American Journal of Plant Sciences, 2012, 3, 1613-1618
http://dx.doi.org/10.4236/ajps.2012.311195 Published Online November 2012 (http://www.SciRP.org/journal/ajps)
1613
Broadleaf Weed Control and Crop Safety with Premixed
Pyrasulfotole and Bromoxynil in Winter Wheat
Seshadri S. Reddy1*, Phillip W. Stahlman1, Patrick W. Geier1, Dallas E. Peterson2
1Agricultural Research Center, Kansas State University, Hays, USA; 2Department of Agronomy, Kansas State University, Manhattan,
USA.
Received August 4th, 2012; revised September 17th, 2012; accepted October 15th, 2012
ABSTRACT
For more than two decades acetolactate synthase (ALS) inhibiting herbicides have been the major weed control tools in
winter wheat which resulted in selection of resistant weeds to those herbicides. Premixed pyrasulfotole & bromoxynil
(Huskie®) is a relatively new herbicide registered for use in wheat in 2008. Pyrasulfotole inhibits 4-hydoxyphenylpy-
ruvate dioxygenase (HPPD) enzyme in susceptible plants and is the first significant new mode of action for use in cere-
als in more than two decades. Field experiments were conducted from 2007 to 2010 at two locations in Kansas, USA to
test the efficacy of pyrasulfotole & bromoxynil for broadleaf weed control and crop safety in winter wheat. Treatments
included pyrasulfotole & bromoxynil alone at 253 g·ai·ha−1 and tank mixtures of pyrasulfotole & bromoxynil at 207
g·ai·ha−1 with MCPA at 280 g·ai·ha−1, dicamba at 140 g·ai·ha−1 or metsulfuron-methyl at 4.2 g·ai·ha−1. Herbicides were
applied postemergence in fall and spring seasons. Pyrasulfotole & bromoxynil alone or in combination with tank-mix
partners, regardless of application time, controlled flixweed, blue mustard, bushy wallflower and field pennycress 98%
or more. Henbit control was better when pyrasulfotole & bromoxynil treatments were applied in fall than spring (≥98%
vs ≥67%). Pyrasulfotole & bromoxynil alone applied in spring was not effective on wild buckwheat, but tank mixing
with dicamba or metsulfuron-methyl controlled wild buckwheat 84% or more. Pyrasulfotole & bromoxynil alone or in
tank mixtures caused little (≤7%) or no injury to wheat and the injury did not influence wheat grain yields. Based on
excellent control of broadleaf weeds evaluated, pyrasulfotole & bromoxynil is an alternative tool to control
ALS-inhibitor resistant weeds in winter wheat. Fall season application and tank mixing with other herbicides are desir-
able for effective broad spectrum weed control.
Keywords: Huskie®; Henbit; Blue Mustard; Flixweed; Bushy Wallflower; Field Pennycress; Wild Buckwheat;
Postemergence; Injury
1. Introduction
Wheat (Triticum aestivum L.) is the most important ce-
real crop in the USA, where it was planted on 23 million
ha in 2012 [1]. Most of the wheat grown in the USA is
winter wheat (17 million ha). Kansas state ranks first in
winter wheat cultivation (4 million ha) in the USA [1].
Winter wheat is not a good competitor with some broad-
leaf weeds even when wheat emerges before weeds [2].
Common weeds found in winter wheat in the US are blue
mustard [Chorispora tenella (Pallas) DC.], henbit (La-
mium amplexicaule L.), flixweed [Descurainia sophia
(L.) Webb. Ex Prantl], bushy wallflower (Erysimum re-
pandum L.), field pennycress (Thlaspi arvense L.), wild
buckwheat (Polygonum convolvulus), shepherd’s purse
[Capsella bursa-pastoris (L.) Medik.], and pinnate tan-
symustard [Descurainia pinnata (Walt.) Britt.]. Their
interference can cause significant yield reduction in win-
ter wheat. Season-long competition of 11, 33, and 98
blue mustard plants·m−2 reduced wheat grain yields by
28%, 42%, and 51%, respectively [2]. Conley and Brad-
ley (2005) [3] reported yield reductions of 13 and 38%
because of henbit interference at 82 and 155 plants·m−2,
respectively. Northam et al. (1993) [4] also reported
wheat grain yield loss of 48% with 221 henbit plants·m−2.
Bushy wallflower at 272 plants·m−2 reduced wheat yields
by 25% [5]. Hence, winter annual broadleaf weed control
is very important for successful wheat production.
For more than two decades acetolactate synthase
(ALS)-inhibiting herbicides have been primary herbi-
cides used in winter wheat, however continuous usage of
those herbicides led to selection of ALS-inhibitor resis-
tant weeds. Currently 126 ALS-inhibitor resistant weed
species have been reported worldwide; 45 in the USA [6].
Bushy wallflower and flixweed, two common broadleaf
*Corresponding author.

Broadleaf Weed Control and Crop Safety with Premixed Pyrasulfotole and Bromoxynil in Winter Wheat 1614
weeds in winter wheat, were reported ALS-inhibitor re-
sistant in 2005 and 2006, respectively in Kansas [6-8].
Rotating herbicides with different modes of action can
avoid selection for weeds biotypes that are resistant to
certain herbicides. Hence, there is a need for herbicides
with alternative modes of action to ALS-inhibitor herbi-
cides in wheat.
Pyrasulfotole is a new herbicidal active ingredient be-
longing to the pyrazoles family of herbicides. Pyrasul-
fotole inhibits 4-hydoxyphenylpyruvate dioxygenase
(HPPD) and blocks the pathway of prenylquinone bio-
synthesis in plants [9]. This leads to decreased levels of
plastoquinone in plant tissue and reduced photosynthetic
yield [10]. Indirect inhibition of phytoene desaturase as a
consequence of blocked plastoquinone biosynthesis sub-
sequently leads to a decrease in carotenoids [11] and
consequently prevents stabilization of the photosynthetic
apparatus so that chlorophyll molecules are destroyed by
excessive light energy. Inhibition of HPPD also prevents
biosynthesis of tocopherols that leads to reduced vitamin
E synthesis, which means loss of protection against oxi-
dative stress and against photo inactivation of the photo-
synthesis apparatus. The whole process will result in
typical bleaching symptoms in the newly developing
leaves during the first week after application. These
bleaching symptoms progress toward necrosis and sus-
ceptible plants generally die within two to three weeks
after treatment. Pyrasulfotale is the first significant com-
pound with a new mode of action for broadleaf weed
control in wheat, barley and triticale in more than 20
years.
The prepacked mixture of pyrasulfotole & bromoxynil
(Huskie®, Bayer CropScience, P.O. Box 12014, 2 T.W.
Alexander Drive, Research Triangle Park, North Caro-
lina 27709, USA) received US registration for use in
wheat in 2008. Bromoxynil belongs to the nitrile group
and inhibits photosynthesis at photosystem II in suscep-
tible plants. The premix also contains the safener me-
fenpyr-diethyl. The recommended dose of pyrasulfotole
& bromoxynil is 207 to 282 g·ai·ha−1 and recommended
stage of application in wheat is first leaf to flag leaf
emergence. The herbicide label recommends tank mixing
pyrasulfotole & bromoxynil with dicamba, MCPA, met-
sulfuron-methyl or 2,4-D for broad spectrum weed con-
trol. Currently not much information on use of pyrasul-
fotole & bromoxynil in winter wheat is available. The
objectives of the study were 1) to evaluate premixed
pyrasulfotole & bromoxynil with and without other her-
bicides for efficacy and safety in winter wheat and 2) to
determine the optimum time for its application.
2. Material and Methods
Field experiments were conducted for two years near
Hays (2007-2009) and for three years near Manhattan
(2007-2010) in Kansas in the central USA. Soil charac-
teristics of the sites are given in Table 1. Experimental
design was a randomized complete block with four treat-
ment replications. The pyrasulfotole & bromoxynil pre-
mix was tested alone or in tank mix combinations with
other herbicides applied postemergence (POST) at two
timings. The rate of pyrasulfotole & bromoxynil used was
253 g·ai·ha−1 without other herbicides and 207 g·ai·ha−1
when tank mixed. Tank mixture partners tested were
MCPA ester at 280 g·ai·ha−1, dicamba at 140 g·ai·ha−1,
Table 1. Soil characteristics and planting and spraying information, Hays and Manhattan, KS, 2007-2010.
Hays, KS Manhattan, KS
2007-2008 2008-2009 2007-2008 2008-2009 2009-2010
Soil type Roxbury silt loam Crete silty clay loamReading silt loam Reading silt loam Reading silt loam
Soil pH 7.7 6.3 5.7 5.7 5.7
Organic matter (%) 2.5 2 2.9 2.9 2.9
Wheat cultivar Danby KS08HW35-1 Overley Overley Fuller
Seed rate (kg·ha−1) 73 63 78 78 78
Planting date 10/02/2007 10/01/2008 10/11/2007 10/08/2008 10/19/2009
Row spacing (cm) 25 25 25 25 19
Plot size 2.5 × 6.7 2.5 × 6.7 1.9 × 6 1.9 × 6 1.9 × 6
Fall-POST spray date 11/04/2007 11/07/2008 11/27/2007 11/25/2008 12/04/2009
Spring-Post spray date 03/13/2008 03/16/2009 03/28/2008 03/17/2009 03/29/2010

Broadleaf Weed Control and Crop Safety with Premixed Pyrasulfotole and Bromoxynil in Winter Wheat 1615
and metsulfuron-methyl at 4.2 g·ai·ha−1. A commercial
standard of premixed triasulfuron & dicamba at 165
g·ai·ha−1 and a non-treated control were also included in
the study. Non-ionic surfactant at 0.5% v/v and 28% urea
ammonium nitrate at 4.7 L·ha−1 were included with all
herbicide treatments. Treatments were applied poste-
mergence to winter wheat at two timings, i.e. fall poste-
mergence (fall-POST) and spring postemergence (spring-
POST). Herbicides were applied broadcast using back-
pack or tractor-mounted plot sprayers, calibrated to de-
liver 121 to 139 L·ha−1 at 172 to 207 kPa. Henbit, flix-
weed and blue mustard were predominate weed species
at Hays, and henbit, flixweed, bushy wallflower, field
pennycress and wild buckwheat were predominate at
Manhattan. Wheat variety, seeding rate, plot size, row
spacing, planting and application dates are presented in
Table 1. Generally, wheat was 5 - 10 cm tall with 1 - 2
tillers at fall-POST application and 7.5 - 15 cm tall with
2 - 5 tillers at spring-POST application. Likewise, except
wild buckwheat, weeds were 1 - 2.5 cm tall at fall-POST
and 2.5 - 7.5 cm at spring-POST application. Wild
buckwheat had not emerged by the time of fall-POST
applications at Manhattan; they emerged in spring and
were at cotyledon to 4 leaf stage when spring-POST
treatments were applied.
Weed control and crop injury were rated based on
composite visual estimations of density reduction, growth
inhibition, and foliar injury on a scale of 0 (no effect) to
100 (plant death). Henbit, flixweed and blue mustard
control ratings were determined 195 to 224 days after
planting (DAP) at Hays. Similarly, henbit, flixweed,
bushy wallflower and field pennycress control ratings
were determined 190 to 206 DAP at Manhattan. Wild
buckwheat control was determined 236 to 258 DAP at
Manhattan. Wheat injury was visually assessed 2 weeks
after fall-POST and spring-POST applications at each
location. Grain yield was determined by harvesting the
six center rows of each plot with a plot combine and ad-
justing seed weight to 12.5% moisture content. Yields
were not determined at Manhattan in 2008 due to hail
damage. Data were analyzed using the general linear
model procedure of SAS (Statistical Analysis Systems
Institute, Cary, NC, USA) and means were separated at
the 5% significance level using Fisher’s protected LSD.
Percent weed control and wheat injury were arcsine
transformed before analysis. The control treatment was
omitted from weed control and crop injury analyses, but
included in the analysis of wheat grain yield. Because
there was significant year by location by treatment inter-
action for henbit and flixweed control data are presented
year wise for each location (Table 2). Year by treatment
interactions were significant for blue mustard, bushy
wallflower and field pennycress and hence data are pre-
sented year wise for respective locations. Wild buck-
wheat control ratings at Manhattan were pooled over
years 2007-2008 and 2008-2009 because year by treat-
ment interaction was non-significant. Wheat injury rating
were pooled over years and presented separately for each
site because site by treatment interactions were significant.
3. Results and Discussion
3.1. Weed Control
3.1.1. Henb it
In 2007-2008, at Hays, fall-POST application of all her-
bicides controlled henbit better than spring-POST treat-
ments (Ta b le 3 ). Complete control of henbit was achieved
with all fall-POST treatments. Among spring-POST
treatments henbit control was lowest with tank mixture
of pyrasulfotole & bromoxynil + metsulfuron-methyl
(86%) and premixed triasulfuron & dicamba (84%).
Henbit control was essentially complete, regardless of
herbicide or application timing at Hays in 2008-2009. At
Manhattan, all pyrasulfotole & bromoxynil treatments
applied fall-POST controlled henbit ≥ 98%, but control
varied significantly among spring-POST treatments (67% -
100%). Lowest henbit control was observed with triasul-
furon & dicamba applied either fall-POST or spring-
POST compared to pyrasulfotole & bromoxynil treat-
ments, however fall-POST treatment was much better
than spring-POST treatment (88% - 95% vs 53% - 63%).
These results indicate that pyrasulfotole & bromoxynil
with or without tank mixtures controlled henbit better
than commercial standard triasulfuron & dicamba. How-
ever, fall applications of pyrasulfotole & bromoxynil
were better than spring applications. This could be due to
the fact that henbit was smaller in size in fall (1 - 2.5 cm)
compared to spring (2.5 - 7.5 cm). Contrary to our results,
Martin et al. (2008) [12] reported complete control of
henbit with pyrasulfotole & bromoxynil alone or in com-
bination with dicamba regardless of application timing
(fall or spring). In our experiment it was also noticed that,
in two instances, pyrasulfotole & bromoxynil + metsul-
furon-methyl applied spring-POST controlled henbit less
compared to pyrasulfotole & bromoxynil alone or in
combination with MCPA. Generally, ALS-inhibiting
herbicides (triasulfuron and metsulfuron-methyl) control
henbit better when applied in fall than spring.
3.1.2. Flixweed, Blue Mustard, Bushy Wallflower and
Field Pennycress
The premix of pyrasulfotole & bromoxynil alone or in
combination with MCPA, dicamba or metsulfuron-
methyl, across locations, controlled flixweed and blue
mustard, 98% or more regardless of application timing
(Table 4). Data on flixweed at Hays in 2008-2009 and at
Manhattan in 2007-2008 and 2009-2010, and on blue
mustard at Hays in 2007-2 08 are not presented here 0

Broadleaf Weed Control and Crop Safety with Premixed Pyrasulfotole and Bromoxynil in Winter Wheat 1616
Table 2. Analysis of variance (ANOVA) results for weed control and crop injurya,b.
Source HenbitFlixweed Blue mustardBushy wallflowerField pennycressWild buckwheat Injury-FInjury-S
Year *** ** * ** *** NS *** NS
Location *** NS - - - - *** ***
Year × location *** *** - - - - *** ***
Treatment *** *** *** *** *** *** ** ***
Year × treatment *** *** *** *** *** NS NS **
Location × treatment *** *** - - - - ** ***
Year × location × treatment *** *** - - - - ** NS
aAbbreviation: NS, not significant; injury-F, injury due to fall treatments; injury-S, injury due to spring treatments; bResults of ANOVA based upon arc-
sine-transformed data; *P = 0.05 - 0.01; **P = 0.01 - 0.001; ***P = 0.001 - 0.0001.
Table 3. Henbit control with POST application of premixed pyrasulfotole & bromoxynil and its tank mixtures, Hays and
Manhattan, KSa.
Hays Manhattan
Rate
2007-2008 2008-2009 2007-2008 2008-2009 2009-2010
Treatmentsb Time of
application
g·ha−1 ---------------------------------------------%---------------------------------------------
Pyrasulfotolec Fall 253 100 99 98 98 100
Pyrasulfotole + MCPA Fall 207 + 280 100 100 99 100 100
Pyrasulfotole + dicamba Fall 207 + 140 100 100 98 100 100
Pyrasulfotole + metsulfuron-methyl Fall 207 + 4.2 100 100 100 100 100
Triasulfuron & dicamba Fall 165 100 100 88 92 95
Pyrasulfotole Spring 253 94 100 99 80 97
Pyrasulfotole + MCPA Spring 207 + 280 95 99 99 82 100
Pyrasulfotole + dicamba Spring 207 + 140 93 99 92 72 100
Pyrasulfotole + metsulfuron-methyl Spring 207 + 4.2 86 100 93 67 97
Triasulfuron & dicamba Spring 165 84 100 63 53 53
LSD (0.05) 4 NS 4 10 5
aAbbreviations: NS, non-significant; bAll herbicide treatments include non-ionic surfactant at 0.5% v/v and 28% urea ammonium nitrate at 4.7 L·ha−1; cPyra-
sulfotole has bromoxynil as premix partner.
because weed control was almost complete and treatment
differences were not significant. These results are con-
sistent with reports of 98% - 99% control of flixweed and
96% - 99% control of blue mustard in Oregon with
spring-applied pyrasulfotole & bromoxynil [13]. In our
study, bushy wallflower and field pennycress were con-
trolled 90% or more, regardless of application time, and
there were no significant differences among treatments
(data not shown). The commercial standard triasulfuron
& dicamba controlled all four weeds completely when
applied fall-POST, but control was occasionally lower
than pyrasulfotole & bromoxynil treatments when ap-
plied in spring. Across locations, triasulfuron & dicamba
applied spring-POST controlled flixweed by 83% - 100%,
blue mustard 89% - 100%, bushy wallflower 90% -
100% and field pennycress 90% - 100%.
3.1.3. Wi ld Buckwh eat
At Manhattan, wild buckwheat emerged late after fall-
POST application and plants were small at the time of
spring-POST application. Spring-applied pyrasulfotole &
bromoxynil alone or in combination with MCPA pro-
vided poor wild buckwheat control (3% and 12%, re-
spectively) (Table 4). However, when pyrasulfotole &
bromoxynil was tank mixed with dicamba or metsulfu-
ronmethyl control of wild buckwheat was 84% or more.
Spring applied triasulfuron & dicamba controlled wild
buckwheat 94%. Even though wild buckwheat had not
emerged at the time of fall-POST application, pyrasul-
fotole & bromoxynil + metsulfuron-methyl and triasul-
furon & dicamba applied in fall controlled wild buck-
wheat 73% and 87%, respectively. This might be due to
residual activity of metsulfuron and triasulfuron in the

Broadleaf Weed Control and Crop Safety with Premixed Pyrasulfotole and Bromoxynil in Winter Wheat 1617
soil. Metsulfuron-methyl and triasulfuron can persist in
the soil up 4 and 12 weeks, respectively [14]. These re-
sults indicated that pyrasulfotole & bromoxynil premix
alone applied in spring has very little effect on wild
buckwheat.
3.2. Crop Injury and Grain Yields
At Hays, averaged over years, pyrasulfotole & bro-
moxynil alone or in combination with MCPA or dicamba
or metsulfuron-methyl applied in fall or spring caused 1
to 4% wheat injury, but the injury was not significant
among treatments (Table 5). At Manhattan, no injury
was obesrved with pyrasulfotole & bromoxynil treat-
ments when applied in fall, but up to 7% injury was ob-
served when applied in spring. Triasulfuron & dicamba
caused 0% to 6% injury. However, injury symptoms dis-
appeared within 3 to 4 weeks and did not influence wheat
grain yields (data not shown). In a study conducted at
Oregon, no wheat injury was observed with pyrasulfotole
& bromoxynil applied in spring at 282 g·ai·ha−1 [13].
This tolerance in wheat might be due to faster metabolic
degradation of the herbicide inside the plant. Wheat grain
Table 4. Fixweed, blue mustar d and wild buckwheat control with POST application of premixed pyrasulfotole & bromoxynil
and its tank mixtures.
Flixweed Blue mustard Wild buckwheat
Hays Manhattan Hays Manhattan Rate
2007-2008 2008-2009 2007-2008 Pooledc&d
Treatmentsa Time of
application
g·ha−1 ------------------------------------------%------------------------------------------
Pyrasulfotoleb Fall 253 100 100 99 0
Pyrasulfotole + MCPA Fall 207 + 280 100 100 99 0
Pyrasulfotole + dicamba Fall 207 + 140 100 100 98 0
Pyrasulfotole + metsulfuron-methyl Fall 207 + 4.2 100 100 100 73
Triasulfuron & dicamba Fall 165 100 100 100 87
Pyrasulfotole Spring 253 100 100 99 3
Pyrasulfotole + MCPA Spring 207 + 280 100 100 100 12
Pyrasulfotole + dicamba Spring 207 + 140 100 100 100 84
Pyrasulfotole + metsulfuron-methyl Spring 207 + 4.2 100 100 100 92
Triasulfuron & dicamba Spring 165 96 83 89 94
LSD (0.05) 1 3 2 12
aAll herbicide treatments include non-ionic surfactant at 0.5% v/v and 28% urea ammonium nitrate at 4.7 L·ha−1; bPyrasulfotole has bromoxynil as premix
partner; cData pooled over years 2007-2008 and 2008-2009; dWild buckwheat did not emerge at the time of fall applications.
Table 5. Wheat injury caused by premixed pyrasulfotole & bromoxynil and its tank mixtures applied in fall and spring sea-
sons, Hays and Manhattan, KSa.
14 DAFT 14 DAST
Rate
Hays Manhattan Hays Manhattan
Treatmentsb
g·ha−1 ------------------------------------------------%------------------------------------------------
Pyrasulfotolec 253 2 0 2 0
Pyrasulfotole + MCPA 207 + 280 1 0 1 0
Pyrasulfotole + dicamba 207 + 140 4 0 1 7
Pyrasulfotole + metsulfuron-methyl 207 + 4.2 3 0 2 1
Triasulfuron & dicamba 165 4 0 0 6
LSD (0.05) NS NS NS 2
aAbbreviations: DAFT, days after fall treatments; DAST, days after spring treatments; NS, non-significant; bAll herbicide treatments include non-ionic surfac-
tant at 0.5% v/v and 28% urea ammonium nitrate at 4.7 L·ha−1. cPyrasulfotole has bromoxynil as premix partner.

Broadleaf Weed Control and Crop Safety with Premixed Pyrasulfotole and Bromoxynil in Winter Wheat 1618
yields were not influenced by any treatment compared to
untreated control (data not shown). High densities of
winter annual broadleaf species often reduce wheat yields,
sometimes dramatically, but controlling low to medium
density weed populations does not always result in higher
grain yields [15,16]. Analysis of 25 experiments con-
ducted over a several year period in Oklahoma found that
effective herbicidal control of weeds did not increase
wheat yields most of the time; yield increased when
bushy wallflower density was as much as 830 plants·m−2
[16]. Still good weed control is necessary in winter wheat
to prevent multiplication of weed density in future.
4. Conclusion
Premixed pyrasulfotole & bromoxynil alone at 253
g·ai·ha−1 or pyrasulfotole & bromoxynil at 207 g·ai·ha−1
in combination with MCPA, dicamba or metsulfuron-
methyl applied postemergence either in fall or spring
controlled blue mustard, flixweed, bushy wallflower and
field pennycress 98% or more. Henbit control with pyra-
sulfotole & bromoxynil treatments was much better when
they were applied in fall than spring (≥98% vs ≥67%).
Pyrasulfotole & bromoxynil applied alone in spring was
not effective on wild buckwheat, but tank mixing with
dicamba or metsulfuron-methyl controlled wild buck-
wheat 84% or more. Hence, tank mixing pyrasulfotole &
bromoxynil with other herbicides is desirable for broad
spectrum of weed control. Minor (≤7%) or no crop injury
was noticed with pyrasulfotole & bromoxynil treatments
regardless of application time. It can be concluded that
the new herbicide pyrasulfotole & bromoxynil can safely
be used in wheat for broadleaf weed control in spring or
fall season, but fall application is desirable for better
weed control. With a new and unique mode of action,
premix of pyrasulfotole & bromoxynil is an effective
alternative herbicide for wheat growers to combat weeds
resistant to ALS-inhibiting herbicides.
5. Acknowledgements
The authors thank Bayer CropScience for their financial
support to this project. Contribution number 13-174-J
from the Kansas Agricultural Experiment Station.
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