27) and WEEDSIM (32) computer modeling programs were used to select
four tank mixtures based on weed densities and herbicide cost. The four herbicide tank

mixtures and full recommend rates selected were: 1) clethodim at 105 g ai/ha +

 

2Deere and Co., Moline, IL 61265-1304.

51

thifensulfuron at 4.4 g ai/ha + lactofen at 105 g ai/ha + nonionic surfactant3 (NIS) at
0.125% WV, 2) the formulated premix of ﬂuazifop-P&fenoxaprop at 186 g ai/ha (140 g
+ 46 g) + thifensulfuron at 4.4 g/ha + fomesafen at 280 g ai/ha + 28% urea
ammonium nitrate (UAN) at 2.3 L/ha + NIS at 0.125% WV, 3) quizalofop at 49 g
ai/ha + a formulated premix of bentazon&aciﬂuorfen at 1032 g ai/ha (841 g + 191 g)
+ crop oil concentrate4 at 2.3 L/ha, and 4) quizalofop at 49 g/ha + thifensulfuron at
4.4 g/ha + chlorirnuron at 5.8 g ai/ha + UAN at 4.6 L/ha + NIS at 0.125% v/v.
Each herbicide tank mixture was applied at four rate/timing combinations: 1) one
quarter of the full recommended rate applied at an early postemergence timing followed
by a second application of one quarter of the full rate applied at a standard
postemergence timing (1/4x E Post + 1/4x Post), 2) one half of the full rate applied at
an early postemergence timing (1/2x E Post), 3) one half of the full rate applied at a
standard postemergence timing (1/2x Post), and 4) the full recommended rate applied at
a standard postemergence timing (1x Post). The early postemergence application timing
was targeted when weeds were 2.5 cm tall. The standard postemergence timing was
targeted when weeds were 10 cm tall.

Early postemergence treatments were applied on May 28, 1994 and May 26,
1995 ( 15 days after planting). Air temperature was 20 C with 48% relative humidity in

1994, and 18 C with 64% relative humidity in 1995. Rainfall in the 10 days prior to

 

3Activator-90, a mixture of alkyl polyoxyethylene ether and free fatty acids. Loveland
Industries, Inc., PO. Box 1289, Greeley, CO 80632.

4Herbirnax, 83% petroleum oil, 17% surfactant, Loveland Industries, Inc., PO. Box
1289, Greeley, CO 80632.

52

application was 0.6 cm in 1994 and 3.4 cm in 1995. Standard postemergence
treatments were applied on June 9 in 1994 and 1995 (27 and 29 days after planting).
Air temperature was 24 C with 37% relative humidity in 1994 and 22 C with 65%
relative humidity in 1995. Rainfall in the 10 days prior to application was 1.0 cm in
1994 and 0.8 cm in 1995. Soybeans were at the cotyledon to unifoliate stage at the
early postemergence application and 1 to 2 trifoliate stage at the standard
postemergence application. Weed heights at herbicide application are presented in
Table 1.

Injury to giant foxtail, common lambsquarters, redroot pigweed, common
ragweed, and velvetleaf was evaluated visually at 10, 24, 38, and 52 days after
postemergence application. The rating scale ranged from 0 (no visual injury) to 100%
(complete plant death). Crop response was evaluated visually at 3, 10, 24, and 38 days
after postemergence application. Injury ratings were based on plant stunting, chlorosis,
and necrosis. Weed biomass was sampled on July 27, 1994 and July 31, 1995 by
harvesting all aboveground portions of each weed species from a 25 by 91 cm
rectangular area arranged perpendicular to the soybean row. Weeds were harvested and
counted by species from two areas per plot, oven dried, and weighed. Sclerotinia stem
rot, or white mold, (Sclerotinia sclerotiorum var. sojae) was measured September 9,
1994 by scoring 30 plants per plot on a 0 to 3 rating scale where 0 equals no symptoms
and 3 equals lesions on the main stem resulting in plant death and poor pod ﬁll.

Soybeans were harvested for seed from the center 1.5 m strip of each plot on October

53

22, 1994 and October 13, 1995 with a Massey 105 plot harvesting combine. Seed yields
were adjusted to 13.0% moisture. In 1994 disease severity index (DSI)‘5 for Sclerotinia
stem rot was calculated and analysis of covariance was used, with DSI as the covariate,
to adjust soybean yield. An economic analysis determined the gross margin of each
treatment by subtracting the variable input costs from the gross income. Gross income
was calculated by multiplying the soybean seed yield by an assumed market price of
$0.20/kg. Variable input costs7 assumed were: $12.36/ha per herbicide application;
$21.15/ha for seeding soybeans in wide rows; $27.72/ha for seeding soybeans in
narrow rows; $60.39/ha for the 1x rate of clethodirn+ thifensulfuron+ lactofen;
$70.79/ha for the 1x rate of ﬂuazifop-P&fenoxaprop+ thifensulfuron-i- fomesafen;
$57.25/ha for the 1x rate of quizalofop+ bentazon&aciﬂuorfen; $57.43 for the 1x rate
of quizalofop+ thifensulfuron+ Chlorimuron; $90.44/ha for metolachlor+ linuron+
metribuzin, $8.60/ha for a cultivation, and $3.61/ha for a rotary hoe operation.
Analysis of variance revealed signiﬁcant year by treatment interactions for
visual weed injury ratings, soybean injury ratings, weed biomass, and soybean yield,
so data is presented separately for 1994 and 1995. Means were separated using
Duncan’s Multiple Range Test at the 5% level of signiﬁcance. Results and discussion

of visual weed injury are based on the rating taken 38 days after standard

 

5Kincaid Equipment Manufacturing, PO. Box 400, Haven, KS 67543.

6DSI was calculated by summing the scoring values for each of the 30 plants and
dividing by 90, the highest possible total score.

7Herbicide prices according to SOYHERB and cultivation and rotary hoe costs
according to Doane’s Agricultural Reports, 1995. v. 58. No.15-5.

54

postemergence application (52 days after early postemergence application). Results and
discussion of soybean injury are based on the rating taken 10 days after early
postemergence application for 1/4x E Post + 1/4x Post and 1/2x E Post, and 10 days

after standard postemergence application for 1/2x Post and 1x Post.

RESULTS AND DISCUSSION

Annual grass. In 1994 the row spacing by herbicide tank mixture interaction was
signiﬁcant (P< 0.05). Control of annual grass (predominantly giant foxtail) was similar
with all herbicide tank mixtures in narrow row soybeans, when averaged over
rate/timing combinations (Table 2). In wide rows, control of annual grass with
quizalofop + bentazon&aciﬂuorfen was greater than with clethodim+ thifensulfuron+
lactofen or with ﬂuazifop-P&fenoxaprop+ thifensulfuron+ fomesafen. Crop oil
concentrate was included in the quizalofop + bentazon&aciﬂuorfen tank mixture, while
nonionic surfactant was included in other tank mixtures. Crop oil concentrate has been
reported to be a more effective adjuvant than nonionic surfactant for grass control with
postemergence soybean herbicides (3).

In 1994 and 1995 the herbicide tank mixture by rate/timing interaction was
signiﬁcant (P< 0.05). In 1994, all herbicide tank mixtures applied at the full
recommended rate (1x Post) provided 92% or greater control of annual grass, averaged
over row spacing (Table 3). Grass control with all four herbicide tank mixtures applied

at 1/2x Post and with three of the four herbicide tank mixtures applied at 1/4x E Post

55

+1/4x Post was equal to 1x Post applications. Grass control with the quizalofop+
thifensulfuron+ chlorirnuron tank mixture applied at 1/4x E Post + 1/4x Post was not
equal to 1x Post applications, but was similar to other tank mixtures applied at 1/4x E
Post + 1/4x Post. Barnyardgrass was present in small patches throughout the ﬁeld and
may be the reason for poor grass control with this treatment. Quizalofop was less
effective on barnyardgrass than other grass herbicides in greenhouse experiments. In
1995, three of the four tank mixtures applied at 1/4x E Post + 1/4x Post, 1/2x Post, or
1x Post provided 90% or greater control of annual grass (Table 3). Grass control with
clethodim+ thifensulfuron+ lactofen applied at 1x Post only reached 71% control.
Reduced control with this tank mixture may have been caused by an antagonistic
interaction between clethodim and lactofen. Vidrine et al. (35) reported an antagonistic
response in barnyardgrass and johnsongrass when lactofen was tank mixed with
clethodim.

Annual grass was not controlled by any herbicide tank mixture applied at 1/2x
E Post in 1994 or 1995. Many grass plants emerged after the early postemergence
herbicide application (personal observation). Because the postemergence herbicides
applied have minimal soil activity, late emerging grasses were not controlled and
resulted in poor season long weed control. DeFelice et al. (10) also reported reduced
control of giant foxtail with l/4x or 1/2x rates applied at an early application timing (7
days after planting) compared to sequential or full rate applications at a later
application timing.

Common lambsquarters. Row spacing by herbicide tank mixture, row spacing by

56

rate/timing, and herbicide tank mixture by rate/timing interactions were signiﬁcant in
both 1994 and 1995.

Control of common lambsquarters from clethodirn+ thifensulfuron+ lactofen
and ﬂuazifop-P&fenoxaprop + thifensulfuron+ fomesafen applied in wide row
soybeans was less than in narrow row soybeans in 1994 (Table 4). In 1995, common
lambsquarters control with clethodim+ thifensulfuron+ lactofen was similar between
row spacings, yet control in wide row soybeans was less than in narrow row soybeans
with the other three tank mixtures, when averaged over all rate/timing combinations.

In 1994, all four rate/timing combinations when averaged over herbicide tank
mixtures provided equal control of common lambsquarters when applied to narrow row
soybeans (Table 5). Control with herbicide tank mixtures applied at 1/2x E Post was
similar in wide and narrow rows (93 and 95 %), but applications of 1/4x E Post + 1/4x
Post, 1/2x Post, and 1x Post to wide rows resulted in reduced control compared to
narrow rows. In 1995, herbicides applied at 1/2x Post or 1x Post resulted in reduced
control in wide rows compared to narrow rows. Very few common lambsquarters
emerged after the E Post application (personal observation). Therefore control was
greater when herbicides were applied at E Post because common lambsquarters were
smaller.

Averaged over row spacings in 1994, all rate/timing combinations of
quizalofop+ bentazon&aciﬂuorfen or quizalofop+ thifensulfuron+ chlorirnuron
provided 93 % or greater control of common lambsquarters (Table 6). Less control

occurred with clethodim+ thifensulfuron+ lactofen at 1/4x E Post + 1/4x Post or

57

ﬂuazifop—P&fenoxaprop+ thifensulfuron+ fomesafen at 1/2x Post or 1x Post. In 1995,
three of four tank mixtures applied at 1/4x E Post + 1/4x Post or 1/2x E Post provided
92% or greater control of common lambsquarters, averaged over row spacing. Control
of common lambsquarters with 1/2x Post or 1x Post applications ranged from 61% to
87 %.

Common lambsquarters is very susceptible to thifensulfuron (2), which was a
component in three of the four tank mixtures. In most cases these tank mixtures
adequately controlled common lambsquarters, however in some cases when
thifensulfuron was tank mixed with lactofen or fomesafen control was less than when
tank mixed with chlorirnuron. This may be evidence of antagonism which can occur
when a diphcnylether herbicide is tank mixed with a sulfonylurea herbicide (38, 39,
40). The tank mixture containing bentazon and aciﬂuorfen also controlled common
lambsquarters at reduced rates in most cases. Bentazon and aciﬂuorfen both have
activity on common lambsquarters. Very few common lambsquarters emerged after the
early postemergence herbicide application (personal observation), therefore control
with herbicides applied at 1/4x E Post + 1/4x Post or 1/2x E Post was equal to or
greater than herbicides applied at 1/2x Post or 1x Post.

Redroot Pigweed. In 1994 the row spacing by herbicide tank mixture by rate/timing
interaction was signiﬁcant. Redroot pigweed was controlled by all rate/timing
combinations in narrow and wide rows except with quizalofop + bentazon&aciﬂuorfen
(Table 7). Control with 1/2x Post or 1x Post applications of this tank mixture were

greater in narrow rows, 84% and 87%, respectively, than in wide rows, 76% and

58

54%, respectively. In 1995, quizalofop+ bentazon&aciﬂuorfen also provided less
control in wide rows compared to narrow rows (Table 8). Applications of quizalofop+
bentazon&aciﬂuorfen at l/4x E Post + 1/4x Post or 1/2x E Post resulted in 82 and
81% control and applications at 1/2x Post or 1x Post resulted in 61% control (Table 9).
When herbicide tank mixtures contained two broadleaf herbicides in which both
have activity on redroot pigweed (thifensulfuron plus lactofen, fomesafen, or
chlorirnuron) control was excellent, regardless of row spacing or rate/timing. The tank
mixture containing bentazon and aciﬂuorfen did not control redroot pigweed. Bentazon
does not control redroot pigweed (1), and apparently the rate of aciﬂuorfen contained
in the formulated premix (191 g/ha) did not provide adequate control.
Common ragweed. In 1994 the row spacing main effect and herbicide tank mixture by
rate/timing interaction were signiﬁcant. Common ragweed control was 82% in narrow
rows and 67% in wide rows, averaged over herbicide tank mixtures and rate/timing
combinations. Fluazifop-P&fenoxaprop+ thifensulfuron+ fomesafen and quizalofop+
thifensulfuron+ chlorirnuron applied at 1/4x E Post + 1/4x Post and 1x Post provided
greater than 90% control, averaged over row spacings (Table 10). With all four
herbicide tank mixtures, applications at 1/4x E Post + 1/4x Post were equal to or
greater than 1x Post applications and were greater than 1/2x E Post or 1/2x Post
applications. Prostko and Meade (24) also reported greater control of common ragweed
with 1/4x + 1/4x sequential applications than with single 1/2x applications. Control
with l/2x E Post applications was less than 1/2x Post applications with three of four

tank mixtures, due to later emerging common ragweed, and not due to regrowth

59

(personal observation).

In 1995, control of common ragweed with three of the four herbicide tank
mixtures applied at 1/4x E Post + 1/4x Post, 1/2x Post, or 1x Post was similar
between row spacings, and ranged from 71% to 98% (Table 13). Control with
quizalofop + thifensulfuron+ Chlorimuron was similar between row spacings when
applied at 1/2x Post or 1x Post but was reduced in wide rows when applied at 1/4x E
Post + l/4x Post.

Differences in common ragweed control between years may be due to the
difference in common ragweed densities. Common ragweed pressure was very high in
1994 (182 plants/m2), however, in 1995 the common ragweed population was low (7
plants/m2). When common ragweed was dense, narrow row soybeans improved
control, regardless of herbicide tank mixture, and a split application (1/4x E Post +
1/4x Post) was most effective. The most effective herbicide tank mixtures were
ﬂuazifop-P&fenoxaprop+ thifensulfuron+ fomesafen and quizalofop+
thifensulfuron + chlorirnuron. Greenhouse rate response studies indicated fomesafen
was one of the most effective herbicides applied at reduced rates for common ragweed
control.

Velvetleaf. Fluazifop-P&fenoxaprop + thifensulfuron+ fomesafen and quizalofop +
thifensulfuron+ Chlorimuron, provided greater velvetleaf control in narrow rows
compared to wide rows, when averaged over rate/timing combinations (Table 12).
Quizalofop+ bentazon&aciﬂuorfen and. quizalofop+ thifensulfuron+ Chlorimuron,

applied at 1x Post controlled velvetleaf, 89% and 97% , respectively, averaged over

60

row spacing (Table 13). Applications of quizalofop+ thifensulfuron+ Chlorimuron at
1/4x E Post + 1/4x Post or 1/2x Post were the only reduced rate treatments which
provided greater than 80% control of velvetleaf, when averaged over row spacing.
Thifensulfuron and chlorirnuron were two of the most effective herbicides at reduced
rates in greenhouse experiments. Control with 1/2x E Post applications was less than
1/4x E Post + 1/4x Post or 1/2x Post applications with all tank mixtures, due to late
emerging velvetleaf. Other researchers have reported reduced control of velvetleaf with
early postemergence applications of reduced rates (10, 31).

Weed Biomass. Data analysis revealed no interactions with row spacing. Planting
soybeans in narrow rows resulted in 30% less total weed biomass (all species
combined) compared to soybeans planted in wide rows. Complete canopy closure
occurred much earlier in narrow row soybeans, approximately 50 days after planting,
compared to 95 days after planting for wide rows. Researchers in Nebraska and Illinois
have reported canopy closure of soybeans planted in 25 cm rows by 35 to 36 days after
planting and in 76 cm rows by 58 to 65 days after planting (7, 36). Slower canopy
closure in Michigan regardless of row spacing, can probably be attributed to
differences in geography, enviromnental conditions, and soybean cultivars.

In 1994, no differences in weed biomass could be detected between tank
mixtures applied at 1x Post (Table 14). In 1995, ﬂuazifop-P&fenoxaprop+
thifensulfuron+ fomesafen, and quizalofop + thifensulfuron+ Chlorimuron applied at
1x Post resulted in less weed biomass than clethodim+ thifensulfuron+ lactofen

applied at 1x Post. In 1994 and 1995, no difference in weed biomass could be detected

61

between tank mixtures applied at l/4x E Post +1/4x Post or 1/2x Post and the same
tank mixture applied at 1x Post, except for clethodim+ thifensulfuron+ lactofen
applied at 1/2x Post in 1995, due to poor annual grass control. All tank mixures
applied at 1/2x E Post resulted in greater weed biomass than at 1x Post, due mainly to
late emerging annual grass, common ragweed, and velvetleaf.

Soybean Injury. Clethodim+ thifensulfuron+ lactofen caused the greatest soybean
injury of the four herbicide tank mixtures 10 days after herbicide application (Table
15). When this tank mixture or quizalofop+ bentazon&aciﬂuorfen, were applied at the
early postemergence timing (1/4x E Post + 1/4x Post and 1/2x E Post), in most cases,
greater soybean injury occurred than when applied at the standard postemergence
timing (1/2x Post and 1x Post). Smaller, younger weeds are more susceptible to
postememergence herbicides than larger, more mature weeds (20, 26). Soybeans which
were in the cotyledon to unifoliate stage at the early postemergence application may
have been more susceptible to injury than soybeans in the 1 to 2 trifoliate stage at the
standard postemergence application. Soybeans treated with ﬂuazifop-P&fenoxaprop +
thifensulfuron+ fomesafen responded similarly regardless of application rate/timing
except at 1/2x Post in 1995 which was greater than other application rate/timing
combinations. Quizalofop+ thifensulfuron+ Chlorimuron applied at the standard
postmergence timing resulted in greater soybean injury than at the early postemergence
timing. By 38 days after herbicide application treatments ranged from 0 to 5% injury in
1994 and 0 to 3% injury in 1995 (data not presented).

Sclerotinia Stem Rot. There was no difference in Sclerotinia stem rot infection of

62

soybeans between row spacings or herbicide tank mixtures. All postemergence
herbicide treatments resulted in greater infection (50 to 75 %) than the untreated control
(24%). Soybeans applied with a herbicide tank mixture at 1/4x E Post + 1/4x Post
resulted in greater infection (71%) than soybeans applied with herbicides at l/2x Post
(58%) or 1x Post (57%), averaged over row spacing and rate/timing combinations. All
yield data for 1994 has been adjusted using analysis of covariance, with DSI as the
covariate. Yield adjustments ranged from O to 256 kg/ha.
Soybean Yield. Planting soybeans in narrow rows resulted in 14% greater soybean
yield than soybeans planted in wide rows, averaged over the sixteen postemergence
herbicide treatments and two years. There was no difference in soybean yield between
row spacings in the weed free control. This suggests the yield advantage in narrow row
soybeans was due mainly to improved weed control in narrow rows. Soybean lodging
occurred in 1994 and 1995 (personal observation) and may be the reason no yield
advantage was seen in narrow row soybeans in the weed free control. Lehman and
Lambert (22) reported greater yield in a weed free environment in narrow rows with a
short growing cultivar, but no difference in soybean yield between row spacing with a
tall growing cultivar which lodged. Soybeans planted in narrow rows at high
populations can result in increased plant height which can increase lodging of soybeans
(13, 37). This can counteract the yield advantage of narrow rows.

The year by herbicide tank mixture by rate/timing interaction was signiﬁcant.
Therefore, treatment data is presented separately for each year and each of the sixteen

postemergence herbicide treatments, but averaged over row spacing. In 1994, no

63

signiﬁcant difference in soybean yield could be detected between tank mixtures applied
at 1x Post (Table 16). In 1995 the highest soybean yield, 4433 kg/ha, was achieved by
applying quizalofop+ thifensulfuron+ chlorirnuron at 1x Post. Soybeans applied with
1x Post rates of quizalofop+ bentazon&aciﬂuorfen or ﬂuazifop-P&fenoxaprop +
thifensulfuron+ fomesafen yielded less (3940 and 3922 kg/ha). Soybeans applied with
clethodim+ thifensulfuron+ lactofen yielded the lowest (3450 kg/ha) of all tank
mixtures applied at 1x Post. Soybean yield was related to weed biomass.

In 1994 and 1995, all four tank mixtures applied at 1/4x E Post + 1/4x Post
resulted in soybean yield similar to the same tank mixture applied at 1x Post. In 1994
three of four tank mixtures applied at 1/2x Post resulted in soybean yield similar to the
same tank mixture applied at 1x Post. Soybeans applied with clethodim+
thifensulfuron+ lactofen at 1/2x Post yielded less than when applied at 1x Post. Low
soybean yield was a result of high weed biomass. In 1995 two of four tank mixtures,
quizalofop + thifensulfuron+ chlorimuron and ﬂuazifop-P&fenoxaprop +
thifensulfuron+ fomesafen, applied at 1/2x Post resulted in soybean yield similar to
soybeans applied with these tank mixtures at 1x Post.

High weed biomass levels present in plots applied at 1/2x E Post resulted in low
soybean yields with all four herbicide tank mixtures in both years. Weed control was
poor due to weeds which emerged after herbicide application and resulted in severe
weed competition.

Economic Analysis. Twelve of the sixteen postemergence herbicide treatments in 1994

and 14 of 16 treatments in 1995 had higher gross margins (25% higher) in narrow row

64

soybeans than wide rows (Table 17). All tank mixtures applied at 1/2x E Post resulted
in low gross margins. Of the 16 reduced rate treatments which were applied at either
1/4x E Post + 1/4x Post or 1/2x Post in wide row soybeans in 1994 and 1995, 3
treatments resulted in higher gross margins (greater than 5%), 8 treatments resulted in
similar gross margins (within 5%), and 5 treatments resulted in lower gross margins
(less than 5%) than the gross margin from the full rate treatment (1x Post) of the same
tank mixture. Of the 16 reduced rate treatments applied at either 1/4x E Post + 1/4x
Post or 1/2x Post in narrow row soybeans in 1994 and 1995, 3 treatments resulted in
higher gross margins, 7 treatments resulted in similar gross margins, and 6 treatments
resulted in lower gross margins than the gross margin from the full rate treatment of
the same tank mixture. Gross margins from full rate treatments varied with herbicide
tank mixture.

The results of this study show that adequate weed control can be achieved using
reduced rates of postemergence herbicide tank mixtures in Michigan. Generally,
herbicides applied at 1/4x E Post + 1/4x Post provided more consistent weed control
than single applications of 1/2x rates, as was previously reported by other researchers
(11, 24, 31). Herbicides applied at 1/2x E Post did not provide season long control of
annual grass, common ragweed, or velvetleaf, because of weed emergence after the
herbicide application. Other researchers have reported reduced weed control when
herbicides were applied at an early application timing (10, 31) or if cool damp soil
conditions delayed weed emergence (11). However some studies have shown that

applying a reduced rate at an early application timing provided weed control equal to a

65

full rate application at a standard timing (11).

Greater weed control was achieved in narrow rows compared to wide rows with
full and reduced rate treatments. The soybean canopy closed much earlier in the season
with narrow rows. This improved weed control by suppressing late emerging weeds
and weeds which were not completely killed by a postemergence herbicide application.
DeFelice et al. (10) commented that better weed control was observed with reduced
rates in drilled soybean studies than in wide row studies. Research has shown that a
timely cultivation can improve weed control with reduced rates (12, 31). A cultivation
probably would have improved weed control in wide row soybeans, however by
planting soybeans in narrow rows, improved weed control was achieved without
requiring the added time and expense of a cultivation. Results from this study also
indicate that a total postemergence herbicide program, using tank mixtures of grass and
broadleaf herbicides, can be used at reduced rates to control a broadspectrum of weeds.
However, careful selection of herbicides must be exercised, in order to avoid
antagonistic interactions between herbicides. The yield advantage observed in narrow
row soybeans can probably be attributed mainly to improved weed control, because no
difference in soybean yield was observed between row spacings in the weed free
control. Even though the potential for higher yields in narrow row soybeans exists (4,
5, 33), often yield enhancement in narrow row soybeans does not occurr in weed free
environments (9, 13, 17, 22). Soybeans treated with a split herbicide application (1/4x
E Post + 1/4x Post) consistently resulted in soybean yields equal to that of full rate

treatments. Other researchers report similar results (10, 11). Applying herbicides in a

66

split application can allow growers to reduce their herbicide input costs and
consistently maintain soybean yield, however the additional time required for the
second application is a drawback.

Calculation of gross margins showed that when using full or reduce rates, in
most cases it was more proﬁtable to plant soybeans in narrow rows. In some cases it
was more proﬁtable to use a reduced rate of a herbicide tank mixture compared to a
full rate of the same tank mixture, justifying the added risk involved. In other cases
gross margins with reduced rates were similar or less than using full rates, indicating
the added risk may not be justiﬁed. However it is difﬁcult to conﬁdently judge
proﬁtability based on a direct comparison of gross margins because the gross margins
were highly inﬂuenced by soybean yield and do not account for the experimental error
associated with the soybean yield data, and because soybean prices and input costs can
vary greatly. Perhaps the best assessment of proﬁtability is to simply compare input

costs of full and reduced rate treatments if soybean yields do not signiﬁcantly differ.

10.

11.

12.

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71

Table 1. Height of weed species at the time of herbicide application.

 

 

 

 

 

 

Weed species Weed height
Early Posta Post
1994 1995 1994 1995
cm

Giant foxtail 3 2 10-15 10-15
Common lambsquarters 3 2 5-10 4-6
Redroot pigweed 3 2 5-10 4-6
Common ragweed 3 2 7-10 4-8
Velvetleaf - 2 - 3-6

 

aEarly Post = early postemergence herbicide application, applied May 28, 1994 and
May 26, 1995. Post = standard postemergence herbicide application, applied June 9,
1994 and 1995.

72

Table 2. Inﬂuence of herbicide tank mixture and soybean row spacing on annual grass
control in 1994‘.

 

 

 

Herbicide tank mixture Annual grass controlz-3

Wide Narrow
%

Clethodirn + thifensulfuron + lactofen + NIS 66 c 73 abc

Fluazifop-P&fenoxaprop + thifensulfuron + 68 bc 77 ab

fomesafen + UAN +NIS

Quizalofop + bentazon & aciﬂuorfen + COC 78 a 69 abc

Quizalofop + thifensulfuron + chlorirnuron + 70 abc 76 ab

UAN + NIS

 

1Means averaged over rate/timing combinations. Means followed by the same letter are
not signiﬁcantly different according to Duncan’s Multiple Range Test. Mean
comparisons are valid within and across columns.

2Visual injury ratings were made 38 days after the standard postemergence herbicide
application timing.

3Wide = soybeans planted in 76 cm rows, Narrow = soybeans planted in 19 cm rows.

73

Table 3. Control of annual grass in soybeans with reduced rates of postemergence

herbicides in 1994 and 1995‘.

 

 

 

 

 

 

 

Herbicide tank mixture Rate/ timing Annual grass control2

1994 1995
%

Clethodirn + thifensulfuron 1/4x+1/4x 88 ab 70 b
+ laC‘Ofe“ + NIS 1/2x E Post 3 d 11 d
1/2x Post 95 a 39 c
1x Post 92 a 71 b
Fluazifop-P&fenoxaprop + l/4x+ 1/4x 91 ab 93 a
Eiﬁfﬁfirg?s+ fomesafen l/2x E Post 19 c 32 c
1/2x Post 87 ab 95 a
1x Post 95 a 94 a
Quizalofop + bentazon & 1/4x+ 1/4x 86 ab 95 a
aCiﬂuorfe“ + CDC 1/2x E Post 24 c 20 d
1/2x Post 92 a 97 a
1x Post 93 a 99 a
Quizalofop + thifensulfuron 1/4x+ 1/4x 78 b 96 a
glghlorimum“ + UAN + l/2x E Post 28 c 10 d
1/2x Post 91 a 90 a
1x Post 94 a 96 a

 

lMeans averaged over row spacing. Means followed by the same letter are not

signiﬁcantly different according to Duncan’s Multiple Range Test. Mean comparisons

are valid only within years.

2Visual injury ratings were made 38 days after the standard postemergence herbicide

application timing.

74

Table 4. Inﬂuence of herbicide tank mixture and soybean row spacing on common
lambsquarters control in 1994 and 1995‘.

 

 

 

Herbicide tank mixture Common lambsquarters controlz-3
1994 1995
Wide Narrow Wide Narrow
%

 

 

Clethodirn + thifensulfuron + lactofen + 86 b 95 a 81 cde 80 cde
NIS

Fluazifop-P&fenoxaprop + thifensulfuron 85 b 94 a 78 de 89 ab
+fomesafen + UAN +NIS

Quizalofop + bentazon & aciﬂuorfen + 94 a 95 a 73 e 87 bc
COC

Quizalofop + thifensulfuron + 95 a 95 a 83 bed 96 a
chlorirnuron +UAN + NIS

 

lMeans averaged over rate/timing combinations. Means followed by the same letter are
not signiﬁcantly different according to Duncan’s Multiple Range Test. Mean
comparisons are valid within and across columns within a year. Comparisons are not
valid between years.

2Visual injury ratings were made 38 days after the standard postemergence herbicide
application timing.

3Wide = soybeans planted in 76 cm rows, Narrow = soybeans planted in 19 cm rows.

75

Table 5. Inﬂuence of application rate and timing, and soybean row spacing on
common lambsquarters control in 1994 and 1995‘.

 

 

 

 

Rate/timing Common lambsquarters controlz-3

1994 1995

Wide Narrow Wide Narrow
%

1/4x + 1/4x 90 b 95 a 88 ab 94 a
1/2x E Post 93 a 95 a 89 a 90 a
1/2x Post 89 b 95 a 68 c 81 b
1x Post 86 c 94 a 70 c 87 ab

 

lMeans averaged over herbicide tank mixtures. Means followed by the same letter are
not signiﬁcantly different according to Duncan’s Multiple Range Test. Mean
comparisons are valid within and across columns within a year. Comparisons are not

valid between years.

2Visual injury ratings were made 38 days after the standard postemergence herbicide

application timing.

3Wide = soybeans planted in 76 cm rows, Narrow = soybeans planted in 19 cm rows.

76

Table 6. Control of common lambsquarters in soybeans with reduced rates of
postemergence herbicides in 1994 and 1995‘.

 

 

 

 

 

 

Herbicide tank mixture Rate/timing Common lambsquarters control2
1994 1995
%
Clethodirn + thifensulfuron 1/4x+1/4x 88 cd 83 abc
+ ‘ac‘O‘en + ”‘3 1/2x E Post 94 ab 78 bed
1/2x Post 91 abc 77 bed
1x Post 90 bc 83 abc
Fluazifop-P&fenoxaprop + 1/4x+ 1/4x 93 ab 94 a
fiftisrgrlﬁfrzig fomesafen 1/2x E Post 93 ab 94 a
1/2x Post 88 cd 77 bed
1x Post 84 d 68 de
Quizalofop + bentazon & 1/4x+ 1/4x 95 a 93 a
”mum” + COC 1/2x E Post 95 a 92 a
1/2x Post 95 a 61 e
1x Post 93 ab 75 cd
Quizalofop + thifensulfuron 1/4x+ 1/4x 95 a 92 a
glgh‘or‘mum“ + UAN + 1/2x E Post 95 a 94 a
1/2x Post 94 a 84 abc
1x Post 95 a 87 ab

 

‘Means averaged over row spacings. Means followed by the same letter are not
signiﬁcantly different according to Duncan’s Multiple Range Test. Mean comparisons

are valid only within years.

2Visual injury ratings were made 38 days after the standard postemergence herbicide

application timing.

77

Table 7. Control of redroot pigweed in soybeans with reduced rates of postemergence

herbicides in 1994‘.

 

 

 

 

 

 

Herbicide tank mixture Rate/ timing Redroot pigweed controlz-3
Wide Narrow
%

Clethodirn + thifensulfuron 1/4x+1/4x 99 a 99 a
+ ‘aC‘O‘en + N‘s 1/2x E Post 99 a 99 a
1/2x Post 99 a 99 a

1x Post 99 a 99 a

Fluazifop-P&fenoxaprop + 1/4x+ 1/4x 99 a 99 a
T‘ﬁﬂﬁ‘ﬁ; “mesafe” 1/2x E Post 84 ab 84 ab
1/2x Post 96 a 98 a

1x Post 98 a 98 a

Quizalofop + bentazon & 1/4x+ 1/4x 40 d 49 d
ac‘ﬂuorfe“ + CDC 1/2x E Post 67 c 44 d
1/2x Post 76 be 84 ab

1x Post 54 d 87 ab

Quizalofop + thifensulfuron 1/4x+ 1/4x 99 a 99 a
glghlorimuron + UAN + l/2x E Post 99 a 99 a
l/2x Post 99 a 99 a

1x Post 99 a 99 a

 

‘Means followed by the same letter are not signiﬁcantly different according to
Duncan’s Multiple Range Test. Mean comparisons are valid within and across

columns.

2Visual injury ratings were made 38 days after the standard postemergence herbicide

application timing.

3Wide = soybeans planted in 76 cm rows, Narrow = soybeans planted in 19 cm rows.

78

Table 8. Inﬂuence of herbicide tank mixture and soybean row spacing on redroot
pigweed control in 1995‘.

 

 

 

Herbicide tank mixture Redroot pigweed controlz-3

Wide Narrow
%
Clethodirn + thifensulfuron + lactofen + NIS 98 a 97 a
Fluazifop-P&fenoxaprop + thifensulfuron + 95 a 98 a
fomesafen + UAN +NIS
Quizalofop + bentazon & aciﬂuorfen + COC 57 c 85 b
Quizalofop + thifensulfuron + chlorirnuron + 96 a 99 a
UAN + NIS

 

‘Means averaged over rate/timing combinations. Means followed by the same letter are
not signiﬁcantly different according to Duncan’s Multiple Range Test. Mean
comparisons are valid within and across columns.

2Visual injury ratings were made 38 days after the standard postemergence herbicide
application timing.

3Wide = soybeans planted in 76 cm rows, Narrow = soybeans planted in 19 cm rows.

79

Table 9. Control of redroot pigweed in soybeans with reduced rates of postemergence
herbicides in 1995‘.

 

 

 

 

 

 

Herbicide tank mixture Rate/timing Redroot pigweed
control2
%
Clethodirn + thifensulfuron + lactofen 1/4x+1/4x 99 a
+ N‘s 1/2x E Post 93 a
1/2x Post 99 a
1x Post 99 a
Fluazifop-P&fenoxaprop + 1/4x+ 1/4x 98 a
thrg‘lelrésulfuron + fomesafen + UAN 1/2x E Post 98 a
1/2x Post 94 a
1x Post 96 a
Quizalofop + bentazon & aciﬂuorfen + 1/4x+ 1/4x 82 b
(3°C 1/2x E Post 81 b
1/2x Post 61 c
1x Post 61 c
Quizalofop + thifensulfuron + 1/4x+ 1/4x 99 a
chlorirnuron + UAN + NIS 1/2x E Post 97 a
1/2x Post 96 a
1x Post 99 a

 

‘Means averaged over row spacing. Means followed by the same letter are not
signiﬁcantly different according to Duncan’s Multiple Range Test.

2Visual injury ratings were made 38 days after the standard postemergence herbicide
application timing.

80

Table 10. Control of common ragweed in soybeans with reduced rates of
postemergence herbicides in 1994‘.

 

 

 

 

 

 

Herbicide tank mixture Rate/ timing Common ragweed
control2
%
Clethodirn + thifensulfuron + lactofen l/4x+l/4x 80 cd
+ ”‘3 1/2x E Post 66 cf
1/2x Post 54 fg
1x Post 85 abc
Fluazifop-P&fenoxaprop + 1/4x+ 1/4x 99 a
thifensulfuron + fomesafen + UAN 1/2x E Post 58 efg
+NIS
1/2x Post 81 cd
1x Post 97 ab
Quizalofop + bentazon & aciﬂuorfen + 1/4x+ 1/4x 84 be
COG 1/2x E Post 34 h
1/2x Post 51 g
1x Post 69 de
Quizalofop + thifensulfuron + 1/4x+ 1/4x 95 ab
chlorirnuron + UAN + NIS 1/2x E Post 69 d e
1/2x Post 81 cd
1x Post 91 abc

 

‘Means averaged over row spacing. Means followed by the same letter are not

signiﬁcantly different according to Duncan’s Multiple Range Test.

2Visual injury ratings were made 38 days after the standard postemergence herbicide

application timing.

Table 11. Control of common ragweed in soybeans with reduced rates of

81

postemergence herbicides in 1995‘.

 

 

 

 

 

 

Herbicide tank mixture Rate/ timing Common ragweed controlz-3
Wide Narrow
%
Clethodirn + thifensulfuron 1/4x+1/4x 93 abc 94 abc
+ ‘ac‘O‘en + N‘s 1/2x E Post 68 cf 92 abc
1/2x Post 86 abcde 88 abcd
1x Post 95 abc 96 ab
Fluazifop-P&fenoxaprop + 1/4x+ 1/4x 97 a 98 a
‘E‘i‘jfl‘gﬁfg‘l‘s” fomesafen 1/2x E Post 92 abc 92 abc
1/2x Post 71 def 81 abcde
1x Post 91 abc 97 a
Quizalofop + bentazon & 1/4x+ 1/4x 84 abcde 92 abc
ac‘ﬂuorfe“ + CDC 1/2x E Post 91 abc 76 cdef
1/2x Post 71 def 78 bcdef
1x Post 79 abcde 83 abcde
Quizalofop + thifensulfuron 1/4x+ 1/4x 61 f 98 a
glgh‘Or‘mm“ + UAN + 1/2x E Post 68 ef 60 1‘
1/2x Post 81 abcde 89 abcd
1x Post 80 abcde 98 a

 

‘Means followed by the same letter are not signiﬁcantly different according to
Duncan’s Multiple Range Test. Mean comparisons are valid within and across

columns.

2Visual injury ratings were made 38 days after the standard postemergence herbicide

application tinting.

3Wide = soybeans planted in 76 cm rows, Narrow = soybeans planted in 19 cm rows.

82

Table 12. Inﬂuence of herbicide tank mixture and soybean row spacing on velvetleaf
control in 1995‘.

 

 

 

 

Herbicide tank mixture Velvetleaf contr012~3

Wide Narrow
%
Clethodirn + thifensulfuron + lactofen + NIS 58 c 63 be
Fluazifop-P&fenoxaprop + thifensulfuron + 41 d 71 b
fomesafen + UAN +NIS
Quizalofop + bentazon & aciﬂuorfen + COC 56 c 56 c
Quizalofop + thifensulfuron + Chlorimuron + 70 b 84 a
UAN + NIS

 

‘Means averaged over rate/timing combinations. Means followed by the same letter are
not signiﬁcantly different according to Duncan’s Multiple Range Test. Mean
comparisons are valid within and across columns.

2Visual injury ratings were made 38 days after the standard postemergence herbicide
application timing.

3Wide = soybeans planted in 76 cm rows, Narrow = soybeans planted in 19 cm rows.

83

Table 13. Control of velvetleaf in soybeans with reduced rates of postemergence
herbicides in 1995‘.

 

 

 

 

 

 

Herbicide tank mixture Rate/timing Velvetleaf control2
%
Clethodirn + thifensulfuron + lactofen 1/4x+1/4x 72 bed
+ N‘s 1/2x E Post 25 f
1/2x Post 67 cde
1x Post 78 be
Fluazifop-P&fenoxaprop + 1/4x+ 1/4x 75 bed
tiulflelrésulfuron + fomesafen + UAN 1/2x E Post 30 f
1/2x Post 60 de
lx Post 60 de
Quizalofop + bentazon & aciﬂuorfen + 1/4x+ 1/4x 51 e
COC 1/2x E Post 24 f
1/2x Post 59 de
1x Post 89 ab
Quizalofop + thifensulfuron + 1/4x+ 1/4x 81 abc
chlorirnuron + UAN + NIS l/2x E Post 34 f
1/2x Post 96 a
1x Post 97 a

 

‘Means averaged over row spacing. Means followed by the same letter are not
signiﬁcantly different according to Duncan’s Multiple Range Test.

2Visual injury ratings were made 38 days after the standard postemergence herbicide
application timing.

84

Table 14. Inﬂuence of various rates of postemergence herbicides on weed biomass in
1994 and 1995‘.

 

 

 

 

 

 

 

 

Herbicide tank mixture Rate/timing Weed biomass
1994 1995
g/m2
Clethodim + thifensulfuron 1/4x+1/4x 43 e 84 bcd
+ ‘aC‘O‘en + ”‘8 1/2x E Post 379 a 212 a
1/2x Post 102 cde 236 a
1x Post 40 e 125 b
Fluazifop-P&fenoxaprop + 1/4x+ 1/4x 30 e 35 d
f‘tﬁl‘jl‘ﬁfg‘l‘; fomesafen 1/2x E Post 193 b 203 a
1/2x Post 53 de 62 cd
1x Post 27 e 48 cd
Quizalofop + bentazon & 1/4x+ 1/4x 74 de 52 cd
“mm” + CDC 1/2x E Post 195 b 242 a
1/2x Post 133 bed 93 be
1x Post 85 de 92 be
Quizalofop + thifensulfuron 1/4x+ 1/4x 127 bed 53 cd
glgh‘orhnmm + UAN + 1/2x E Post 178 be 223 a
1/2x Post 37 e 57 cd
1x Post 53 de 35 d
Untreated 338 461
Weedfree 0 0
Preemergence standard 363 139
Mechanical control only 286 289

 

‘Means averaged over row spacing. Means followed by the same letter are not
signiﬁcantly different according to Duncan’s Multiple Range Test. Mean comparisons
are valid only within years.

85

Table 15. Injury to soybean from postemergence herbicides in 1994 and 1995‘.

 

 

 

 

 

 

Herbicide tank mixture Rate/timing Soybean visual injury2
1994 1995
%
Clethodim + thifensulfuron 1/4x+1/4x 16 b 22 ab
+ ‘acwfen + N‘s 1/2x E Post 22 a 24 a
1/2x Post 12 c 14 def
1x Post 11 c 18 be
Fluazifop-P&fenoxaprop + 1/4x+ 1/4x 6 d 9 hi
‘iﬁfsﬂffg‘l‘; “mesafen 1/2x E Post 7 d 13 efg
1/2x Post 6 d 8 hi
1x Post 5 d 8 hi
Quizalofop + bentazon & 1/4x+ 1/4x 7 d 15 cde
ac‘ﬂuorfe“ + COC 1/2x E Post 13 be 17 cd
1/2x Post 8 d 11 fgh
1x Post 11 c 9 ghi
Quizalofop + thifensulfuron 1/4x+ 1/4x 0 e 3 jk
glghlorimuron + UAN + 1/2x E Post 0 e 2 k
1/2x Post 5 d 6 ij
1x Post 8 d 14 cdef

 

‘Means averaged over row spacing. Means followed by the same letter are not
signiﬁcantly different according to Duncan’s Multiple Range Test. Mean comparisons

are valid only within years.

2Visual injury ratings presented are taken 10 days after the early postemergence
herbicide application for 1/4x E Post + 1/4x Post and 1/2x E Post timings, and 10
days after standard postemergence herbicide application for 1/2x Post and 1x Post

timings.

86

Table 16. Inﬂuence of various rates of postemergence herbicides on soybean yield in
1994 and 1995‘.

 

 

 

 

 

 

 

 

Herbicide tank mixture Rate/timing Yield2
1994 1995
kg/ha
Clethodim + thifensulfuron 1/4x+1/4x 2255 abcd 3469 d
+ ‘a°‘°‘e“ + N‘s 1/2x E Post 1318 e 2485 e
1/2x Post 1755 de 2555 e
1x Post 2413 ab 3450 d
Fluazifop-P&fenoxaprop + 1/4x+ 1/4x 2357 abc 4260 ab
f‘ﬁfl‘gffﬁ‘l‘; fomesafen 1/2x E Post 1747 de 2679 e
1/2x Post 2403 ab 3740 cd
lx Post 2634 a 3922 be
Quizalofop + bentazon & 1/4x+ 1/4x 2430 ab 4163 abc
ac‘ﬂuorfe“ + CDC 1/2x E Post 1938 bed 2412 e
1/2x Post 1929 bed 3470 d
1x Post 2340 abc 3940 be
Quizalofop + thifensulfuron 1/4x+ 1/4x 2510 a 4140 abc
glgh‘or‘mum“ + UAN + 1/2x E Post 1828 cde 2549 e
1/2x Post 2372 abc 4126 abc
1x Post 2326 abc 4433 a
Untreated 1050 1030
Weedfree 2867 461 1
Preemergence standard 1219 4097
Mechanical control only 1651 2562

 

‘Means averaged over row spacing. Means followed by the same letter are not
signiﬁcantly different according to Duncan’s Multiple Range Test. Mean comparisons
are valid only within years.

21994 data presented is the adjusted yield. Soybean yields were adjusted using analysis
of covariance, with Sclerotinia stem rot disease severity index rating as the covariate.

87

Table 17. Gross margin of soybean treatments in 1994 and 1995.

 

 

 

 

 

 

 

 

 

Herbicide tank mixture Rate/timing Gross margins‘
1994 1995
Wide Narrow Wide Narrow
$/ha
Clethodim + thifensulfuron 1/4x+1/4x 320‘ 433+ 596+ 636‘
+ ‘aC‘Ofen + N‘s 1/2x E Post 139- 257- 399- 468-
1/2x Post 262‘ 309' 417‘ 474‘
1x Post 379 397 531 660
Fluazifop-P&fenoxaprop + 1/4x+ l/4x 339‘ 437‘ 711+ 827+
:ﬁthT—rﬁtrl; fomesafen 1/2x E Post 275- 283- 468- 467-
1/2x Post 391= 428‘ 656‘ 698‘
1x Post 397 447 637 721
Quizalofop + bentazon & 1/4x+ 1/4x 398‘ 421+ 724+ 788‘
ac‘ﬂuorfe“ + COG 1/2x E Post 285- 365- 366- 475-
1/2x Post 322‘ 322‘ 637‘ 623'
1x Post 378 375 624 769
Quizalofop + thifensulfuron 1/4x+ 1/4x 440“ 410‘ 698‘ 804‘
glgh‘orhnm“ + UAN + 1/2x E Post 230- 377- 401- 498-
1/2x Post 408+ 412‘ 717‘ 804‘
1x Post 317 424 715 875
Preemergence standard 104 189 711 742
Mechanical control only 349 266 656 327

 

‘(+) indicates the gross margin of the reduced rate treatment was greater than (5% or
more) the gross margin of the same tank mixture applied at a full rate; (=) indicates
the gross margin of the reduced rate treatment was within 5 % of the gross margin of
the same tank mixture applied at a full rate; (-) indicates the gross margin of the
reduced rate treatment was lower than (5 % or more) the gross margin of the same tank
mixture applied at a full rate. Comparisons of gross margins using these symbols are
valid only within a tank mixture, row spacing, and year.

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