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This is to certify that the
thesis entitled

Common Ragweed Control with Bentazon
as Influenced by Imazethapyr and
Thifensulfuron Tankmixes

presented by

Aaron Gregory Hager

has been accepted towards fulfillment
of the requirements for

M.S. degree in Crop and Soil Sciences

 

 

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Major professor

Date W
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cztcircWMS-o.‘

 

COMMON RAGWEED CONTROL WITH BENTAZON
AS INFLUENCED BY
IMAZETHAPYR AND THIFENSULFURON TANKMIXES
By

Aaron Gregory Hager

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

MASTER OF SCIENCE

Department of Crop & Soil Sciences

1993

ABSTRACT
COMMON RAGWEED (Ambrosia artemisiifolia) CONTROL WITH BENTAZON
AS INFLUENCED BY
IMAZETHAPYR AND THIFENSULFURON TANKMIXES
By

Aaron Gregory Hager

Several herbicides control common ragweed postemergence in soybeans. However,
when herbicides are tankmixed, synergistic or antagonistic weed responses may occur,
and additive selection may influence weed response. Imazethapyr and thifensulfuron
can be tankmixed with bentazon for broadleaf weed control, and visual crop injury and
cost hectare“ may be reduced compared to a bentazon plus acifluorfen tankmix.
Experiments determined common ragweed control from these herbicides and herbicide
combinations when applied with various additives. In the greenhouse bentazon at 560
g ha‘l reduced common ragweed dry weight 95 to 97%. Common ragweed control with
35 g ha'1 of imazethapyr increased 13 and 7% when urea ammonium nitrate (UAN) was
applied with nonionic surfactant (N18) or petroleum oil adjuvant (POA), respectively.
Tankmixing thifensulfuron at 4.5 g ha‘l or imazethapyr at 35 or 71 g ha‘1 with bentazon
did not enhance nor reduce dry weight of common ragweed compared with bentazon
alone. In field research bentazon at 1120 g ha'l reduced common ragweed dry weight
78 to 81%. Tankmixing bentazon with thifensulfuron at 4.5 g ha‘l or imazethapyr at
71 or 35 g ha‘l reduced common ragweed dry weight by 89 to 91%, thus enhancing

control compared to each herbicide applied alone. Adding 28% UAN to bentazon plus

ii

imazethapyr or bentazon plus thifensulfuron tankmixes did not increase control
compared to NIS or POA alone. By 28 days after treatment (DAT) only 35 or 71 g ha‘l
of imazethapyr plus 28% UAN plus N18 or POA and bentazon plus imazethapyr
tankmixes provided 85% or more control of common ragweed in 1991, and only 71 g
ha'1 of imazethapyr plus 28% UAN plus POA or 71 g ha’1 of imazethapyr plus bentazon
provided greater than 78% common ragweed control in 1992. Soybean yields were
equal to that of the handweeded control only with the bentazon plus 71 g ha" of
imazethapyr tankmix treatments. Research was conducted to determine the influence
of relative humidity (RH), soil moisture, and the presence of bentazon on the absorption
and translocation of 14C-imazethapyr in common ragweed (Ambrosia artemisiifolia L.).
Low soil moisture (-3 bars) reduced l4C-imazethapyr absorption by 6% 24 h after
treatment, but did not influence translocation. The absorption and translocation of 14C-
imazethapyr when applied alone was not effected by RH 48 and 168 h after treatment.
However, when bentazon was present in the spray solution absorption 24 h after
treatment was reduced by 51% at 65% RH but was not reduced at 85% RH.
Translocation of l4C-imazethapyr out of the treated leaf was reduced significantly at

65% RH, but not at 85% RH in the presence of bentazon.

iii

ACKNOWLEDGEMENTS

I would like to express my sincere thanks and appreciation to Dr. Karen Renner for
her guidance, support, and friendship during my two years here at Michigan State while
completing this degree. I also would like to express gratitude to Dr. Penner for serving
as a committee member and his patience in answering the many questions I had as a
new graduate student. I would also like to thank Dr. Bernard Zandstra for serving as
a committee member and for his suggestions in preparing the data for presentation. A
large thanks is extended to Gary Powell for his help in all aspects of field work. Your
patience and understanding was greatly welcomed after my first encounter in laying out
a study. Gratitude is also extended to Dr. Jim Kells for his friendship and the many
instances of good advice. I would also like to thank Patrick Svec, Eric McPherson, and
Sharon Grant for their many hours of assistance in completing this research. I have
also had the privilege of working with and becoming friends with some very talented
fellow graduate students. Troy ’The Ghandi’ Bauer deserves a special thanks for all
his help and assistance during my two years here. Boyd ’The Lure’ Carey was
someone who I could always count on no matter how busy he was. I "enjoyed" grading
all the weed collections with you, Boyd. Karen ’The Door Bender’ Novosel can always
be counted on to bring laughter to the darkest situation. Rick (RICH) ’The
Schemenkanator’ Schmenk was always willing to play nine and sometimes he actually
thought he won. Thanks also to former graduate students Jason ’Woodsie’ Woods, Joe
’I have no Tolerance’ Bruce, and Steve ’The Kid’ Hart. Of course, no one can seem

to forget the antics of Andy Chomas.

iv

I also would like to give my most sincere thanks and appreciation to my dear wife
Amy who put up with so much while I was completing this degree and to whom I will

be forever grateful for her love and understanding.

 

TABLE OF CONTENTS

Chapter 1. Review of Literature.

Chapter 2.

Chapter 3.

Imazethapyr ........................................ l
Acifluorfen ........................................ 8
Thifensulfuron ..................................... 1 7
Bentazon ......................................... 23
Interactions ....................................... 32
Additives ......................................... 34
Common Ragweed .................................. 35
Interference ....................................... 39
Literature Cited .................................... 41

Common Ragweed (Ambrosia artemisiifolia) Control in Soybeans with
Bentazon as Influenced by Imazethapyr and Thifensulfuron
Tankmixes.
Abstract ......................................... 50
Introduction ....................................... 5 1
Materials and Methods ............................... 54
Greenhouse study ................................ 54
Field study .................................... 55
Results and Discussion ............................... 57
Greenhouse study ................................ 57
Field study .................................... 5 8
Literature Cited .................................... 62

Influence of Relative Humidity and Soil Moisture on Absorption and
Translocation of Imazethapyr in Common Ragweed (Ambrosia
artemisiifolia).
Abstract ......................................... 70
Introduction ....................................... 71
Materials and Methods ............................... 73
Results and Discussion ............................... 76
Literature Cited .................................... 83

vi

 

LIST OF TABLES

Chapter 2. Common Ragweed Control in Soybeans with Bentazon as Influenced
by Imazethapyr and Thifensulfuron Tankmixes.

Table 1. Soil characteristics of field studies conducted

in 1991 and 1992 .......................... 64
Table 2. Rainfall accumulation at East Lansing
in 1991 and 1992 .......................... 65

Table 3. Common ragweed dry weight reductions following
bentazon, thifensulfuron, and tankmix combinations
in the greenhouse .......................... 66
Table 4. Common ragweed dry weight reductions following
bentazon, imazethapyr, and tankmix combinations

in the greenhouse .......................... 67
Table 5. Common ragweed dry weight reductions, visual control
28 DAT, and soybean yield in the field ........... 68

Chapter 3. Influence of Relative Humidity and Soil Moisture on Absorption and
Translocation of Imazethapyr in Common Ragweed.

Table 1. Effect of soil moisture, relative humidity, and bentazon

on absorption and translocation of l4C-imazethapyr

in common ragweed ......................... 87
Figure 2. Effect of relative humidity and bentazon on

absorption and translocation of 1"C-imazethapyr

in common ragweed ......................... 88

vii

LITERATURE REVIEW

Imazethapyr. Imazethapyr [2- [4,5-dihydro-4-methyl-4-( 1-methylethyl)-5-oxo-1H-

imidazol-Z-yl]-5-ethyl-3-pyridinecarboxylic acid] is a selective herbicide used to control
a wide spectrum of broadleaf and grassy weeds in soybeans (Glycine max (L.) Merr.)
and other leguminous crops. Imazethapyr belongs to the herbicide family known as the
imidazolinones. Soybeans have shown excellent tolerance to imazethapyr applied
preemergence, preplant incorporated, or postemergence. Cantwell et al. (1989) reported
no yield reduction in soybeans from preplant incorporated, preemergence, or
postemergence imazethapyr applications of 0.05 to 0.14 kg ha". Mills and Witt (1989)
reported that soybeans treated with 70 g ha‘1 of imazethapyr applied preemergence had
yields equivalent to the weed free control. Wilson and Miller (1991) observed no yield
reduction in dry edible bean (Phaseolus vulgaris L.) from imazethapyr applied preplant
incorporated and postemergence at 0.07 and 0.10 kg ha".

In a review of the structural requirements for imidazolinone herbicidal activity,
Ladner (1990) concluded that a carboxylic acid group or a group which could be readily
transformed to a carboxylic acid was necessary for activity and that this group must be
ortho to the imidazolinone ring, with. methyl and isopropyl substituents on the
imidazolinone ring affording superior activity. A pyridine ring as the backbone ring
shows the highest level of activity.

The mode of action of a herbicide can be defined as being the manner in which a

 

 

 

2

action of many herbicides is not known, the biochemical pathway which is inhibited by
the imidazolinone herbicides has been identified. In 1984, the site of action of the
imidazolinone herbicides was discovered to be the enzyme acetolactate synthase,
commonly referred to as ALS (Shaner et a1. 1984). ALS is the first enzyme in the
pathway leading to the synthesis of the branched-chain amino acids valine, leucine, and
isoleucine. For valine and leucine synthesis, two molecules of pyruvate are condensed
to form 2-acetolactate, while for isoleucine formation one molecule of pyruvate reacts
with 2-ketobutyrate in a similar reaction (Dumer et a1. 1991). Like many of the amino
acid biosynthetic enzymes, ALS is nuclear encoded and located in the plastid (Miflin
1974). The inhibition of the ALS enzyme by the imidazolinone herbicides causes a
disruption in protein synthesis which in turn interferes with DNA synthesis and cell
growth (Shaner and Reider 1986). The meristematic tissues of the plant die first
followed by a slow necrosis of mature tissue.

Numerous studies have investigated the inhibitory action of the imidazolinone
herbicides with agreement that ALS is the site of action. Anderson and Hibbard
(1985) reported that the analysis of free amino acid levels of corn (Zea mays L.)
suspension cell cultures indicated that levels of leucine and valine were substantially
decreased in cells exposed to 10 ”M of irnazapyr [(i)-2-[4,5-dihydro-4—methyl-4-(1-
methylethyl)-5-oxo-lH—imidazol-Z-yl]-3-pyridinecarboxylic acid], another
imidazolinone herbicide. Quantities of leucine and valine after 48 hours of exposure
to the herbicide were 52 and 27%, respectively, of the levels seen in untreated cell

cultures. Supplementation of suspension culture growth media with leucine and valine

 

 

3

plus isoleucine was carried out to determine if reversion of the growth inhibition would
occur. These supplemented amino acids reversed the growth inhibitory effects of
imazapyr at levels ranging from 10 nM to 1 mM. These results lead Anderson to
suggest a specific interaction of the herbicide with leucine, valine, and isoleucine
synthesis. Muhitch et a1. (1987) found that ALS activity in crude extracts of excised
maize leaves and cultured suspension cells was reduced 85 and 58%, respectively, by

incubation of the tissue with 100 uM (excised leaves) and 5 uM (suspension cultures)

of irnazapyr. Muhitch also found that when ALS activity was measured over a 4 hour
assay period in the presence of various irnazapyr concentrations, inhibition was found
to increase with time, an indication of a tight binding inhibitor. Stidman and Shaner
(1990) reached similar conclusions when they demonstrated that the level of inhibition
by three concentrations of irnazapyr increased over a 4 hour time period, approaching
a final steady-state level of inhibition between 3 and 4 hours. From their results the
authors postulated two explanations of this slow-binding behavior: 1) the enzyme
undergoes conformational changes upon initial binding with the herbicide and the
change results in an increased stabilization of the enzyme-herbicide complex, and 2) the
inhibitor experiences some transformation on the surface of the enzyme that results in
a modified inhibitor. Pillmoor and Caseley (1987) found that imazamethabenz-methyl

[(i)-2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-Z-yl]-4(and5)-
methylbenzoic acid (3:2)] inhibited the ALS enzyme, however the inhibition appeared
to be dependent upon the conversion of the parent herbicide to the fi'ee acid form.

LaRossa et a1. (1987) theorized that a build-up of oc-ketobutyrate to toxic levels caused

by a blockage of the ALS enzyme may be the actual cause of plant death, not depletion

 

 

of amino acid levels.

Van Ellis and Shaner (1988) investigated the mechanism of cellular absorption of
the imidazolinone herbicides in soybean leaf discs. Since the amino acid synthesis
pathway is located in the plastid the herbicide must transverse the plasmalemma by l)
passive diffusion, 2) active uptake via a carrier, or 3) ion trapping of a weak acid. The
uptake of the three imidazolinone herbicides irnazapyr, imazethapyr, and imazaquin [2-
4,5-dihydro—4-methyl-4-(1-methylethyl)-5-oxo- lH—imidazol-Z-yl]-3-quinolinecarboxylic
acid] was sensitive to the pH of the uptake solution, uptake into the leaf discs was
linear with respect to external concentration, and metabolic inhibitors and uncouplers
decreased uptake 58 to 85%. The Q10 for uptake between 13 and 23 C was 1_.7. There
were differences in uptake of the three herbicides across the pH ranges from pH 4 to
pH 7 and these differences were postulated to be reflective of the differences in
lipophilicity of the herbicides at pH 4 and pH 7. The authors final conclusion was that
the uptake of these imidazolinone herbicides could best be explained by ion trapping.

To reach the site of action, postemergence herbicides must be absorbed by the plant
foliage. The main barrier to herbicide absorption is the plant cuticle and diffusion of
the herbicide through the cuticle appears to be the rate-limiting step in foliar absorption
(Wanamarta and Penner 1989). The thickness of the cuticle varies by plant species and
environmental conditions. Differences in the amount of foliar absorption of the
imidazolinone herbicides has been observed for different plant species. Shaner and
Robson (1985) reported that l4C-imazaquin did not penetrate velvetleaf (Abutz’lon

theophrasti Medicus) foliage as well as it did soybean or common cocklebur (Xanthium

5

strumarium L.) leaves. Absorption of imazaquin by velvetleaf was decreased when the
herbicide was applied to the leaves as compared to application to the cotyledons.
Wilcut et a1. (1988) reported differences in l4C-imazaquin absorption 3 hours after
application for the weed species evaluated. The order of absorption from greatest to
least was soybean 78%, sicklepod (Cassia obtusifolia L.) 71%, Florida beggarweed
(Desmodium tortuosum (Sw.) DC.) 61%, peanut (Arachis bypogaea) 59%, and common
cocklebur 53%. Differences in absorption by species were no longer evident after 24
hours, and by 72 hours the amount absorbed exceeded 90% of that applied for all
species with no differences between species. Cole et a1. (1989) noted differences in
absorption of l4C-imazethapyr 3 hours following foliar application. Soybeans had
absorbed 80% of the amount applied and this maximum amount was followed by
sicklepod which absorbed 77% and then by peanut, Florida beggarweed, and redroot
pigweed (Amaranthus retroflexus L.), the three of which absorbed an average of 55%.
By 24 hours after application, Florida beggarweed had only absorbed 77%, while
absorption among the remaining species was 92% with no differences between species.
Acropetal and basipetal movement of l4C-imazethapyr away from the treated leaf was
apparent in all species (soybean, peanut, sicklepod, Florida beggarweed, and redroot
pigweed) evaluated. At 72 hours after treatment, approximately 6% of the applied
imazethapyr had been translocated out of the treated leaf of peanut, sicklepod, and
Florida beggarweed, while 12 and 15% were translocated out of the treated leaf of
redroot pigweed and soybean, respectively. Following root uptake of soil applied l"C-

imazethapyr, all species except peanut had translocated greater than 85% of the

6

absorbed imazethapyr into the shoot. Peanut had translocated 72% of the absorbed l4C-
imazethapyr. Shaner and Robson (88) found that based on the distribution of '4C-
imazaquin applied to the foliage or roots, imazaquin was both xylem and phloem
mobile in velvetleaf, common cocklebur, and soybean. Wilcut et al. (99) found both
symplastic and apoplastic translocation of MC-imazaquin in soybean, peanut, Florida
beggarweed, common cocklebur, and sicklepod.

The mechanism of plant selectivity to a herbicide could be the result of differences
in the rate of absorption, translocation, metabolism, and/or sensitivity of the site of
action to the herbicide. Bauer et al. (11) demonstrated that enzyme sensitivity
differences to imazethapyr could not explain the difference in sensitivity of two pinto
bean varieties. Studies by Shaner and Robson (88), Cole et al. (25), and Wilcut et al.
(99) have shown that crop selectivity is based upon differential rates of metabolism
among various species. The half-life of imazethapyr in soybeans, a tolerant crop, is 1.6
days', while the half-life in redroot pigweed, a susceptible species, was shown by Cole
et al. (25) to be 32.1 days. Wilcut et al. (99) determined the half-life of imazaquin in
soybeans to be 4.4 days, while the half-life in common cocklebur is 39.8 days. The
metabolic pathway of imazethapyr in soybeans has been suggested by Lee et al. (1991).
Based on metabolic profiles of extractable residues from soybean plants, oxidative

hydroxylation at the oc-carbon of the ethyl substituent on the pyridine ring is believed

to be the primary site for initial metabolic conversion of the parent compound, yielding

oc-hydroxyethyl imazethapyr which then reacts with glucose, yielding a glucose

 

lAC 263,499 Experimental herbicide technical information report. 1985. American
Cyanamid Co., Princeton, NJ 08540.

conjugate as the final metabolite.

The imidazolinone herbicides possess soil activity in addition to foliar activity. The
soil activity and persistence of this class of herbicides may have a negative effect on
subsequent crops planted where imidazolinone herbicide applications were made during
a previous growing season. Various factors contribute to the field persistence of
imazethapyr. Goetz et al. (1990) examined the effects of photodecomposition,
volatilization, temperature, soil type, and soil moisture on the degradation and
persistence of imazethapyr. Volatilization accounted for less than 2% of the dissipation
of imazethapyr, and losses from photodecomposition were less than 10% when
imazethapyr was applied to the soil. Photodecomposition losses up to 52% occurred
from application to a glass slide, free of soil. Degradation of imazethapyr increased
under conditions of increased soil moisture which was attributed to greater soil
microorganism activity. Elevated soil temperatures conducive to enhanced microbial
activity and soil types which were low in clay and organic matter were also found to
be conditions for enhanced degradation. Stougaard et al. (1990) evaluated the effects
of soil type and pH on adsorption, mobility, and efficacy of imazaquin and imazethapyr,
finding that adsorption of both herbicides increased as soil pH decreased. These
observations were attributed to protonation of the basic quinoline and pyridine moieties
of imazaquin and imazethapyr, respectively, resulting in ionic binding to soil colloids
at the lower soil pH. Consistent with the findings of Goetz et al.(1990), Stougaard
observed greater mobility of both herbicides in soils of low clay and organic matter

content. Renner et al. (1988) concluded that adsorption of imazaquin and imazethapyr

8

increased as soil pH decreased from pH 8.0 to 3.0 in laboratory studies. The authors
postulated that protonation of the imidazolinone nitrogens, or the nitrogen of the
quinoline or pyridine ring of imazaquin and imazethapyr, respectively, may have
resulted in catiohic binding of the herbicides to soil at low soil pH values. Cantwell
et a1. (1989) investigated abiotic and biotic factors influencing the biodegradation
characteristics of imazaquin and imazethapyr. Using gamma-irradiated soil as a
sterilized soil, 95% of the applied radioactivity could be extracted as parent herbicide
following 12 weeks of incubation. Extensive degradation of the parent herbicide
occurred in the unsterilized soil, leading the authors to conclude that the primary
mechanism of imidazolinone degradation in soil was microbial.

Acifluorfen. Acifluorfen [5-[2-chloro-4~(trifluoromethyl)phenoxy]-2-nitrobenzoic
acid] is a selective herbicide used for the preemergence and/or postemergence control
of broadleaf weeds and grasses in soybeans, peanuts, and rice (Oryza sativa L.).
Acifluorfen is active as a preemergence treatment but high rates are required for
effective weed control (Johnson et al. 1978).

Acifluorfen belongs to the class of herbicides known as the diphenyl ether
herbicides. This class of herbicides is characterized by two phenyl rings joined by an
ether linkage. The members of this herbicide family differ from one another by the
different substituent groups present at various locations on the phenyl rings. The
patterns of the various substituent groups are the cause of the different effects
demonstrated by the diphenyl ethers (Kunert et a1. 1987).

The major use of acifluorfen is as a postemergence herbicide. The application rate

 

 

9

table for acifluorfen begins at a rate of 285 g ha‘l and concludes at a maximum use rate
of 850 g ha". Several annual grass species are included in the label, such as fall
panicum (Panicum dichotomiflorum Michx.), shattercane (Sorghum bicolor (L.)
Moench), and giant foxtail (Setaria faberi Herrm.), but the majority of acifluorfen is
applied for broadleaf weed control.

Lee and Oliver (1982) studied the efficacy of acifluorfen on broadleaf weeds. Their
research concentrated on ascertaining the best time and method for postemergence
acifluorfen applications. They concluded that the most efficient control of the weed
species evaluated occurred when acifluorfen was applied when the weeds were in the
early stages of growth. If applications were delayed until the weeds were at a later
stage of growth, increased application rates of acifluorfen were required to obtain
satisfactory control. They observed greater control of hemp sesbania (Sesbania exaltata
(Raf) Rydb. ex A. W. Hill), pitted morningglory (Ipomoea lacunosa L.), and smooth
pigweed (Amaranthus hybridus L.) when acifluorfen was applied in the dark (2100b)
than at sunup (0600b) or mid-day (1200b). The postulation was put forth that the
greater control from dark applications may have resulted from the observed increase in
relative humidity that may have enhanced herbicide penetration. Volume of spray
solution was evaluated also. The conclusions were that higher spray volumes are
required when complete coverage of foliage is necessary for acceptable control in
species such as pitted morningglory, but lower volumes are necessary to prevent runoff
of spray solution from the waxy leaf surfaces of species such as hemp sesbania.

Ritter and Coble (1984) examined the influence of crop canopy, weed maturity, and

10

rainfall on acifluorfen activity. Consistent with Lee and Oliver (58), Ritter and Coble
found more consistent weed control when applications of acifluorfen were made when
the weed species were small. Topical applications of acifluorfen plus surfactant gave
the greatest control. Rainfall occurring shortly after herbicide application resulted in
less control than when rainfall was delayed several hours. They concluded that several
factors influenced acifluorfen activity, including amount, length, intensity of rainfall,
and size of droplet.

In a separate study, Ritter and Coble (1981) examined the influence of temperature
and relative humidity on acifluorfen activity. Control of the weed species evaluated
was greater at high temperatures (32 C day, 22 C night) than at low temperatures (26

C day, 16 C night). Treatments made at high relative humidity (85% i 5%) provided
better control than treatments made at low (50% i 5%) relative humidity. When the

same herbicide treatments were compared across all four regimes, high temperature and
high humidity provided the best control. They concluded, however, that relative
humidity was a more important factor than temperature, as relative humidity affects the
ability of the herbicide to penetrate the plant leaf cuticle. This conclusion is in
agreement with that reached by Willingham and Graham (1988) who found that relative
humidity had the greatest effect on penetration of acifluorfen into velvetleaf.
Penetration into velvetleaf increased from 1 to 10% as relative humidity increased from
40 to 90%. Temperature was the second most important factor. They theorized that
higher temperatures may soften the leaf cuticle and aid in penetration, but higher
temperatures may also accelerate the drying of the spray droplets and cause more loss

due to volatility.

ll

Wills and McWhorter (1981) considered the effect of environment on the
translocation and toxicity of acifluorfen to showy crotalaria (Crotalaria spectabilis
Roth). They discovered that acifluorfen applied postemergence provided greater control
at high (100%) than at low (40%) relative humidity. The control was also greater at
high (27 and 35 C) than at low (18 C) temperature. Translocation of radiolabeled
acifluorfen was influenced more by air temperature than by relative humidity. ‘

Several studies have examined the use of mefluidide [N-[2,4-dimethyl-5-
[[(trifluoromethyl)sulfonyl]amino]phenyl]acetamide] as a pretreatment or in conjunction
with acifluorfen treatments. Mefluidide is classified as a plant growth regulator (107).

Hook and Glenn (1984) examined penetration, translocation, and metabolism of MC-
acifluorfen following pretreatment with mefluidide in soybean and various weed
species. Results varied between weed species and soybeans. In ivyleaf morningglory
(Ipomoea hederacea (L.) Jacq.), pretreatment with mefluidide 3, 5, or 7 days prior to
radiolabeled acifluorfen application increased penetration, decreased metabolism, and
did not affect translocation of acifluorfen. In velvetleaf, pretreatment 0, 3, 5, or 7 days
increased penetration while the 3, 5, and 7 day pretreatrnents decreased metabolism of
acifluorfen by velvetleaf. Penetration and translocation of radiolabeled acifluorfen in
soybeans was unaffected by pretreatment of melfuidide, but metabolism of acifluorfen
was decreased by the 0 day pretreatment. In a separate study, Hook and Glenn (1984)
investigated the interactions of mefluidide and acifluorfen. No statistical differences
(P>0.05) were found when the data were averaged over all acifluorfen rates and

sequences of application in either injury or dry weight reduction of ivyleaf

12

morningglory, velvetleaf, or common cocklebur using either 0.1 or 0.3 kg ha’I of
mefluidide. Treatments included individual applications at each herbicide rate as well
as tank mixtures and sequential applications. This study was conducted in the
greenhouse. Field studies by Glenn et al. (1985) evaluated control of velvetleaf and
common cocklebur in soybeans with sequential applications of mefluidide and
acifluorfen. Field results from 1981 and 1983 showed that 6 weeks after application
there was no difference in velvetleaf control between sequential early-post treatments
and acifluorfen applied alone. In 1981, all sequential late-post treatments of mefluidide
and acifluorfen were more effective in controlling velvetleaf than late-post applications
of acifluorfen alone. There were no differences in common cocklebur control between
sequential early-post treatments and acifluorfen applied early-post alone by 6 weeks
after application in 1981. All sequential late-post treatments increased common
cocklebur control 3 and 6 weeks after application, compared to control obtained with
late-post applications of acifluorfen.

Surfactants have been shown to increase the phytotoxicity of acifluorfen. Lee and
Oliver (1982) noted that increasing the surfactant concentration enhanced the control
of entireleaf morningglory (Ipomoea hederacea var. integriuscula) and Texas gourd
(Cucurbita texana (Scheele) Gray). Ritter and Coble (84) showed that the addition of
a nonionic surfactant enhanced control of common cocklebur and common ragweed
(Ambrosia artemisiifolia L.). Willingham and Graham (103) and Banks et al. (1988)
also noted greater control of weed species with the addition of a surfactant to the spray

solution with acifluorfen.

 

13

The selectivity of acifluorfen is related to crops metabolizing the herbicide to a
larger extent than do susceptible species. Ritter and Coble (1981) examined the
metabolism of acifluorfen in soybean, common ragweed and common cocklebur. At
7 days after application of radiolabeled acifluorfen, soybeans metabolized more of the
parent compound than common cocklebur which metabolized more of the parent
compound than common ragweed. The authors also noted that penetration and
translocation were more rapid in susceptible weed species as compared to tolerant
soybean. Higgins et al. (1988) observed the absorption, translocation, and metabolism
of l4C-acifluorfen in pitted morningglory and ivyleaf morningglory, showing
metabolism and translocation of radiolabeled acifluorfen were minimal in both species.
Penetration of acifluorfen was greater in pitted than in ivyleaf morningglory, and this
fact may account for the greater tolerance of ivyleaf morningglory to acifluorfen. The
pubescent leaf surface of ivyleaf morningglory appears to be more restrictive to
absorption of acifluorfen than the glabrous leaf surface of pitted morningglory.

Frear et al. (1983) examined and defined the pathway of soybean metabolism of
acifluorfen. From 85 to 95% of the absorbed radiolabeled acifluorfen was metabolized
in less than 24 hours. The first reaction in this metabolic pathway is the cleavage of
the diphenyl ether bond. This cleavage results in a reactive phenolic intermediate and
a homoglutathione conjugate. The phenolic intermediate is conjugated with glucose and
further acylated to form a malonyl-B-D-glucoside, which was labeled metabolite I. The
homoglutathione conjugate was identified at metabolite 11. Further metabolism of 11

results in a cysteine conjugate, identified as metabolite III.

14

The ortho-substituted diphenyl ether herbicides require light for activation
(Matsunaka 1969). This activation process appears to be a photobiochemical process.
Matsunaka (1969) examined the susceptibility of white or yellow mutant rice plants to
nitrofen [2,4-dichloro-1-(4-nitrophenoxy)-4-(trifluoromethyl)benzene] to determine
which pigments were responsible for the photoactivation process. The author found that
green or yellow rice plants were susceptible to nitrofen, but white plants were tolerant.
From the results, Matsunaka concluded that chlorophylls or yellow pigments, especially
xanthophylls, were the acceptors of light energy in the photoactivation of nitrofen.
Fadayomi and Warren (1976) found results in agreement with Matsunaka (1969) when
they Observed that nitrofen and oxyfluorfen [2-chloro-1-(3-ethoxy-4-notrophenoxy)-4-
(trifluoromethyl)benzene] were much less active on white mutants Of corn, but active
on normal and greenish yellow plants. The authors also found that plants kept in the
dark after herbicide application showed no signs of injury, but when these plants were
moved into the light, injury symptoms appeared after 4 hours. Devlin et a1. (1983)
conducted a study to lend support to the hypothesis of carotenoid pigments being the
photoactivators of diphenyl ether herbicides. The author theorized that if carotenoid
pigments were in fact responsible for the photoactivation process, plants treated with
norflurazon [4-chloro-5-(methylamino)-2-(3-(trifluoromethyl)phenyl)-3(2H)-
pyridazolinone], which inhibits carotenoid synthesis, should be tolerant to diphenyl ether
herbicides. The results of this study revealed that com seedlings treated with
norflurazon were partially tolerant to oxyfluorfen, lending evidence to the proposed

theory of carotenoid involvement.

15

Many theories have been proposed and examined concerning the mode of action of
the diphenyl ether herbicides, and in particular, acifluorfen. Orr and Hess (1981)
characterized the herbicide injury by acifluorfen in excised cucumber (Cucumis sativus
L.) cotyledons. Using 86Rb+ to monitor membrane disruption, the authors found that
upon exposure to light, “Rb" efflux increased, indicating a disruption of the cellular
membranes. Significant increases in efflux occurred within ten to fifteen minutes if
plants were kept in the dark after application, owing to the fact that the longer dark
period allowed more herbicide to reach its site(s) of action. The efflux of various
substances after treatment seemed to indicate that the disruption of the membranes was
general in nature. In a separate study, Orr and Hess (1982) attempted to investigate if
the formation of lipophilic free radicals, formed after acifluorfen application, were
responsible for the membrane disruptions. The authors found that acifluorfen was much
less active in cotyledons held in an atmosphere of nitrogen, and that with the addition
of alpha-tocopherol, a known scavenger of lipophilic free radicals, membrane damage
was reduced. Duke et al. (1984) proposed that after an initial interaction of acifluorfen
with a carotenoid, the mitochondria was involved in the production of free radical
species that caused the membrane disruption. Ensminger and Hess (1985) concluded
that photosynthesis was not involved in the mechanism of action of diphenyl ether
herbicides, which is in disagreement with Gillham and Dodge (1987) who found that
diphenyl ether herbicides were activated by photosynthetic electron transport in the
vicinity of ferredoxin. After accepting an electron from ferredoxin, the reduced

diphenyl ether then went on to initiate lipid peroxidation. Kenyon et al. ( 1985)

l6

monitored the lipid peroxidation indicators malondialdehyde and ethane after treatment
of cucumber cotyledon discs with acifluorfen and discovered an increase in their
concentration within 1 hour after exposure of the treated discs to light. The authors
also noted early decreases in photosynthesis after acifluorfen application, and proposed
that the chloroplast envelope may be a primary target of acifluorfen in the green
cucumber cotyledons. Matringe and Scalla (1988) proposed that the phytotoxicity of
diphenyl ether herbicides is caused by their ability to induce abnormal accumulations
of tetrapyrroles, which induce lethal photooxidative reactions. Tetrapyrroles are a
chemical group consisting of four pyrrole rings joined either in a straight chain or in
a ring. In their study, Matringe and Scalla found that acifluorfen exerts most of its
phytotoxicity in the blue region of the light spectrum, which matches the absorption
spectrum of protoporphyrin IX, a tetrapyrrole. This finding suggested that
protoporphyrins are the photoreceiptors that activate acifluorfen. These tetrapyrroles
are light-dependent generators of singlet oxygen which is responsible for the
peroxidation of membrane fatty acids. Witkowski and Halling (1988) also found an
increased level of photodynamic tetrapyrroles after treatment with acifluorfen. In a
different study, Matringe and Scalla (1988) enhanced the evidence of acifluorfen
induced porphyrin accumulation when it was demonstrated that cucumber cotyledons
treated with 4,6-dioxheptanoic acid, an inhibitor of tetrapyrrole biosynthesis, were more
resistant to the effects of diphenyl ether herbicides.

Protoporphyrinogen, an intermediate common to the heme and chlorophyll synthesis

pathways, is oxidized to protoporphyrin (Jacobs and Jacobs 1987). The enzyme that

17

catalyses this reaction is protoprophyrinogen oxidase, and Matrine et. a1. (1989) have
proposed that its activity is a target site of diphenyl ether herbicides. Jacobs et al.
(1991) found that protoporphyrinogen oxidase activity in barley root mitochondria and
etioplast extracts was more than 90% inhibited by assay in the presence of acifluorfen.
Sato et al. (1991) also found that acifluorfen enhanced the accumulation of
protoporphyrin IX.

Thifensulfuron. Thifensulfuron [3-[[[[(4-methoxy-1,3,5-triazin-2-
yl)amino]carbonyl]amino]sulfonyl]-2-thiophenecarboxylic acid] is a selective
postemergence herbicide used for control of broadleaf weeds in cereals and soybeans.
It soon may also be registered for use in corn.

Thifensulfuron belongs to the class of herbicides known as the sulfonylureas. This
class of herbicides is characterized by controlling a large spectrum of weeds at low use
rates. The potential for this chemical class to be used as herbicides was first reported
in 1966 (Beyer et al. 1987), but further research was minimal until the mid-1970’s
when George Levitt began to investigate the herbicidal properties once again. This
work led to further development of the sulfonylurea chemical class to be used as
herbicides.

The sulfonylurea molecule consists of three parts: (1) an aryl group, (2) a
sulfonylurea bridge, and (3) a heterocyclic ring (Hay 1990). Each of these three parts
contributes to the herbicidal activity of the compound. Those compounds with an
ortho-substituent on the aryl ring show the highest degree of herbicidal activity. The

changing of various substituent groups to different positions on the aryl, sulfonylurea

 

 

 

18

bridge, or heterocyclic parts of the molecule results in the diversity of compounds with
herbicidal activity within the sulfonylurea class.

Sulfonylureas are weak acids with pKa values ranging from 3 to 5 (43). Because
sulfonylureas are weak acids, pH greatly influences their water solubility and partition
coefficients. As the pH of the solution increases, more of the herbicide molecule will
exist in the deprotonated or anionic form which is more water soluble. This increase
in water solubility at higher pH values has the concomitant effect of lowering the
herbicide partitioning into octonal.

Sulfonylureas undergo two major chemical reactions that are relevant to their use
as herbicides: (1) hydrolysis and (2) salt formation (13). As weak acids, sulfonylureas
form stable metal salts in the presence of bases, alkali or alkaline earth hydroxides, or
carbonates (43). The hydrolysis reaction of sulfonylureas is greatly dependent on pH.
Hydrolysis is more rapid under acidic conditions when the herbicide molecule is in the
protonated form. Cleavage of the sulfonylurea bridge part of the molecule is the
predominant hydrolysis reaction, which results in the formation of the sulfonamide, the
aminoheterocycle, and carbon dioxide from the parent molecule.

In the soil environment, the sulfonylurea herbicides are subject to chemical
hydrolysis and microbial attack as the two main degradation pathways (Brown, 1990).
Blair and Martin (1988), in their review of the sulfonylurea class of herbicides, referred
to evaluation experiments using chlorsulfuron [2-chloro-N-[[(4-methoxy-6-methyl-1,3,5-
triazin-Z-yl)amino]carbonyl]benzenesulfonamide] to determine soil conditions optimal

for the degradation of the herbicide. The determining factor of chlorsulfuron

19

persistence was soil pH, with the most rapid degradation occurring at acid pH values.
It was concluded that under acid soil conditions, both hydrolysis and microbial activity
were functioning to degrade the chlorsulfuron, but under basic soil conditions,
hydrolysis was greatly reduced and microbial activity became the dominant factor, thus
resulting in a longer persistence of the chlorsulfuron. Brown (16) discussed a model
for sulfonylurea herbicide degradation originally proposed by Hamaker and Goring (41).
According to the model, when first applied the herbicide initially undergoes microbial
and chemical (hydrolysis) breakdown. Over time, a small amount of the herbicide
moves into soil compartments too small for microbial entry. Thus, after the microbial
populations have degraded all herbicide available to them, long term soil degradation
is dependent upon chemical breakdown, which is again a function of soil pH.
Numerous studies have been conducted to determine the mode of action of the
sulfonylurea herbicides. Conclusive evidence has been presented that identifies the
mode of action as the inhibition of the biosynthesis of the branched-chain amino acids
valine, leucine, and isoleucine. The sulfonylurea herbicides inhibit the first common
enzyme in the biosynthetic pathway of these amino acids; this enzyme is acetolactate
synthase, also known as acetohydroxyacid synthase. Beyer et al. (1988) described the
function of the acetolactate synthase enzyme as follows: acetolactate synthase catalyzes
the condensation of two molecules of pyruvate to form CO2 and (Jr-acetolactate, which
leads to valine and leucine synthesis, and the condensation of one molecule of pyruvate
with or-ketobutyrate to form CO2 and a-aceto-OL-hydroxybutyrate, which leads to

isoleucine formation. Chaleff and Mauvais (1984) conducted experiments in which

20

sulfonylurea susceptible and resistant tobacco plants were crossed and the ALS enzyme
from the progeny extracted and tested for sensitivity to chlorsulfuron and sulfrneturon
methy1[2-[[[[(4,6-dimethyl-2-pyrimidinyl)amino]carbonyl]amino]sulfonyl]benzoic acid].
Through the crosses, it was shown that the ALS enzyme proved to be the site of action
of the sulfonylureas as resistance or susceptibility was not accounted by another enzyme
or group of enzymes. Ray (1984) identified the site of action of chlorsulfuron to be the
ALS enzyme. Using excised pea (Pisum sativum) roots, as little as 2.8 uM of

chlorsulfuron inhibited root growth while supplementation of the media with 100 uM

each of valine and isoleucine protected the roots from growth inhibition. These results
showed that chlorsulfuron interfered with the synthesis of these amino acids, but
supplementation of. pyruvate and threonine in the presence of chlorsulfuron did not
alleviate growth inhibition so it was reasoned that the site of action was an earlier step
in the synthesis of these amino acids. Following extraction of the ALS enzyme,
treatment with chlorsulfuron was shown to strongly inhibit the activity of the enzyme,
with the resultant inhibition of the biosynthesis of valine, leucine, and isoleucine.
Research has determined the basis of selectivity of sulfonylurea herbicides to crop
species. Sweeter et al. (1982) investigated the selectivity of cereal crops to
chlorsulfuron. No correlations occurred between the amount of 14C-chlorosulfuron
penetration and translocation and the selectivity of tolerant and susceptible species. The
authors found a good correlation between plant sensitivity and the rate of herbicide
metabolism. Using sugar beet (Beta vulgaris L.) as a sensitive plant and wheat as a

tolerant plant, l"C-chlorsulfuron was applied to a leaf of each species. By 24 hours

21

after treatment, 97% of the radioactivity in the sugar beet leaf was recovered as parent
chlorsulfuron, while only 5% of the radioactivity in the wheat leaf was unmetabolized
chlorsulfuron. Brown and Neighbors (1987) investigated soybean tolerance to
chlorimuron ethyl [2-[[[[(4-chloro-6-methoxy-2-
pyrimidinyl)amino]carbonyl]amino]sulfonyl]benzoic acid]. Their results also point to
metabolism by tolerant species as the major factor determining selectivity. Soybeans
were shown to metabolize chlorimuron ethyl rapidly, with a half-life of only 1 to 3
hours. Susceptible species such as common cocklebur and redroot pigweed metabolized
the chlorimuron ethyl slower, with half-lives over 30 hours. Brown et al. (1990) then
investigated soybean tolerance to thifensulfuron methyl. The authors found the half-life
of thifensulfuron methyl in soybean seedlings to be 4 to 6 hours, while the half-life in
susceptible species such as velvetleaf, pigweed, and lambsquarters (Chenopodium album
L.) was typically greater than 36 hours. The conclusion was that rapid metabolism by
a deesterification reaction to yield herbicidally inactive thifensulfuron acid was the basis
for soybean tolerance.

Fielding and Stoller (1990) investigated the effects of various additives on
thifensulfuron methyl efficacy, uptake, and translocation using soybean, velvetleaf, and
common lambsquarters as the test species. The results showed that 28% UAN gave
greater control of velvetleaf than treatments without 28% UAN, but that adding 28%
UAN increased soybean injury and control of common lambsquarters was not improved.
In controlled environment studies, uptake of MC-thifensulfuron into velvetleaf without

the addition of an additive was less than 4% by 84 hours after treatment, but with the

22

addition of a nonionic surfactant or 28% UAN, uptake was increased to 33 and 45%,
respectively, 84 hours after treatment. When both nonionic surfactant and 28% UAN
were included in the spray solution, uptake was 76% by 84 hours after treatment.
Translocation was also enhanced when both additives were present. Beckett and Stoller
(1991) examined the effects of methylammonium and 28% UAN on foliar uptake of
thifensulfuron by velvetleaf and found that in the absence of nitrogen only 15% of the
applied l4C-thifensulfuron was taken up into the plant 24 hours after treatment, but with
the addition of UAN or methylammonium nitrate, uptake was increased to 38 and 59%,
respectively. Following these increases in uptake, translocation of l"C-thifensulfuron
also increased. Ahrens (1990) reported an enhancement of soybean injury and weed
control when thifensulfuron was tank-mixed with various organophosphate or carbamate
insecticides. Since thifensulfuron is metabolized in soybean by a deesterification
reaction, and the organophosphate and carbamate insecticides are esterase inhibitors,
the author postulated that the insecticides partially hindered the normal metabolic
detoxification process of thifensulfuron in soybean. Ahrens also speculated that if the
proper rate of herbicide and insecticide could be found, this interaction could enhance
weed control from thifensulfuron.

Corn has shown tolerance to applications of thifensulfuron and Eberlein et a1.
(1989) examined the tolerance of several corn genotypes to thifensulfuron. A tolerant
line (A671) and a susceptible line (A619) were selected to evaluate the mechanism of
tolerance by this crop. Differences in whole plant spray retention, differential

susceptibility of ALS, or differences in absorption did not explain the differential

23

tolerance between the two lines. However, 23% of the parent compound remained in
the treated leaves of the tolerant line (A671) 5.5 hours after treatment compared to 78%
of the parent compound remaining in the treated leaves of the susceptible line (A619).
Therefore, it was concluded that differential rates of metabolism was the basis for
different corn genotype tolerance of thifensulfuron.

Zhao et al. (1990) examined factors that affect thifensulfuron efficacy, using
velvetleaf and corn as test species. Velvetleaf was more susceptible to foliar
applications versus root exposure, and greatest control was achieved when treatments
were applied to young plants. Addition of nonionic surfactant increased velvetleaf
control and reduced loss of activity when rainfall occurred shortly after application.
Gast et al. (1990) examined the best application timing of thifensulfuron for wild garlic
(Allium vineale L.) control. Applications in late March to early April or when offset
bulb initiation began provided the best control of wild garlic and reduced the number
of new bulbs formed. Kells (1989) applied thifensulfuron for control of mayweed
chamomile (Anthemis cotula L.) in winter wheat (Triticum aestivum L.). Thifensulfuron
at 8.8 g ha'l controlled 89% or more mayweed chamomile at heights from 3 to 18 cm
with no visible wheat injury. Ostermeyer and Meier (1989) investigated thifensulfuron
use in pasture for control of Rumex species. Rumex species control exceeded 90% with
little damage to grasses in pastures.

Bentazon. Bentazon [3-(1-methylethyl)-(lH)-2,l,3-benzothiadiazin-4(3H)-one 2,2-
dioxide] is a selective postemergence herbicide used in a wide variety of crops for

broadleaf weed control. Bentazon is effective through contact action and thus thorough

24

spray coverage of the target plants is essential for good control.

Bentazon selectivity to tolerant crops has been investigated extensively and it
appears that no one factor completely governs this differential selectivity. Connelly et
al. (1988) examined the basis for soybean tolerance to bentazon among soybean
genotypes considered tolerant and those considered susceptible. Following the
application of 1 kg ha‘l of bentazon, the photosynthetic rate of all genotypes was
significantly reduced at 3 and 6 hours after bentazon application compared to the
untreated control plants. By 9 hours after application the photosynthetic rate of the
tolerant genotypes was not significantly different than the controls, but the susceptible
genotypes never recovered and photosynthesis stopped after 48 hours. Tolerant and
susceptible genotypes could not be separated on the basis of differences in the amount
of absorption or translocation of l4C-bentazon. Metabolites of l4C-bentazon were
compared from the tolerant and susceptible genotypes to determine if differential
metabolism was the basis of tolerance. The tolerant genotypes metabolized bentazon
to form glycosyl conjugates of 6- and 8- hydroxybentazon. Neither of these metabolites
were extracted from the susceptible genotypes. Thus, bentazon tolerance in soybean
was a function of a particular genotype’s ability to metabolize bentazon to these two
glycosyl conjugates. Wills (1976) determined the effect of environment on bentazon
toxicity to resistant and susceptible soybean cultivars. ’Hurrelbrink’ soybeans were
found to be more sensitive to bentazon than ’Hill’ soybeans in greenhouse studies.
Overall translocation of l4C-bentazon increased from 11 to 19% in Hill and from 48 to

62% in Hurrelbrink as the temperature increased from 24 to 35 C. There was also

25

greater basipetal translocation of l4C-bentazon in the Hurrelbrink cultivar than in the
Hill cultivar, giving overall greater translocation and a wider distribution of the
radiolabel in the susceptible Hurrelbrink cultivar. The author concluded that the
tolerance level of these two soybean cultivars was directly associated with the amount
of herbicide translocation occurring in the plant. Mahoney and Penner (1975) studied
the translocation and metabolism of bentazon in soybean and navy bean. F oliar
application of 14C-bentazon to the unifoliate leaves of navy bean resulted in both
acropetal and basipetal movement of the bentazon, while foliar applications made to the
center leaflet of the first trifoliate resulted in acropetal movement only. Metabolism of
the l4C-bentazon was much slower in the unifoliate leaves than in the center leaflet of
the first trifoliate after 1 day and it was theorized that both these factors, translocation
and metabolism, may explain why injury occurred in the field following bentazon
applications to unifoliate navy beans. Movement of l4C-bentazon in soybean was
primarily acropetal with some basipetal movement. Translocation and metabolism were
considered as the basis of selectivity. In a separate study, Mahoney and Penner (1975)
found that the trifoliate leaves of navy bean retained less bentazon from foliar
applications than the unifoliate leaves. Bentazon metabolism was similar in the
unifoliate and trifoliate leaves, however, a significantly greater amount of l4C-bentazon
remained as unmetabolized bentazon in the unifoliate leaves as compared to the
trifoliate leaves. Baltazar et al. (1984) reported on the selectivity of bentazon in hot
pepper (Capsicum chinense L.) and sweet pepper (Capsicum annuum L.). Yields in

1982 showed that all rates of bentazon, 0.6 to 6.7 kg ha" reduced sweet pepper yield,

26

but only the highest rate of bentazon significantly reduced hot pepper yields. Hill
reaction sensitivity was equally susceptible in both species due to similar Iso values, and
thus the authors concluded that hot pepper tolerance to bentazon was not due to
resistance at the chloroplast level. Irons and Burnside (1982) examined the absorption,
translocation, and metabolism of bentazon in sunflower (Helianthus annuus L.). The
addition of a surfactant to the spray solution greatly increased the absorption and
translocation of l4C-bentazon. Mine et al. (1975) examined the mechanism of bentazon
selectivity between the tolerant rice crop and the susceptible Cyperus serotinus.
Differences in absorption and translocation did not provide evidence for differential
selectivity as the tolerant rice crop actually absorbed more 1"C-bentazon than the
susceptible Cyperus serotinus and the translocation patterns were similar in both plants.
Metabolism of bentazon was then investigated as the possible mechanism of selectivity.
The tolerant rice plant metabolized 80-85% of the radioactivity within the first 24 hours
after treatment while the susceptible Cyperus serotinus metabolized only 20-30% of the
radioactivity after 7 days. Thus, difference in the rates of metabolism of the parent
compound between tolerant and susceptible species was the mechanism of bentazon
selectivity. Sterling and Blake (1988) examined bentazon metabolism using suspension-

cultured cells of soybean and velvetleaf. Following 6 hours of incubation in 1 uM of

bentazon, soybean and velvetleaf cells accumulated similar amounts of bentazon.
Extraction of the radiolabel revealed that all the radiolabel remained as parent bentazon
in velvetleaf cells whereas only 21% of the radiolabel remained as parent bentazon in

soybeans. Differential rates of metabolism explained the selectivity of these two species

27

to bentazon. Penner (1974) investigated the selectivity of bentazon between soybean
and Canada thistle (Cirsium arvense (L) Scop.). Soybeans tolerated 5.6 kg ha‘I of
bentazon under greenhouse conditions, but when plants were grown under conditions
of excess soil moisture, soybean injury from bentazon applications became apparent.
It was suggested that under excessive soil moisture conditions root absorption of
bentazon deposited on the flooded soil surface from the spraying application may have
caused the observed soybean injury. The photosynthetic rate of Canada thistle 3 hours
after bentazon application was reduced by 70% and respiration was also reduced one
day after treatment in both plant species. However, soybean respiration began to
recover by 6 days after treatment, possibly indicating metabolism of the bentazon to a
noninjurious metabolite. Spray retention was much greater in Canada thistle than
soybeans and the selectivity of‘bentazon between soybean and Canada thistle may be
due in part to differences in spray retention and rate of metabolism.

Mine and Matsunaka (1975) examined the mode of action of bentazon, classifying
it as a photosynthetic inhibitor from the results of their research. Bentazon was applied
to tolerant rice and susceptible Cyperus serotinus plants under flooded conditions as a
foliar or flooded-water treatment at a rate of 2 kg ha", which is a common application
rate for bentazon in Japan. Injury from bentazon application to Cyperus serotinus
appeared 6 to 9 days after a flooded-water treatment. Time needed for translocation
was the explanation given for the slow appearance of injury. Desiccation and death of
the treated leaves occurred within 2 to 4 days following foliar applications of bentazon.

The effects of bentazon on CO2 fixation and the Hill reaction were investigated. By 4

 

28

hours after a foliar application of 1000 ppm bentazon, inhibition of CO2 fixation was
approximately 90% in both species. The susceptible Cyperus serotinus showed no
recovery of photosynthetic capacity and the plants were killed in 9 days. Bentazon was
found to inhibit the Hill reaction in isolated chloroplasts by 50% at 4.0 x 10‘3 to 4.8 x
10'5 M concentration. If bentazon was actually inhibiting photosynthesis, the authors
questioned if an exogenous supply of carbohydrate material would lessen the effects of
bentazon] to the plant. Susceptible Cyperus serotinus plants that were supplied with
exogenous sucrose were much more tolerant to bentazon than were plants that received
no sucrose. Since bentazon inhibited the Hill reaction of photosynthesis and rapidly
inhibited photosynthetic CO2 fixation, herbicidal injury developed slowly in flooded-
water treatments, and exogenous carbohydrate supplies protected plants from bentazon
injury, Mine and Matsunaka concluded that the mode of action of bentazon was the
inhibition of photosynthesis. This conclusion was also given by BOger et al. (1977)
after studying the long-term effects of bentazon on the photosynthetic system in isolated
chloroplasts from the algae Bumilleriopsis filiformis.

Suwanketnikom et al. (1982) used isolated spinach (Spinacea oleracea L.)
chloroplasts to determine the location of electron transport inhibition . by bentazon.
From the results, the authors postulated that the site of inhibition was at the reducing
side of photosystem 11, between the primary electron acceptor Q and plastoquinone.

Potter and Wergin (1975) examined the role of light in bentazon toxicity to
cocklebur. The first visible signs of injury following an application of 0.25 kg ha'1 of

bentazon were necrotic spots on the leaves. The appearance of these spots was

 

29

accelerated by increasing the amount of light the plants received or increasing the rate
of bentazon. Plants that were placed in darkness for 48 hours after application never
developed necrosis. Net photosynthesis stopped within 1 hour after treatment and this
length of time was independent of the amount of illuminance. However, this time
period could be reduced further with increased rates of bentazon. The data suggested
that cellular damage was caused by photoinduced toxic by-products that resulted from
photosynthesis cessation.

Sterling et al. ( 1990) examined the uptake and accumulation of bentazon by
cultured soybean and velvetleaf cells in an attempt to determine the mechanism of
uptake and accumulation into cells. Uptake of bentazon into the cells was linearly
related to the external bentazon concentration and bentazon was able to freely diffuse
out of the cells, implying that movement into the cells was not carrier mediated. From
these results, the authors concluded that bentazon entered the cell by passive diffusion,
a process not requiring energy. Accumulation of bentazon in the cell did, however,
require metabolic energy and was also pH dependent, with greatest accumulation of
bentazon occurring at pH 4.6 and the least accumulation occurring at pH 6.6. Since
bentazon is a weak acid with a pKa of 3.45 and accumulation was found to be pH
dependent, the authors proposed an ion trapping phenomenon to explain bentazon
accumulation. Bentazon would diffuse across the cell membrane as an undissociated
molecule due to the lower pH of the extracellular environment, and then dissociate and
become "trapped" once inside the cell due to the higher pH environment.

In 1974, a report by Andersen et a1. presented the results of early field research

30

with bentazon concerning weed control and soybean tolerance. Their results showed
that optimal application timing for control of wild mustard (Brassica kaber (DC.) L. C.
Wheller), common ragweed, velvetleaf, Pennsylvania smartweed (Polygonum
pensylvanicum L.), common cocklebur, and common sunflower was when the soybeans
were in the first trifoliate growth stage. Addition of a nonionic surfactant to 0.84 kg
ha'1 of bentazon did not improve weed control because of the exceptionally good
control achieved with bentazon alone except when rainfall occurred after application.
In eight yield studies, soybean yields were not reduced by bentazon at up to 3.36 kg
ha‘1 as compared to the weed free control.

Growth chamber and greenhouse experiments by Nalewaja et al. (1975) determined
the influences of environmental factors on redroot pigweed control with bentazon with
and without oil (linseed and petroleum) adjuvants. Control of redroot pigweed by 0.4
kg ha“ of bentazon was reduced by simulated rainfall up to 24 hours after application
compared to the control of plants receiving no rainfall after application. Redroot
pigweed control was 60, 65, and 70% for bentazon alone, bentazon plus petroleum oil,
and bentazon plus linseed oil, respectively. The addition of oil adjuvants reduced the
influence of rainfall on the control of redroot pigweed. Control was reduced 21, 12,
and 7% for bentazon alone, bentazon plus petroleum oil, and bentazon plus linseed oil,
respectively. Wetting the plants after treatment with 180 L ha" of water increased
control of redroot pigweed, possibly by redissolving the herbicide which would allow
for further plant uptake. In growth chambers, the greatest control of redroot pigweed

was obtained under conditions of high relative humidity at 10 C. Oil adjuvants did not

 

31

increase redroot pigweed control at any temperature when the humidity level was high.
The addition of an oil adjuvant reduced the deleterious influence of low humidity on
redroot pigweed control. Nalewaja and Adamczewski (1977) found that a water-soluble
linseed oil formulation enhanced absorption and translocation of l4C-bentazon by
redroot pigweed more than did emulsifiable linseed oil, petroleum Oil, or a surfactant.
Uptake and translocation of l4C-bentazon was greater at 30 C than at 10 C and at high
humidity compared to low humidity. Doran and Andersen ( 1975) attempted to
determine the rain-free period required for acceptable weed control by bentazon and
also examined the possibility of using oil adjuvants to reduce the critical rain-free
period. The authors concluded that to avoid loss of activity from bentazon on velvetleaf
and common cocklebur, applications should be avoided if rainfall is expected within 24
hours. In greenhouse studies, adjuvants (vegetable and petroleum oil) reduced the
detrimental effects of rainfall after application of bentazon, and soybean yields were not
reduced by the application of 1.68 kg ha'l of bentazon plus an oil adjuvant.

Campbell and Penner (1982) reported enhanced injury to soybean and navy bean
from postemergence applications of bentazon plus organophosphate insecticides. Injury
occurred if the applications were split by as much as 48 hours. No corn injury was
observed from postemergence applications of bentazon plus insecticide or soil applied
insecticide prior to bentazon application. The proposed theory to explain the enhanced
soybean injury was the possibility of the insecticides interfering with bentazon
metabolism in soybean and navy bean.

Andersen and Koukkari (1978) examined the movement of velvetleaf leaves in

 

32

greenhouse and growth chamber studies during various periods of the day to determine
if these changes in leaf orientation might affect bentazon phytotoxicity. The leaves
were found to move from a horizontal position during the day to a near vertical position
at night. As the leaves departed from the horizontal position, the amount of bentazon
retained on the leaf surface decreased, with a concomitant decrease in velvetleaf control.
These results are similar to those obtained by Doran and Andersen (1976) who found
that bentazon effectiveness in controlling common cocklebur and velvetleaf was
partially dependent on the time of day the application was made. Velvetleaf control
was influenced more by the time of day the application was made than was common
cocklebur.

Herbicide Interactions. In recent years, applying two or more herbicides in
combination has become a common practice with crop producers. Several reasons exist
for combining herbicides including reduced application costs, an increase in the
spectrum of weeds controlled, increased crop safety by using lower amounts of each
herbicide, and to delay the appearance of resistant weed species. Many times, when
two or more herbicides are combined, the weed control spectrum appears as expected
based on the performance of each herbicide applied separately. However, in some
instances, the results of combining two or more herbicides may not be as expected. In
the latter case, the herbicides may be said to have interacted. In statistical terms, an
interaction may be defined as occurring when the effect of one factor is not independent
in the presence of another factor (Zar 1984). In other words, the difference in response

between the levels of one factor is not the same at all levels of the second factor.

 

 

33

Two scenarios of herbicide interaction are commonly encountered. In the first, a
particular herbicide may control a particular weed species when applied alone, but not
when tankmixed with a second herbicide. Alternatively, when two herbicides are
combined weed control can be far superior than when either of the herbicides are
applied alone. The first situation represents an interaction described as antagonistic,
while the second situation may be described as synergistic. Hatizos and Penner (42)
defined antagonism as "a type of joint action of two agrochemicals such that the
observed response of a test organism to their combined application appears to be less
thanthe response predicted to occur by an appropriate reference model." Synergism
was defined as the "cooperative action of two agrochemicals such that the observed
response of a test organism to their joint application appears to be greater than the
response predicted to occur by an appropriate reference model." The two reference
models commonly used in weed science are the additive model and the multiplicative
model. The additive model equates the response to the application of a combination
of chemicals to the sum of the response when each chemical is applied alone. The
multiplicative model equates the percentages or proportions of response to the
application of a combination of chemicals to the product of the corresponding
percentages or proportions when the two chemicals are applied alone.

The most widely used equation in weed science to describe the effect of a herbicide
mixture on plants is one proposed by Colby (24). The formula is:

_X*Y
100

E

 

34

where E, X, and Y are the expected response as a percent of control, response as a
percent of control obtained from herbicide A applied singly, and response as a percent
of control from herbicide B applied singly, respectively. If the observed responses are
equal, greater than, or less than the expected value, the herbicide combination exhibits
an additive, synergistic, or antagonistic response, respectively.

With increasing use of herbicides to control weeds, more herbicide combinations
will be developed. These combinations may interact, producing responses previously
described. An understanding of the mechanisms of interactions is essential in order to
provide crop producers with the necessary information required to choose herbicide
options for the weed spectrum in the field.

Additives. The efficacy of postemergence herbicides can often be enhanced by the
addition of an additive. Adjuvants may be defined as "any substance in a herbicide
formulation or added to the spray tank to improve herbicide activity or application
characteristics (WSSA Herbicide Handbook). Adjuvants are commonly used to enhance
foliar penetration of a herbicide, improve spray delivery to the plant foliage, and
increase retention of spray droplets on the plant foliage (76). The effectiveness of an
additive can be specific for a particular herbicide or weed species. Nalewaja and
Adamczewski (70) found greater absorption and translocation of l“C-bentazon in redroot
pigweed when applied with oil additives as compared to applications with no additive.
Irons and Burnside (47) Observed an increased rate and quantity of 14C-bentazon

absorption in common sunflower (Helianthus annuus L.) when applied with a

35

surfactant. Fielding and Stoller (35) noted improved velvetleaf control with
thifensulfuron when 28% UAN was added, and Kent et a1. (52) found enhanced
absorption and translocation of l“C-imazethapyr in pitted morningglory (Ipomoea
lacunosa L.) when applied with ammonium sulfate.

ConLnon lggweed. Common ragweed is a native species of North America (Bassett
and Terasmae 1962). The plant is an erect annual with a tap root system. Stems are
branched and hairy, bearing leaves on short petioles. The plant is monoecious with
male flowers born on terminal inflorescences and female flowers born in the leaf axils
(Ackerly and Jasieriski 1990). Ragweed are short-day plants, withholding flowering
until the day length has shortened to their requirements (Allard 1943).

Common ragweed is a pioneer weed species in abandoned fields and roadsides.
Raynal and Bazzaz (1975) studied the life-cycle strategy of common ragweed in
abandoned fields in Illinois. The plant typically occurred in disturbed sites for a single
growing season, during which time it produced seeds that provided a source of plants
for future years. When common ragweed was observed growing in competition with
neighboring winter annuals, there was no measurable increase in mortality, but there
was a measurable reduction in the size of the plants, which the authors termed as
phenotypic plasticity. Seed production of common ragweed was determined to be 18
seeds/plant when the ragweed was growing in competition with winter annuals and 182
when the winter annuals were removed. Raynal and Bazzaz (1975) also examined the
interference of the winter annuals Erigeron annuus and Conyza canadensis on common

ragweed in early successional fields. The winter annuals were shown to overwinter in

36

a rosette stage and when growth resumed in the spring, these weeds had a distinctive
spacial advantage both above and below the ground. This advantage translated into
enhanced light interception, water extraction, and nutrient usage early in the season by
the developed winter annuals. After establishment of common ragweed in fields with
winter annuals, the ragweed was observed to undergo population regulation through
phenotypic plasticity that enabled the available resources to be utilized by the existing
ragweed population. No density-dependent thinning was observed in common ragweed.
The ragweed that underwent this phenotypic plasticity, although smaller in size, did
produce seeds that would remain viable for many years, thus ensuring future
generations of common ragweed.

Willemsen and Rice (1972) investigated the mechanism of seed dormancy of
common ragweed, focusing on the relationship between germination inhibitors and
promotor compounds within the seed. Stratification for 8 weeks at 5 C broke seed
dormancy, while leaching and scarification did not. Germination was greater in light
than in darkness. Working with the hypothesis that auxin and gibberellin were
promoters of germination and that abscisic acid was a germination inhibitor, extractions
and assays were performed to determine if changes in concentration of these compounds
before and after stratification could explain the germination response. Following the
period of stratification, the concentration of abscisic acid declined while the
concentrations of auxin and gibberellin increased. The germination inhibitor was absent
in ragweed seeds placed in a favorable germination environment after the stratification

period. Exogenous gibberellin increased germination of dormant seeds and this effect

37

was greatly enhanced following scarification which suggested that the seed coat was
impervious to gibberellin. Since leaching did not enhance germination, the authors
concluded that the pericarp may contain a nonleachable inhibitor. The increase in
gibberellin following stratification may increase the activity of hydrolytic enzymes
which would make stored carbohydrates available to the developing embryo, while the
increased auxin content may stimulate protein synthesis. The inhibitor, abscisic acid,
may cause a reduction in enzyme activity necessary for germination. Willemsen (1975)
determined the effects of stratification and germination temperature on the germination
and induction of secondary dormancy in common ragweed seeds. The stratification
temperatures evaluated were -5, 4, and 10 C, with percent germination determined in
continuous darkness and light at 5/15, 10/20, 15/25, and 20/30 C, with the higher
temperatures corresponding to a 16 hour light period. The time of stratification ranged
from 3 to 15 weeks. The results indicated that optimal germination occurred after
stratification at 4 C in both light and darkness and at all temperatures. Stratification at
-5 C resulted in the least germination, and germination in light was superior to that in
darkness at all germination and stratification temperatures. The ability of the seeds to
germinate was enhanced by increased length of time of the stratification period up to
12 weeks. It was noted that germination decreased after 15 weeks of stratification when
seeds were placed at high germination temperatures, especially when germinated in the
dark. This was attributed to the onset of a secondary dormancy period which required
an additional period of stratification to break. Willemsen used this observation to

propose a model of how common ragweed succeeds in invading fields. Mechanical

38

disturbance of the soil in the spring exposed seeds to light and seeds germinated that
were stratified over the winter. This exposure in early spring when temperatures are
low may enhance common ragweed germination. Seeds that did not receive adequate
light for germination may enter secondary dormancy and thus insure a source of viable
seed in the following years. Willemsen (1975) continued his work by evaluating
common ragweed germination in the field. Stratified seeds were placed at 3 intervals
in the soil: on the soil surface, a depth of 5 cm, and a depth of 10 cm. There appeared
to be no differences in the time required for stratification at any of the soil levels used,
likely resulting from small differences in soil temperature at the various levels during
January. When the seeds began to germinate, germination was initially higher at the
higher temperatures on the soil surface. When secondary dormancy occurred, it was
first noted with seeds on the soil surface and decreased with increasing depth of burial.
The author did not attribute this to lack of light, low oxygen levels, or volatile-
inhibitors, but rather due to low moisture levels. With the ability of seeds near the soil
surface being readily able to germinate after the stratification over the winter and the
secondary dormancy of seed buried several centimeters below the soil surface, common
ragweed insures that there will be a supply of viable seeds in the soil for many years.
Baskin and Baskin (1980) found that germination of viable common ragweed seeds was
low when the seeds were buried in the soil while those seeds on the soil surface showed
good germination. The results indicated that failure of the buried seeds to germinate
in the spring was due to the low temperatures of the soil at depths below the soil

surface and the lack of light. Buried seeds did not germinate because they were

39

induced into secondary dormancy when the soil temperature was optimal for
germination but the seeds still lacked the required light. The seeds that entered
secondary dormancy required another period of stratification before they were once
again able to germinate, and by this mechanism of secondary dormancy, viable seed
could remain in the soil for several years.

Interference. Coble et al. (1981) described common ragweed interference in soybean.
Various densities of common ragweed (0, 2, 4, 8, or 16 equidistantly spaced plants/10
m of soybean row) were established to determine damage threshold populations for full-
season weed infestations. Based on linear regression calculations for soybean yield
reduction, a damage threshold of 4 weeds 10 ml Of soybean row was found to reduce
soybean yields compared to the weed-free control plots. Soybean yields were not
reduced if common ragweed were kept out of the crop for the first 4 weeks of the
growing season, or if common ragweed were allowed to grow from the time of crop
emergence and then eliminated prior to 8 weeks into the season. Plant height
measurements made at various times after crop emergence showed that common
ragweed were taller than the soybeans at all times measured. At 8 weeks after crop
emergence, the common ragweed plants intercepted 24% of the photosynthetically
active radiation, leading the authors to conclude that light competition may be a
contributing factor to soybean yield losses. Shurtleff and Coble (1985) also found that
common ragweed were substantially taller than soybean when grown at densities of 0,
2, 4, 8, and 16 weeds/10 m of row. Consistent with Coble et al. (23), Shurtleff and

Coble concluded that aboveground factors such as competition for light may be an

40

important contributing factor in the competitive ability of common ragweed.

10.

11.

12.

41

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47

Penner, D. 1989. The impact of adjuvants on herbicide antagonism. Weed
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48

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49

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50

Common ragweed (Ambrosia artemisiifolia) control in soybeans
(Glycine max ) with bentazon as influenced by imazethapyr or
thifensulfuron tankmixes.

AARON HAGER and KAREN RENNER

Abstract. Several herbicides control common ragweed postemergence in soybeans.
However, when herbicides are tankmixed, synergistic or antagonistic weed responses
may occur, and additive selection may influence weed response. Imazethapyr and
thifensulfuron can be tankmixed with bentazon for broadleaf weed control, and visual
crop injury and cost hectare" may be reduced compared to a bentazon plus acifluorfen
tankmix. Experiments determined common ragweed control from these herbicides and
herbicide combinations when applied with various additives. In the greenhouse
bentazon at 560 g ha’l reduced common ragweed dry weight 95 to 97%. Common
ragweed control with 35 g ha" of imazethapyr increased 13 and 7% when urea
ammonium nitrate (UAN) was applied with nonionic surfactant (NIS) or petroleum oil
adjuvant (POA), respectively. Tankmixing thifensulfuron at 4.5 g ha" or imazethapyr
at 35 or 71 g ha‘1 with bentazon did not enhance nor reduce dry weight of common
ragweed compared with bentazon alone. In field research bentazon at 1120 g ha'l
reduced common ragweed dry weight 78 to 81%. Tankmixing bentazon with
thifensulfuron at 4.5 g ha‘l or imazethapyr at 71 or 35 g ha“1 reduced common ragweed
dry weight by 89 to 91%, thus enhancing control compared to each herbicide applied

alone. Adding 28% UAN to bentazon plus imazethapyr or bentazon plus thifensulfuron

5 l
tankmixes did not increase control compared to N18 or POA alone. By 28 days after

treatment (DAT) only 35 or 71 g ha‘l of imazethapyr plus 28% UAN plus N18 or POA
and bentazon plus imazethapyr tankmixes provided 85% or more control of common
ragweed in 1991, and only 71 g ha‘1 of imazethapyr plus 28% UAN plus POA or 71
g ha" of imazethapyr plus bentazon provided greater than 78% common ragweed
control in 1992. Soybean yields were equal to that of the handweeded control only
with the bentazon plus 71 g ha"I of imazethapyr tankmix treatments.

Nomenclature: Bentazon, 3 -(methylethyl)-( lI-I)-2, 1 ,3 -benzothiadiazin-4(3H)—one

2,2,dioxide; imazethapyr, 2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-irnidazol-
2-yl]-5-ethyl-3-pyridinecarboxylic acid; thifensulfuron, 3[[[[(4-methoxy-6-methyl-
1,3,5-triazin-2-yl)amino]carbonyl]amino]sulfonyl]-2-thiophenecarboxylic acid;
acifluorfen, 5-[2-chloro-4-trifluromethyl)phenoxy]-2-nitrobenzoic acid; common

ragweed, Ambrosia artemisiifolia L., soybean, Glycine max (L.) Merr.

INTRODUCTION

No-till or reduced tillage soybean production are viable alternative soybean
production systems. Reduced tillage practices may increase the need for herbicides
since mechanical control options are limited. Postemergence weed control has become
an attractive option to many producers in reduced tillage systems.

Common ragweed is a predominant weed problem in soybeans and several

52

postemergence herbicides provide control. Bentazon at 1120 g ha'l controls four-leaf
common ragweed (20) and control can be improved by the addition of acifluorfen (19).
Acifluorfen also controls redroot pigweed (A maranthus retroflexus L.) and eastern black
nightshade (Solanum ptycanthum Dun.), two weed species not controlled by bentazon
(10, 17). Two other postemergence herbicides have recently been marketed that could
potentially replace acifluorfen in a tankmix with bentazon. Thifensulfuron controls
redroot pigweed (l, 5, 21), while imazethapyr controls redroot pigweed and eastern
black nightshade (3). Common ragweed control may not be adequate if these herbicides
antagonize the control provided by bentazon alone.

A reduction in weed control may be overcome by the addition of an additive (8,
13). Nalewaja and Adamczewski (16) found greater absorption and translocation of
14C-bentazon in redroot pigweed when bentazon was applied with oil additives as
compared to applications with no additive. Irons and Burnside (9) observed an increase
in the rate and quantity of I"C-bentazon absorption in common sunflower (Helianthus
annuus L.) when applied with a nonionic surfactant. Velvetleaf control with
thifensulfiiron was enhanced when 28% UANl was applied along with a nonionic
surfactant (6), and absorption and translocation of l4C-irnazethapyr in pitted
morningglory (Ipomoea lacunosa L.) increased when ammonium sulfate was applied
along with a surfactant (11).

Postemergence applications of diphenyl ether herbicides control common ragweed.

However, soybean injury often occurs and producers may desire to use a postemergence

 

1UAN, urea ammonium nitrate.

 

53

herbicide such as thifensulfiiron or imazethapyr that is not as injurious to the crop.
Tankmixing bentazon with thifensulfuron or imazethapyr may also lower the cost per
hectare compared to bentazon plus acifluorfenz.

The objectives of this research were to 1) determine if the addition of imazethapyr
or thifensulfuron to bentazon enhanced or reduced the common ragweed control
achieved by bentazon alone, and 2) determine if common ragweed control was

enhanced by the addition of various additives or additive combinations.

 

2Renner, K. A. 1993. SOYHERB: An integrated decision support computer
program. Version 2.2. CP-022, Michigan State University.

 

54

MATERIALS AND METHODS

Greenhouse studies. Locally collected common ragweed seed were planted in Baccto3
greenhouse potting soil in 946-ml plastic pots. Following emergence, the plants were
thinned to one plant per pot. Environmental conditions were maintained at 25 C i- 4
C with the plants grown in a 16-hour photoperiod of natural lighting supplemented with

metal halide lighting giving a midday photosynthetic photon flux density of 1000 “E

m’2 s". Pots were surface watered daily and fertilized once prior to herbicide
application with 0.1 g of water soluble fertilizer solution (20% N, 20% P205, 20%
K20). All herbicides were applied using a continuous link-belt sprayer equipped with
a single 8001 even flat fan nozzle4 calibrated to deliver 206 L ha'l at a spray pressure
of 248 kPa. Bentazon at 560 g ha", thifensulfuron at 4.5 g ha", imazethapyr at 35 or
71 g ha", and bentazon plus thifensulfuron and bentazon plus imazethapyr tankmix
combinations were applied to four-leaf common ragweed. Additives or additive

combinations included with the herbicide treatments were POAS, POA plus 28% UAN,

 

3Baccto Professional Potting Mix. Michigan Peat Company, PO Box 980129,
Houston, TX 77098.

4Teejet flat fan tips. Spraying Systems Co., North Ave. and Schmale Road,
Wheaton, IL 60188.

sHerbimax, 83% petroleum oil, 17% adjuvant, Loveland Industries, Inc. Greeley, Co
80632.

 

55
NIS", or NIS plus 28% UAN. Control of common ragweed was determined 14 DAT

by harvesting, drying, and weighing the common ragweed plants.

The experiment was a randomized complete block design with four replications,
repeated in time. Treatments were applied as two separate factorials with bentazon and
thifensulfuron and bentazon and imazethapyr. Data were converted to a percent of the
untreated control and subjected to arcsine transformation. ANOVA was performed and
means separated where appropriate using Fisher’s Protected LSD at the 5% level of
significance. Non-transformed means are presented. Analysis of the data revealed no
significant differences between experiments and the data are combined for discussion.
Field studies. Field experiments were conducted on a sandy clay loam soil at
Michigan State University in 1991 and 1992 (Table 1). Field plots were chisel-plowed
the previous fall and then spring disked and field cultivated. Elgin 87 soybeans were
planted May 14, 1991 and May 13, 1992 at a seeding rate of 407,000 seeds ha". Plot
size was 3 by 10 m in 1991 and 3 by 8.5 m in 1992 with a crop row spacing of 76 cm.
Herbicide treatments were applied with a tractor-mounted compressed airresearch plot
sprayer equipped with 80015 flat fan nozzles spaced 50 cm apart on a boom 56 cm
above the crop canopy. Herbicide treatments were applied in a total volume of 187 L
ha" at a spray pressure of 345 kPa. Herbicide treatments consisted of bentazon at 1120
g ha“, thifensulfuron at 4.5 g ha", imazethapyr at 35 or 71 g ha", and bentazon plus

thifensulfuron and bentazon plus imazethapyr tankmix combinations. Additives used

 

6X-77 Nonionic Surfactant. A mixture of alkylarylpolyoxy-ethylene glycols, free
fatty acids, and isopropanol. Valent U.S.A. Corp., Walnut Creek, CA 94956.

56

for the field experiments were the same as those in the greenhouse. Common ragweed
were at the four to six leaf growth stage (2 to 4 cm) at the time of herbicide
application. The density of common ragweed ranged from 40 to 70 plants m'2 in 1991
and 110 to 160 plants In2 in 1992. Air temperature was 18 C in 1991 and 24 C in
1992 and relative humidity was 65% in 1991 and 45% in 1992 at the time of treatment.
Rainfall in the 7 days prior to herbicide application totalled 0.6 cm in 1991 and 6 cm
in 1992 (Table 2). Treatments were applied 22 and 28 days after planting in 1991 and
1992, respectively.

Visual injury to soybeans and common ragweed were taken 14 and 28 DAT,
respectively. The rating scale ranged from 0 (no visible injury) to 100% (complete
plant death). Injury ratings were based on plant stunting, discoloration, chlorosis, and
necrosis. In addition, six common ragweed plants per plot that were at a uniform
growth stage (plant height and leaf number) were selected prior to herbicide application
and marked using plastic garden stakes’. These plants were harvested 14 DAT and dry
weights recorded. Each year the center two soybean rows were harvested with a
Massey 10 plot harvesting combine (October 10, 1991 and October 27, 1992) and seed
yields adjusted to 13.5% moisture.

The experimental design each year was a randomized complete block with four
replications. The results and discussion of common ragweed control will be based on
the dry weight data collected from plants marked prior to herbicide application unless

otherwise stated. ANOVA was performed on all data collected and means were

 

7Pylow Plastics, Inc. 211 Ogden Ave., PO. Box 505, Lisle, IL 60532.

 

57

separated where appropriate using Fisher’s Protected LSD at the 5% level of
significance. The common ragweed dry weight data were converted to a percent of the
untreated control prior to analysis and subjected to arcsine transformation. Analysis of
the dry weight reduction and soybean yield data revealed no significant differences
between years and the data are combined for discussion. Analysis of the common
ragweed dry weights and 28 DAT visual rating of common ragweed were subjected to
arcsine transformation and revealed differences between years and the data are therefore

presented separately. Non-transformed means are presented.

RESULTS AND DISCUSSION

Greenhouse studies. Bentazon reduced dry weight of common ragweed by 95 to 97%
with no difference between POA and POA + 28% UAN (Table 3). Ideal growing
conditions in the greenhouse may have improved bentazon efficacy, as such large
reductions in dry weight would not be expected under field conditions with 560 g ha"
of bentazon.

Thifensulfuron reduced common ragweed dry weight by 72 to 77% when applied
with POA, POA + 28% UAN, or NIS + 28% UAN (Table 3). Only a 58% reduction
in common ragweed dry weight resulted from thifensulfuron applied with NIS.
Tankmixing thifensulfuron with bentazon did not improve control over that of bentazon

alone, regardless of additive.

58

Common ragweed control was greater from 71 g ha'1 of imazethapyr compared with
35 g ha“l when applied with POA, POA + 28% UAN, or NIS (Table 4). Greatest
common ragweed dry weight reduction occurred from 71 g ha" of imazethapyr plus
POA + 28% UAN. Tankmixing imazethapyr with bentazon did not improve dry weight
reduction compared to bentazon alone or imazethapyr at 71 g ha“l plus POA + 28%
UAN.

These greenhouse results show that common ragweed control with bentazon was

not adversely effected when imazethapyr was added, regardless of additive. When
bentazon was tankmixed with thifensulfuron the addition of POA, POA + 28% UAN
or NIS + 28% UAN was necessary for common ragweed control to equal that of
bentazon alone.
Field studies. Only slight discoloration (yellowing) of plant tissue was observed 14
DAT from bentazon, thifensulfuron, imazethapyr, and tankmix treatments. Visual injury
from these treatments did not exceed 5% in 1991 and 11% in 1992 (data not presented).
Acifluorfen at 560 g ha’l caused 7% injury (leaf chlorosis and necrosis) in 1991 and
20% injury in 1992. The increased soybean injury in 1992 from acifluorfen may have
been the result of the higher temperature at the time of application. Higher
temperatures increase soybean and weed response to acifluorfen (12, 18).

Common ragweed dry weight 14 DAT was reduced 78 and 81% from 1120 g ha'1
of bentazon + POA and bentazon + POA + 28% UAN, respectively (Table 5). This
application rate of bentazon was twice that applied in the greenhouse, yet common

ragweed dry weights were 15% greater in this field study. Thifensulfuron reduced dry

59

weight of common ragweed 71 to 77%. There were no significant differences in
common ragweed dry weight reductions due to additive selection, as NIS alone was as
effective as the other additives (Table 5), thus differing from the greenhouse results
(Table 3). When bentazon was tankmixed with thifensulfuron dry weight of common
ragweed was reduced 89 to 91% which was significantly greater control than when
either of these herbicides were applied alone. The addition of 28% UAN did not
improve control over that of POA or NIS alone.

Imazethapyr at 35 or 71 g ha'l reduced dry weight of common ragweed 79 to 81%
with no differences between additives (Table 5). Increasing the imazethapyr rate did
not reduce common ragweed dry weight more, unlike the greenhouse studies. When
bentazon was tankmixed with 35 or 71 g ha‘1 of imazethapyr, dry weight of common
ragweed was reduced 89 to 91% which was significantly greater than that achieved with
either of these herbicides alone (Table 5). No differences were observed between
additives.

Visual evaluations of common ragweed control 28 DAT were less than 58% for
bentazon or thifensulfuron in 1991 and 1992, with no differences due to additives. This
lack of common ragweed control in the field by bentazon has been observed in previous
years (personal observation). Tankmixing bentazon with thifensulfuron + NIS or NIS
+ 28% UAN improved common ragweed control over that of bentazon or thifensulfuron
alone in both years (Table 5). However, common ragweed control was still less than
72%, and regrth of many common ragweed plants was evident.

Common ragweed control 28 DAT for imazethapyr alone was 76 to 87% in 1991

 

60
and 61 to 78% in 1992 (Table 5). Imazethapyr at 35 g ha" plus POA provided less

control both years, and only in 1991 did the addition of 28% UAN improve control to
equal that of 71 g ha‘1 of imazethapyr. Therefore, by 28 DAT an imazethapyr rate
response was evident that was not observed 14 DAT.

Imazethapyr plus bentazon controlled common ragweed longer than thifensulfuron
plus bentazon. This could be due to more complete control of the common ragweed
emerged at the time of herbicide application and/or greater soil residual control of the
common ragweed that germinated after herbicide application. The half-life of
thifensulfuron in the soil has been reported to be less than 1 week (4) while the half-life
of imazethapyr in the soil has been reported to be up to ten months (7). Since
acifluorfen which has limited soil activity provided 84 and 93% control 28 DAT in
1991 and 1992, respectively, regrowth of existing common ragweed is the more likely
explanation.

Soybeans treated with thifensulfuron, bentazon, and thifensulfuron plus bentazon
yielded less than the handweeded control (Table 5). Little soybean injury was observed
from these herbicide treatments and reduced yields were most likely a reflection of poor
common ragweed control at 28 DAT. Soybeans treated with 35 or 71 g ha'l of
imazethapyr or 35 g ha’1 of imazethapyr plus bentazon also yielded less than the
handweeded control (Table 5). Only soybeans treated with 71 g ha" of imazethapyr
plus bentazon plus POA, NIS, or NIS + 28% UAN had yields equivalent to the
handweeded control or the acifluorfen plus NIS treatment.

In the field studies, little visible soybean injury was observed 14 DAT from any

 

61

bentazon or thifensulfuron treatment. However, common ragweed control from
bentazon plus thifensulfuron, regardless of additive selection, would not be acceptable
since soybean yields were at least 18% below that of the handweeded control. Only
bentazon plus 71 g ha‘1 of imazethapyr plus POA, NIS, or NIS + 28% UAN controlled
common ragweed and resulted in soybean yield equal to the acifluorfen treatment or the
handweeded control. A bentazon plus imazethapyr tankmix would provide
broadspectrum weed control with little soybean chlorosis or necrosis but would not
reduce the cost per hectare as compared to that of bentazon plus acifluorfen since 71

ha'1 is not a reduced rate of imazetha yr.
3 P

62

LITERATURE CITED

Amuti, K. S., D. C. Drake, and G. L. Leek. 1985. DPX-M63l6: Postemergence
broadleaf weed control in corn. Proc. North Cent. Weed Cont. Conf. 40:47.

Andersen, R. N., W. E. Leuechen, D. D. Warnes, and W. W. Nelson. 1974.
Controlling broadleaf weeds in soybeans with bentazon. Weed Sci. 22:136-142.

Cantwell, R. S., R. A. Liebl, and F. W. Slife. 1989. Imazethapyr for weed control
in soybeans (Glycine max). Weed Technol. 31596-601.

Brown, H. M., M. M. Joshi, and A. Van. 1987. Rapid soil degradation of DPX-
M6316. Abstr. Weed Sci. Soc. Am. 27:75.

Farney, D. R., K. S. Amuti, J.S. Claus, J. D. Long, J. M. Green, L. L. Saari, H. M.
Brown, and S. Sebastian. 1988. DPX-M6316, A new postemergence soybean
herbicide: activity, selectivity, and soil residual considerations. Abstr. Weed Sci.
Soc. Am. 28:17.

Fielding, R. J. and E. W. Stoller. 1990. Effects of additives on the efficacy,
uptake, and translocation of the methyl ester of thifensulfuron. Weed Sci. 39:412-
416.

Goetz, A. J., T. L. Lavey, and E. E. Gbur Jr. 1990. Degradation and field
persistence of imazethapyr. Weed Sci. 38:421-428.

Hatzios, K. K. and D. Penner. 1985. Interactions of herbicides with other
agrochemicals in higher plants. Rev. Weed Sci. Vol. 121-63.

Irons, S. M. and O. C. Burnside. 1982. Absorption, translocation, and metabolism
of bentazon in sunflower (Helianthus annuus). Weed Sci. 30:255-259.

. Johnson W. 0., G. E. Kollman, C. Swithenbank, and R. Y. Yih. 1977. RH-6201

(Blazer): A new broadspectrum herbicide for postemergence use in soybeans. J.
Agric. Food Chem. 26:285-286.

. Kent, L. M. , G. D. Wills, and D. R. Shaw. 1991. Influence of ammonium

sulfate, irnazapyr, temperature, and relative humidity on the absorption and
translocation of imazethapyr. Weed Sci. 39:412-416.

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

 

13.

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63

Leuschen, W. E. and T. R. Hoverstad. 1991. Imazethapyr for weed control in no-
till soybeans (Glycine max). Weed Technol. 5:845-851.

Malefyt, T., P. A. Marc, K. Umeda, M. Los, and P. L. Orwick. 1984. AC
263,499 - A new broad spectrum herbicide for soybeans and other leguminous
crops. Abstr Weed Sci. Soc. Am. 24:18.

McWhorter, C. G. 1983. The use of adjuvants. pp 119-132. In R. H. Hodgson,
ed. Adjuvants for herbicides. Weed Sci. Soc. Am., Champaign, IL.

Nalewaja, J. D. and K. A. Adamczewski. 1977. Uptake and translocation of
bentazon with additives. Weed Sci. 25:309-315.

Quakenbush, L. S., and R. N. Andersen. 1985. Susceptibility of five species of
the Solanum nigrum complex to herbicides. Weed Sci. 33:386-390.

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

Sorensen, V. M., W. F. Meggitt, and D. Penner. 1987. The interaction of
acifluorfen and bentazon in herbicidal combinations. Weed Sci. 35:449-456.

WSSA Herbicide Handbook. 1989. Herbicide Handbook. 6th ed. Champaign, IL.

Zhao, C. C., J. R. Teasdale, and C. B. Coffinan. 1990. Factors affecting the
activity of thifensulfuron. Weed Sci. 38:553-557.

 

II

64

Table 1. Soil characteristics for field studies conducted in 1991 and 1992.

 

 

 

 

 

 

 

Year
1991 1992
Sandy clay loam Sandy clay loam
pH 7.0 pH 6.7
2.1% organic matter 2.5% organic matter
53% sand 25% silt 23% clay 55% sand 25% silt 23% clay
Capac Loam Capac Loam

 

 

65

Table 2. Rainfall accumulation at East Lansing in 1991 and 1992.

 

 

 

1991 1992

Time
cm

7 DBPa 0.4 0.7
Planting to 7 DBPO 4.0 6.0
7 DBPO to Day of PO 0.6 6.0
0-14 DAPO 3.0 3.0
14-28 DAPO 6.0 2.0

 

 

 

“Abbreviations: Days before planting (DBP), Days before posting (DBPO), Days after
posting (DAPO).

66

Table 3. Common ragweed dry weight reduction following application of bentazon,
thifensulfuron, and tankmix combinations in the greenhouse.

 

 

 

 

 

 

 

 

 

 

Additive
NIS POA
+ 28% - 28% + 28% - 28%

Rate UAN UAN UAN UAN
Herbicide -g ha"- ........ % of control ........
Bentazon 560 97 a" 95 a 97 a 96 a
Thifensulfuron 4.5 76 c 58 d 77 c 72 c
Bentazon + 560 + 4.5 97 a 79 be 97 a 93 ab
Thifensulfuron

 

 

 

bMeans followed by the same letter are not significantly different by Fisher’s
Protected LSD at the 0.05 level of significance. All mean comparisons within the table

are valid.

 

 

67

Table 4. Common ragweed dry weight reduction following application of bentazon,
imazethapyr, and tankmix combinations in the greenhouse.

 

 

 

 

 

 

 

—i'=
Additive
NIS POA
+ 28% - 28% + 28% - 28%
Rate UAN UAN UAN UAN
. . -g ha"-

Herbrcrde ------- % of control- ......
Bentazon 560 97 ac 95 ab 97 ab 96 ab
Imazethapyr 71 84 c 79 cd 91 b 81 c
Imazethapyr 35 81 c 68 e 80 c 73 de
Bentazon + 560 + 71 96 ab 97 ab 96 ab 96 ab
Imazethapyr
Bentazon + 560 + 35 97 ab 93 ab 97 ab 95 ab
Imazethapyr 4L

 

 

 

 

 

 

 

 

°Means followed by the same letter are not significantly different by Fisher’s
Protected LSD at the 0.05 level of significance. All mean comparisons within the table
are valid.

68

Table 5. Common ragweed dry weight reduction, visual evaluations 28 DAT, and soybean yield.

 

 

 

Treatment Rate (g ha") Dry wt Control 28 Soybean

reduction DAT yield

14 1991 1992
DATd
% -kg ha"-

Bentazon + POA 1120 + 1% 78 48 50 2700
Bentazon + POA + 28% 1120 + 1% + 4% 81 53 46 2820
Thifensulfuron + POA 4.5 + 1% 75 47 52 2870
Thif" + POA + 28% 4.5 + 1% + 4% 77 53 57 2930
Thifensulfuron + NIS 4.5 + 0.125% 73 51 57 2840
Thif + NIS + 28% 4.5 + 0.125% + 4% 71 53 53 2760
Bent + Thif + POA 1120 + 4.5 + 1% 89 57 66 2790
Bent + Thif + POA + 28% 1120 + 4.5 + 1% + 4% 89 70 63 2940
Bent + Thif + NIS 1120 + 4.5 + 0.125% 91 61 66 3030
Bent + Thif + NIS + 28% 1120 + 4.5 + 0.125% + 4% 91 71 66 3060
Imazethapyr + POA 71 + 1% 80 83 70 3390
Imazethapyr + POA + 28% 71 + 1% + 4% 80 87 78 3260
Imazethapyr + NIS 71 + 0.25% 81 83 76 3050
Imazethapyr + NIS + 28% 71 + 0.25% + 4% 81 85 74 3190
Imazethapyr + POA 35 + 1% 80 76 63 3320
Imazethapyr + POA + 28% 35 + 1% + 4% 79 85 61 3280
Bent + Imep + POA 1120 + 71 + 1% 91 88 85 3560
Bent + Imep + POA + 28% 1120 + 71 + 1% + 4% 90 91 82 3390
Bent + Imep + NIS 1120 + 71 + + 0.25% 90 95 81 3500
Bent + Imep + NIS + 28% 1120 + 71 + + 0.25% + 4% 89 92 84 3450
Bent + Imep + POA 1120 + 35 + 1% 91 85 69 3290
Bent + Imep + POA + 28% 1120 + 35 + 1% + 4% 91 86 75 3380

 

dDAT = Days after treatment.

cBent, Thif, and Imep are the approved codes for bentazon, thifensulfuron, and
1991. NCWSS Proc. 46:164.

imazethapyr, respectively.

69

Table 5 (cont).

 

Acifluorfen + NIS 560 + 0.125% 91 84 93 3450
Handweeded control 0 0 0 3730

 

LSD 0.,, 6 7 7 290

 

 

70

Influence of Relative Humidity and Soil Moisture on Absorption and
Translocation of Imazethapyr in Common Ragweed (Ambrosia
artemisiifolia).

AARON HAGER, KAREN RENNER, and DONALD PENNER

Abstract. Research was conducted to determine the influence of relative humidity
(RH), soil moisture, and the presence of bentazon on the absorption and translocation
of I4C-imazethapyr in common ragweed (Ambrosia artemisiifolia L.). Low soil
moisture (-3 bars) reduced l4C-imazethapyr absorption by 6% 24 h after treatment, but
did not influence translocation. The absorption and translocation of 14C-imazethapyr
when applied alone was not effected by RH 48 and 168 h after treatment. However,
when bentazon was present in the spray solution absorption 24 h after treatment was
reduced by 51% at 65% RH but was not reduced at 85% RH. Translocation of l"C-
imazethapyr out of the treated leaf was reduced significantly at 65% RH, but not at

85% RH in the presence of bentazon.

Nomenclature: Bentazon, 3-(1-methylethyl)-(1H)-2,1,3-benzothiodiazin-4(3H)-one 2,2-
dioxide; imazethapyr, 2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-lH-imidazol-

2-yl]-5-ethyl-3-pyridinecarboxylic acid; common ragweed, Ambrosia artemisiifolia L.

Additional index words: absorption, relative humidity, soil moisture, translocation

71

INTRODUCTION

For postemergence herbicides to exert a toxic affect, the herbicide must reach its
site of action. F oliar absorption of the herbicide through the plant cuticle proceeds by
passive diffusion (32). Environmental conditions, eg. RH and soil moisture, prior to
or at the time of herbicide application may affect the diffusion process by altering the
plant cuticle. Generally, as relative humidity and soil moisture increase the absorption
of herbicides also increases.

Wills (37) observed greater absorption and translocation of l4C-sethoxydim [ 2-[1-
ethoxyimino)butyl]-5-[2-(ethylthio)propyl]-3-hydroxy-2-cyclohexen- 1 -one] in common
bermudagrass (Cynodon dactylon (L.) Pers.) at 100% RH compared to 40% RH.
Absorption of l4C-acifluorfen [5-[2-chloro-4-(trifluoromethyl)phenoxy]-2-nitrobenzoic
acid] in showy crotalaria (Crotalaria spectabilis Roth) was three to four fold greater at
100% than at 40% RH (38). Ritter and Coble (28) observed a significant increase in
acifluorfen phytotoxicity and a decrease in common ragweed and common cocklebur
(Xanthium strumarium L.) dry weight at high RH (85%) as compared to lower RH
(50%). High RH (95-100%) increased control of kochia (Kochia scoparia (L.) Schrad.)
with imazethapyr [2-[4,5-dihydro-4-methyl-4-(1-methylethyl)-5-oxo-1H-imidazol-2-yl]-
5-ethyl-3-pyridinecarboxylic acid] compared to low RH (45-50%) (25). Absorption and
translocation of l“C-imazethapyr in pitted morningglory (Ipomoea lacunosa L.) was

greater at 100% RH than at 40% (18), and control of redroot pigweed (Amaranthus

 

72
retroflexus L.) with bentazon was greater at high RH (100%) than at low RH (30-52%)

(24).

Plants under moisture stress have been shown to absorb less herbicide than plants
grown under adequate soil moisture. Baker and Procopiou (3) found that a prolonged
period of soil moisture stress induced formation of a thicker plant cuticle. This
increased cuticular development may adversely effect the absorption of foliar applied
herbicides. Kells et al. (17) reported reduced quackgrass (Elytrigia repens (L.) Nevski)

control with fluazifop-butyl [{butyl ester of (i)-2-[4-[[5-(trifluoromethyl)-2-

pyridinyl]oxy]phenoxy]propanoate}] under moisture stress (6-10% (w/w)).
Autoradiographs of plants treated with MC-fluazifop-butyl suggested that more extensive
distribution of the herbicide occurred under adequate soil moisture. Low soil moisture
(-6.5 bars) following application of the methyl ester of diclofop [{2-[4-(2,4-
dichlorophenoxy)phenoxy]propanoic acid}] to wild oat (Avena fatua L.) reduced
herbicide activity as compared to the control achieved at higher moisture levels (-0.3
bars) (2). Dortenzio and Norris (8) noted a similar reduction in control of yellow
foxtail (Seteria glauca (L.) Beauv), wild oat, littleseed canarygrass (Phalaris minor
Retz.), and barnyardgrass (Echinochloa crus-galli (L.) Beauv.) at low soil moisture
conditions following the application of the methyl ester of diclofop. Reduced
absorption and control of barnyardgrass under low soil moisture conditions following
the application of glyphosate [N-(phosphonomethyl)glycine] has also been reported (1).

When herbicides are tankmixed, a reduction in absorption and translocation of one

of the herbicide components may occur. Several studies have reported reduced

 

73

herbicide absorption in the presence of bentazon (4, 5, 11, 16, 33, 34). Formation of
sodium salts of anionic herbicides when bentazon is present has been implicated as the
mechanism involved in reduced absorption (33).

The objectives of this research were to I) examine the effects of soil moisture and
RH on the absorption and translocation of imazethapyr in common ragweed, and 2)
determine what effect the addition of bentazon had on imazethapyr absorption and

translocation in common ragweed.

MATERIALS AND METHODS

Greenhouse studies were conducted to examine the effects of RH and soil moisture
level on the absorption and translocation of l4C-irnazethapyr applied alone or tankmixed
with bentazon. Treatments were arranged in a factorial design with two humidity levels
and two soil moisture levels. Locally collected common ragweed seeds were planted
in 0.5-L plastic pots containing a sandy clay loam soil (64% sand, 21% silt, 12% clay)
with a pH of 6.9 and 2.1% organic matter. Plants were initially grown under

greenhouse conditions at 25 C :4 C with a 16 hour photoperiod of natural lighting
supplemented with metal halide lighting giving a midday photosynthetic photon flux

density of 1000 uE m‘2 s". Pots were surface watered daily.

The soil used in these experiments was air dried for 7 days prior to planting, and

 

74

450 g of soil was then placed into each pot. Through previous research by Zollinger‘,
a soil moisture retention curve was developed for this particular soil. Using this curve,
the soil water content (g/g) was determined for the high soil moisture level (- 1/3 bars
(field capacity)) and the low soil moisture level (-3 bars). Seven days prior to herbicide
application the plants were transferred into growth chambers with day/night

temperatures of 26/22 C. Experiments at high RH were conducted at 85% i- 4% while
experiments at low RH were conducted at 65% i 4%. The grth chamber was

initially programmed at 85% RH. The experiment was repeated at this humidity, and
then the chamber was programmed to the lower RH (65%) and the experiment
conducted twice. The chamber was maintained at a 16-hour photoperiod from
fluorescent and incandescent lighting with a photosynthetic photon flux density of 750
[LE m‘2 s“. Pots were watered daily to 19 and 9% (wt/wt) soil moisture which
corresponds to soil moisture potentials of -‘/3 and -3 bars, respectively. These values
were established from the soil moisture curve described previously.

At the time of herbicide application the common ragweed plants were at the 6 leaf
growth stage. A single 2 ”L droplet containing 1110 Bq of l“C-imazethapyr was
applied to one of the four true leaflets of the common ragweed using a microsyringez.
The spotting solution contained l‘lC-irnazethapyr (pyridine ring-labelled, 6th position with
a specific activity of 784.4 MBq/g) with the appropriate amounts of formulation blank,

petroleum oil adj uvant, bentazon (when appropriate) and distilled water to simulate a

 

lZollinger, Richard K. 1989. Perennial sowthistle (Sonchus arvensis L.) distribution,
biology and control in Michigan. Ph.D. Dissertation, Michigan State Universrty.

2Hamilton microsyringe. Hamilton Co. Reno, NV 89520-0012.

75

spray solution of 71 g ha" of imazethapyr, 1120 g ha‘1 of bentazon (when appropriate),
and POA at 1% (v/v) in a total spray volume of 206 L ha".

Plants were harvested at 0, 24, 48, and 168 hours after treatment. At each harvest
time, the treated leaf was excised and rinsed in a 20 ml glass scintillation vial
containing 3 ml of methanol : distilled water (2:1 v/v) for 45 seconds to remove
herbicide not absorbed into the cuticular wax. The unabsorbed l4C-imazethapyr was
quantified using liquid scintillation counting3. Plants were then removed from the soil
and the soil was removed from the roots by gentle shaking. Plants were sectioned into
the following parts: treated leaf, above the treated leaf, growing point, below the treated
leaf, and roots. The plant parts were combusted in a biological oxidizer4 and the 14CO2
trapped in a solution of scintillator : CO2 absorber5 (2:1 v/v). All samples were
quantified by liquid scintillation counting. Herbicide absorption was calculated by
subtracting the amount of unabsorbed l4C-herbicide recovered in the leaf wash from the
total amount l"C-herbicide applied. Herbicide translocation out of the treated leaf was
calculated by summation of the total radioactivity recovered in all plant parts, which
represents 100% of the radioactivity recovered within the plant. The quantity of
radioactivity remaining on the treated leaf was then subtracted from the total amount
of radioactivity recovered within the plant to determine translocation out of the treated

leaf. Recovery of 14C averaged 84% for the experiments.

 

3Model 1500. Packard Instrument Corp. Downers Grove, IL 60515.
4OX-300. R.J. Harvey Instrument Corp. Patterson, NJ 07642.

5Carbo-Sorb II. Packard Instrument Corp. Meriden, CT 06450.

 

Ill

76

The experiment was arranged as a completely randomized design with four
replications, repeated in time. All data were subjected to AN OVA and data presented
are combined over time because there were no treatment by time interactions. Mean

comparisons were made by Fisher’s Protected LSD at the 5% level of significance.

RESULTS AND DISCUSSION

Common ragweed plants grown at field capacity and -3 bars were indistinguishable
in physical appearance when RH was 85%. When the RH was decreased to 65%,
plants grown at -3 bars were reduced in size (plant height and leaf area but not growth
stage) compared to those grown at field capacity in the 7 day period prior to herbicide
application. Within a given soil moisture, the plants were uniform.
Absorption. Low soil moisture limited absorption of l4C-imazethapyr in common
ragweed. When averaged over the presence of bentazon and RH, absorption of 1"C-
imazethapyr decreased by 6, 13, and 7% at -3 bars 24, 48, and 168 h after treatment,
respectively (Table 1). Bruce et al. (6) found enhanced absorption of l4C-nicosulfuron
[2-[[[[(4,6-dimethoxy-2-pyrimidinyl)amino]carbonyl]amino]sulfonyl]-N,N-dimethyl-3-
pyridinecarboxamide] in quackgrass as soil moisture increased from -5 bars to -0.3 bars
at day/night temperatures of 26/16 C, but no differences in absorption at 11/6 and 31/26
C. Greater absorption of l4C-glyphosate in johnsongrass (Sorghum halepense (L) Pers.)

and soybean (Glycine max (L)) occurred at field capacity than at a soil moisture near

 

77

the permanent wilting point (23). Absorption of 14C-glyphosate by barnyardgrass was
20% of that applied at 10% soil moisture whereas absorption was 62% at 40% soil
moisture (1).

Absorption of l4C-imazethapyr was influenced by RH and the presence of bentazon
(Tables 1 and 2). At 24 h after treatment, 16% more l4C-imazethapyr was absorbed at
65% RH than at 85% RH in the absence of bentazon (Table 2). These results are in
contrast to those of Kent et al. (18) who found greater absorption of l"C-imazethapyr
in pitted morningglory at 100% RH than at 40% RH. By 48 and 168 h after treatment
absorption of l‘lC-imazethapyr in the absence of bentazon was similar at 85 and 65%
RH. This increased absorption of l4C-imazethapyr at low RH cannot readily be
explained.

The presence of bentazon in the spray solution decreased the absorption of MC-
imazethapyr 51, 35, and 35% at 24, 48, and 168 h after treatment, respectively, at 65%
RH (Table 2). Absorption of l"C-imazethapyr was only reduced 14% 48 h after
treatment when bentazon was applied with 1“C-irnazethapyr at 85% RH. Absorption of
I“C-imazethapyr in the presence of bentazon reached a maximmn of 57% of that applied
at 168 h after treatment at 85% RH.

Translocation. There were no differences in translocation of l4C-irnazethapyr due to
soil moisture (Table 1). Kells et al. (17) reported no differences in translocation of 1"C-
fluazifop-butyl in quackgrass under adequate soil moisture (field capacity) compared
to plants grown under moisture stress (6-10% (w/w)), and Akey and Morrison (2)

reported that soil moisture level did not effect the translocation of l4C-diclofop in wild

 

78

oat. Similarly, Dortenzio and Norris (8) reported no consistent changes in l"C-diclofOp
translocation in wild oat at differing soil moisture levels.

Translocation of l4C-irnazethapyr out of the treated leaf was influenced by RH and
the presence of bentazon 48 and 168 h after treatment (Tables 1 and 2). In the absence
of bentazon, no differences in 1"C-imazethapyr translocation occurred. McWhorter (21)
reported no differences in translocation of l"C-metriflufen [2-[4-(4-
trifluoromethy1phenoxy)phenoxy]propanoic acid] in johnsongrass at 100 or 40% RH at
35 C. Ritter and Coble (28) concluded that humidity level was not a critical factor
affecting translocation of l4C-acifluorfen in common cocklebur. Lauridson et al. (20)
observed no differences in translocation of l‘lC-picloram [4-arnino-3,5,6-trichloro-2-
pyridinecarboxylic acid] or 14C-dicamba [3,6-dichloro-2-methoxybenzoic acid] in
Canada thistle (Cirsium arvense (L) Scop.) at three soil moisture levels ranging from
-6.6 to -15 bars.

Bentazon reduced the translocation of l4C-imazethapyr out of the treated leaf at 65%
RH only (Table 2). At 65% RH, 29 and 26% more l"C-imazethapyr remained in the
treated leaf in the presence of bentazon than when l"C-imazethapyr was applied alone
at 48 and 168 h after treatment, respectively (Table 2).

These results of decreased absorption and translocation of imazethapyr in the
presence of bentazon are in agreement with those reached by Bauer et al. who reported
reduced absorption and translocation of l‘lC-imazethapyr in pinto bean (Phaseon
vulgaris L.) (4) and redroot pigweed (5) when tankmixed with bentazon. Numerous

other studies have also reported reduced herbicide absorption in the presence of

79

bentazon (11, 16, 33, 34). It has been proposed that the formation of a sodium salt of
sethoxydim in the presence of sodium bentazon may be the mechanism responsible for
the reduced absorption of sethoxydim in quackgrass (33). Bauer et al. (4) demonstrated
a similar reduction in absorption of l4C-imazethapyr in pinto bean when applied with
either sodium acetate or sodium bentazon. Imazethapyr is a phloem mobile herbicide
and translocation of a phloem mobile herbicide may be reduced when the production
and translocation of photoassimilates is inhibited (10, 7). Bentazon inhibits
photosynthesis, thereby inhibiting the non-cyclic flow of electrons in photosystem II (9,
31). This block of electron flow would inhibit the reduction of NADP+ to NADPH at
the reducing end of photosystem I, thus depleting the source of reducing equivalents for
CO2 fixation during the carbon assimilation reactions of photosynthesis. Ultimately,
photoassimilation and movement into the phloem would cease and concomitantly reduce
the translocation of a phloem mobile herbicide such as imazethapyr.

Experiments investigating the effects of RH on herbicide absorption are typically
conducted at only two levels, a low level (generally less than 40%) and a high level
(generally greater than 85%) (13, 15, 18, 21, 22, 23, 28, 29, 35, 36, 37, 38, 40). The
herbicides and weed species evaluated have been diverse, however, none of these
experiments have reported decreased herbicide absorption at lower RH. From these
studies, the response of herbicide absorption to increasing RH appears to proceed in a
linear manner. However, evaluations using only two levels of humidity may not
correctly indicate a linear response. Smith and Buchholtz (30) and Wills et al. (39)

demonstrated reduced rates of transpiration in resistent and susceptible species using

 

80
high rates of atrazine [6-chloro-N-ethyl-N’ -(1-methylethyl)-1,3,5-triazine-2,4-diamine].

In contrast, increases in plant growth and plant protein have been reported following
exposure to micromolar concentrations of s-triazine herbicides in laboratory studies (12,
14, 27). Ladlie et al. (19) examined the effect of atrazine on soybean tolerance to
metribuzin [4-amino-6-(1 , 1-dimethylethyl)-3-(methylthio)-1 ,2,4-triazin-5(4H)-one].
Atrazine at 10‘6 M was found to reduce translocation and stomatal opening in soybean.
This reduced the uptake of 14C-metribuzin into the shoot of the soybean. When the
atrazine concentration was reduced to 10‘7 or 10'8 M, transpiration was increased over
that of the control and more metribuzin moved into the shoot. While others had
demonstrated reduced rates of transpiration at high atrazine concentrations (30, 39),
Ladlie et al. (19) demonstrated that over a wide range of atrazine concentrations,
atrazine actually reduced and enhanced transpiration rates in soybean. The response of
transpiration to atrazine concentration would not be explained as a linear response, but
rather as a quadratic response.

In studies examining the effects of RH, weed species, herbicide selection, and spray
additives may all influence the pattern of herbicide absorption or level of weed control.
This was most clearly demonstrated in the work of Reed et al. (26) who investigated
the influence of RH and temperature on the phytotoxicity of thifensulfuron [3-[[[[(4-
methoxy-6-methyl-1,3,5-triazin-2-yl)amino]carbonyl]amino]sulfonyl]2-
thiophenecarboxylic acid] and surfactant applications on two soybean cultivars (Asgrow
1937, Williams 82) and three weed species (velvetleaf (A butilon theophrasti Medicus),

ivyleaf morningglory (Ipomea hederacea (L) Jacq.), common lambsquarters

 

81

(Chenopodium album L.). Examination of fresh weight data averaged over all
thifensulfuron treatments indicated that ivyleaf morningglory fresh weight was
significantly reduced at 30 C and 35% RH over that at 30 C and 80% RH. This
observation was reversed at 15 C. Velvetleaf fresh weight was reduced more at 30 C
and 80% RH than at 30 C and 35% RH, however, there were no differences at 15 C.
Furthermore, no differences in common lambsquarters fresh weight were observed at
either RH regardless, of temperature. When the data was examined for all
thifensulfuron treatments and environmental conditions, more differences became
apparent. Greater reductions in fresh weight of velvetleaf occurred at lower RH with
thifensulfuron plus 0.25% (v/v) NIS at 15 C and thifensulfuron plus 0.25% NIS plus
28% N (4% (v/v)) at 30 C. Similarly, greater reduction in fresh weight of ivyleaf
morningglory occurred at low RH with thifensulfuron plus 0.25% NIS at 30 C,
thifensulfuron plus 0.125% NIS at 15 C, and thifensulfuron plus 0.25% NIS plus 28%
N at 15 C, while common lambsquarters fresh weight was reduced more at low RH
with thifensulfiiron plus 0.25% NIS at 15 C and thifensulfuron plus 28% N at 30 C.
It was demonstrated by Kent et al. (18) that the reduced absorption of 14C-imazethapyr
in pitted morningglory at low RH could be increased 32% by the addition of
ammonium sulfate.

It can be concluded that herbicide absorption as influenced by RH is dependent on
the herbicide, additive, and weed species evaluated. It would be difficult to conclude
from studies of RH at only two levels that herbicide absorption proceeds as a linear

response or as some other response such as quadratic. If herbicide absorption actually

 

82

proceeds as a quadratic response, maximal absorption of l4C-imazethapyr in common

ragweed may occur at approximately 65% and then decline slightly at higher RH.

 

10.

ll.

12.

83

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and water stress on barnyardgrass (Echinochloa crus-galli) control and on
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Akey, W. C. and I. N. Morrison. 1983. Effect of moisture stress on wild oat
(Avena fatua) response to diclofop. Weed Sci. 31:247-253.

Baker, E. A. and J. Procopiu. 1980. Effect of soil moisture status on leaf surface
wax yield of some drought-resistant species. J. Hort. Sci. 55:85-87.

Bauer, T. A., K. A. Renner, and D. Penner. 1993. ’Olathe’ pinto bean (Phaseolus
vulgaris) response to postemergence imazethapyr and bentazon. Weed Sci.
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Bauer, T. A., K. A. Renner, and D. Penner. 1993. Response of selected weed
species to postemergence imazethapyr and bentazon. Weed Sci. (Submitted).

Bruce, J. A., D. Penner, and J. J. Kells. 1993. Foliar absorption, translocation,
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Devine, M. D., H. D. Bestrnan, and W. H. Vanden Born. 1990. Physiological
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Dorenzio, W. A. and R. F. Norris. 1980. The influence of soil moisture on the
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Fuerst, E. P. and M. A. Norman. 1991. Interactions of herbicides with
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Geiger, D. R. and H. D. Bestrnan. 1990. Self-limitation of herbicide mobility by
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Gerwick, B. C. III. 1988. Potential mechanisms for bentazon antagonism with
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Hiranpradit, H., C. L. Foy, and G. M. Shear. 1972. Effects of low levels of
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Jordan, T. N. 1977. Effects of temperature and relative humidity on the toxicity
of glyphosate to bermudagrass (Cynodon dactylon). Weed Sci. 25:448-451.

Jordan, D. L., A. C. York, and F. T. Corbin. 1989. Effect of ammonium sulfate
and bentazon on sethoxydim absorption. Weed Technol. 3:674-677.

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

Kent, L. M., G. D. Wills, and D. R. Shaw. 1991. Influence of ammonium sulfate,
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87

Table 1. Effect of soil moisture, RH, and bentazon on absorption and translocation
of imazethapyr in common ragweed.

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Absorption Translocation
----------- Time (h)----------
Factor 24 48 68 24 48 168
l Soil Moisture

-1/3 bars 47 49 56 22 28 28

-3 bars 41 36 49 17 25 27
Significance * ** ** NS NS NS
Relative Humidity

65% 41 40 43 21 20 27

85% 47 45 61 18 32 29
Significance * * ** NS ** NS
Herbicide Trmt

Imazethapyr 58 54 63 27 34 36

Imazethapyr + Bentazon 30 30 42 12 18 19
Significance ** ** *il! *3? *Ik **
Soil Moisture "‘ RH NS NS NS NS NS NS
Soil Moisture * Herb NS NS NS NS NS NS
RH * Herb ** *III *II‘ NS #43 *
Soil Moist * RH * Herb NS NS NS NS NS NS

 

Table 2. Effect of RH and bentazon on absorption and translocation of imazethapyr in

common ragweed.

88

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

 

Absorption Translocation
Time (h)

Factor 24 48 168 24 48 168 1

RH * Herb Trmt I
Imep 65% RH 66 57 61 30 35 40
Imep + Bent 65% RH 15 22 26 12 6 14
Imep 85% RH 50 52 65 24 34 33
Imep + Bent 85% RH 45 38 57 12 31 24
Significance 12 12 1 1 NS 6 14

 

 

 

1'1 HIGRN STRTE LIBRQRIES

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