American Journal of Plant Sciences, 2012, 3, 1625-1631 Published Online November 2012 (
Weed Control and Crop Safety with Premixed
S-Metolachlor and Sulfentrazone in Sunflower
Seshadri S. Reddy1*, Phillip W. Stahlman1, Patrick W. Geier1, Curtis R. Thompson2
1Agricultural Research Center, Kansas State University, Hays, USA; 22014 Throckmorton Hall, Kansas State University, Manhattan,
Email: *
Received August 16th, 2012; revised September 20th, 2012; accepted October 7th, 2012
A preliminary study conducted in the central USA near Colby and Hays, Kansas (KS) in 2010 indicated a premix of
S-metolachlor & sulfentrazone codenamed F7583 (Broadaxe®) had good potential for use in sunflower (Helianthus an-
nuus L.). Additional studies were conducted in 2011 at Colby, Hays, and Manhattan, KS to refine rate and application
timing of F7583 for weed control and crop safety. Four rates of F7583 (860, 1100, 1350 and 1840 g·ha1) were com-
pared to single rates of S-metolachlor and pendimethalin, and applied 21 days preplant versus preemergence (PRE).
F7583 at 1100 g·ha1 applied preplant or PRE controlled Palmer amaranth (Amaranthus palmeri S. Wats.) and kochia
[Kochia scoparia (L.) Schrad.] 95% and 100%, respectively in neutral pH soils. In slightly acidic soils, PRE applica-
tion of F7583 was more effective against Palmer amaranth and grass weeds compared to preplant application. No bene-
fit was gained by increasing the rate of F7583 from 1100 to 1350 g·ha1 at either application timing. Puncturevine
(Tribulus terrestris L.) control was not commercially satisfactory with F7583 at any rate or time of application. Both
S-metolachlor at 1070 g·ha1 and pendimethalin at 1600 g·ha1 applied either preplant or PRE were considerably less
effective on all three broadleaf weeds compared to F7583 treatments. Individually, S-metolachlor and pendimethalin
were more effective when applied PRE compared to preplant application. F7583 did not reduce sunflower plant popula-
tion or visibly injure sunflower anytime during the season.
Keywords: Broadaxe; Sulfentrazone; S-Metolachlor; Sunflower; Crop Injury; Kochia; Palmer Amaranth
1. Introduction
Sunflower (Helianthus annuus L.) is a major oilseed crop
in the United States where it was planted on 0.6 million
ha in 2011 [1]. North Dakota (0.28 million ha), South
Dakota (0.20 million ha) and Kansas (0.05 million ha)
are the three major sunflower-growing states in the coun-
try. Weeds are one of the major yield limiting factors of
sunflower [2]. Canada thistle [Cirsium arvense (L.)
Scop.], kochia [Kochia scoparia (L.) Schrad.], redroot
pigweed (Amaranthus retroflexus L.), green foxtail [Se-
taria viridis (L.) Beauv.], and Palmer amaranth (Ama-
ranthus palmeri S. Watson) are among the most common
weeds interfering with sunflowers in the central Great
Plains [2].
Sunflowers grow slowly the first few weeks after
planting which makes the crop vulnerable to weed inter-
ference. Johnson (1971) [3] reported maximum seed
yields when sunflower was kept weed free 4 to 6 weeks
after planting. In the central US Great Plains, most sun-
flowers are planted in rows spaced greater than 50 cm
apart [2]. Wide row spacing initially provides weeds an
advantage over sunflowers. Durgan et al. (1990) [4]
found that season-long competition by kochia at densities
of 0.3, 1, 3, and 6 plants·m1 of row decreased sunflower
seed yield by 7%, 10%, 20%, and 27%, respectively.
Holman (2004) [5] reported as much as 57% yield loss in
sunflowers due to Persian darnel (Loliumpersicum Boiss.
& Hohen. ex Boiss.) interference. Hence, weed manage-
ment has great importance in sunflower production.
There are relatively few herbicides registered for use
in sunflower. Sulfentrazone belongs to the phenyl tria-
zolinone herbicide group and is a protoporphyrinogen
oxidase (PPO) inhibitor [6]. Sulfentrazone is widely used
in sunflower in the US for control of broadleaf weeds,
but it has little activity on grass weeds. Olson et al. (2011)
[7] reported only 57% and 23% large crabgrass [Digi-
taria sanguinalis (L.) Scop.] control in sunflower with
sulfentrazone at 105 g·ha1 at Manhattan and Hays, KS,
respectively. Hence, ulfentrazone is usually tank mixed
with S-metolachlor or pendimethalin for broad spectrum
weed control [8]. The prepacked mixture of S-meto-
lachlor & sulfentrazone (9:1 ratio, Broadaxe®, FMC
*Corresponding author.
Copyright © 2012 SciRes. AJPS
Weed Control and Crop Safety with Premixed S-Metolachlor and Sulfentrazone in Sunflower
Corporation, 1735 Market Street, Philadelphia, PA 19103,
USA) received US registration for use in sunflowers in
2012. S-metolachlor belongs to chloroacetamide family
of herbicides. The primary mode of action of S-meto-
lachlor is not clear, but the most recent evidence suggests
that it blocks the formation of very long chain fatty acids.
The objective of the study was to evaluate the premix of
S-metolachlor & sulfentrazone (codenamed F7583) for
efficacy and safety in sunflowers compared to some
commercial standard herbicides.
2. Materials and Methods
2.1. Year I
A preliminary experiment on F7583 was conducted at
two locations near Colby and Hays, KS in the central
USA in 2010. Soil type at Colby was a Keith silt loam
with pH 7.2 and 2% organic matter (OM). At Hays, soil
was a Crete silty clay loam with pH 6.5 and 1.5% OM.
Experimental design was a randomized complete block
with four treatment replications. Treatments include three
rates of F7583 (896, 1120 and 1400 g·ai·ha1) applied
PRE to crop and weeds. For comparison, two rates of
tank mixed sulfentrazone plus S-metolachlor (90 + 806
and 112 + 1008 g·ha1), three rates of sulfentrazone (90,
112 and 140 g·ha1), two rates of S-metolachlor (806 and
1008 g·ha1) and single rate of pendimethalin (1600
g·ha1) were also included as PRE treatments. A non-
treated control was also included. At Colby, before
planting, 120 kg N ha1 as 32-0-0 liquid fertilizer was
applied in early spring and 15 kg·N and 45 kg P2O5 ha1
as 10-34-0 liquid fertilizer was injected 5 cm away and 5
cm below the seed row at the time of planting. Similarly
at Hays, 10 kg N and 30 kg P2O5 ha1 as 10-34-0 liquid
fertilizer was injected at the time of planting. Sunflower
hybrids Mycogen 8N453DM and 8N386CL were planted
no-till into wheat stubble at Colby and Hays, respectively.
The seeding rates were 43,200 and 49,000 seeds ha1 at
Colby and Hays, respectively, with a row spacing of 76
cm. Plots were 3 by 6.7 m with four rows of sunflower.
Preemergence herbicides were applied immediately after
planting using a CO2-powered backpack sprayer deliver-
ing 115 L·ha1 at 220 kPa.
Palmer amaranth, kochia, green foxtail, puncturevine
(Tribulus terrestris L.) and tumble pigweed (Amaranthus
albus L.) were predominate weed species at Colby. All
these weeds except kochia also were predominate at
Hays. Weed control was rated 62 and 56 days after plant-
ing (DAP) based on composite visual estimations of den-
sity reduction, growth inhibition, and foliar injury on a
scale of 0 (no effect) to 100 (plant death). Sunflower re-
sponse was visually rated 14 DAP. Seed yield was de-
termined only at Colby by harvesting the two center rows
of each plot with a plot combine and adjusting seed
weight to 10% moisture content.
Data were analyzed using the general linear model
procedure of the Statistical Analysis System (Statistical
Analysis Systems Institute, Cary, NC, USA) and means
were separated at the 5% significance level using
Fisher’s protected LSD. Weed control ratings were arc-
sine transformed before analysis, but original values are
presented in this paper. The control treatment was omit-
ted from weed control analyses, but included in the
analysis of sunflower seed yield. Data are presented by
year or location when significant (P 0.05) year and or
location interactions occurred.
2.2. Year II
Field experiments were conducted at three sites in 2011
near Colby, Hays and Manhattan, KS. Soil type at Colby
was a Keith silt loam with pH 7.2 and 2% OM. Soil type
at Hays was a Roxbury silt loam with pH 7.8 and 2% OM
and soil type at Manhattan was a Belvue silt loam with
pH 6.8 and 1.1% OM. The experimental design was a
randomized complete block with an unbalanced factorial
treatment arrangement. Treatments were replicated four
times. Treatment structure was modified from year I.
However, core treatments of F7583 remained similar
with minor changes in application rate. Treatments in-
cluded four rates of F7583 (860, 1100, 1350 and 1840
g·ha1) and single rates of S-metolachlor (1070 g·ha1),
and pendimethalin (1600 g·ha1). All treatments except
F7583 at 1840 g·ha1 were applied 21 days preplant and
PRE (not sequentially). F7583 at 1840 g·ha1 was ap-
plied only 21 days preplant. A non-treated control also
was included. At Colby, 110 kg·N·ha1 was applied as
anhydrous ammonia in fall 2010 and was followed by 20
kg N and 67 kg P2O5 ha1 in the form of 10-34-0 liquid
fertilizer injected 5cm away and 5cm below the seed row
at the time of planting. At Hays, 95 kg·N·ha1 was ap-
plied at the time of planting as urea ammonium nitrate.
At Manhattan, 80 kg·N·ha1 was applied as urea 9 days
after planting. The experimental areas at Colby and Hays
were over seeded with Palmer amaranth and kochia be-
fore herbicide application. Sunflower hybrids Mycogen
8N435DM, Mycogen 8N358CL and Pioneer 63N82 were
planted at Colby, Hays and Manhattan, respectively.
Sunflower planting, herbicide application, weed control
and crop response ratings and data analyses were done as
explained in year I, except the Hays trial was conven-
tionally tilled. Planting and herbicide application dates
are shown in Table 1. Weed control was determined 49,
61 and 46 DAP at Colby, Hays and Manhattan, respec-
tively. Crop response was rated 14 DAP at all sites. At
Manhattan, seed yield was determined by harvesting the
two center rows of each plt with a plot combine and o
Copyright © 2012 SciRes. AJPS
Weed Control and Crop Safety with Premixed S-Metolachlor and Sulfentrazone in Sunflower
Copyright © 2012 SciRes. AJPS
Table 1. Sunflower planting and herbicide application dates, 2010 and 2011.
 Year IYear II
Colby, KSHays, KSColby, KSHays, KSManhattan, KS
Planting June 10June 12June 14June 17June 13
Preplant--May 23May 27May 23
applicationPreemergenceJune 11June 12June 14June 17June 14
adjusting seed weight to 10% moisture content. The
Colby trial was terminated 7 weeks after planting after
severe defoliation and lodging caused by hail. Seed
yields of the Hays trial was not determined because of
late-season crop damage caused by wildlife.
3. Results and Discussion
3.1. Year I
Nearly 3.8 cm rainfall was received at Colby within three
days after PRE herbicide application (data not shown).
At Hays, a total of 3.6 cm rainfall was received over a 7
day period after PRE herbicide application. F7583 at 896
g·ha1 controlled Palmer amaranth, tumble pigweed, ko-
chia and green foxtail 89%, 99%, 94% and 89%, respec-
tively (Table 2). Tank-mixed sulfentrazone plus S-me-
tolachlor at 90 + 806 g·ha1 was similarly effective on all
species compared to F7583 at 896 g·ha1. Increasing
F7583 rate from 896 to 1120 g·ha1 and tank mix rates of
sulfentrazone plus S-metolachlor from 90 + 806 to 112 +
1008 g·ha1 did not improve weed control. This indicates
that at equivalent rates of active ingredient, the premixed
and tank-mixed treatments performed similarly on all
Sulfentrazone alone at 90 g·ha1 controlled Palmer
amaranth 83% and control increased to 93% with in-
crease in rate to 140 g·ha1, but sulfentrazone at 112
g·ha1 did not control Palmer amaranth nearly as well as
F7583 at 1120 g·ha1. Regardless of rate, sulfentrazone
controlled tumble pigweed, kochia and puncturevine
similar to F7583. However, foxtail millet control was
significantly lower with sulfentrazone treatments (54% to
76%) compared to F7583 (89% to 97%). These results
indicated that sulfentrazone alone is effective on broad-
leaf weeds but not on grass weeds and the S-metolachlor
present in the premix of F7583 contributed to increased
green foxtail control.
Among PRE treatments, lowest broadleaf weed control
was observed with S-metolachlor at 806 g·ha1. S-meto-
lachlor at 1008 or 1266 g·ha1 controlled Palmer ama-
ranth, puncturevine, tumble pigweed and green foxtail as
well as F7583 at 1120 g·ha1, but was less effective on-
kochia. Pendimethalin at 1600 g·ha1 provided similar
control of all weed species as provided by the 896 g·ha1
rate of F7583 except tumble pigweed. No treatment con-
trolled puncturevine more than 85% and there was no
significant difference among treatments.
Premixtures or tank mixtures of sulfentrazone con-
taining treatments did not visibly injured sunflower any-
time during the season (data not shown). Olson et al.
(2011) [7] also observed negligible injury in sunflowers
at Colby with sulfentrazone at 105 or 140 g·ha1. On a
silt loam soil, Kniss (2011) [9] reported 25% and 36%
sunflower injury with sulfentrazone at 105 and 140
g·ha1, respectively at 28 DAP; however, seed yields
were not affected. Commercial standards S-metolachlor
and pendimethalin also did not cause any injury.
At Colby, sunflower treated with F7583 treatments
averaged across rates yielded 2.6-times more compared
to sunflower in the non-treated control (Table 2). Sun-
flower treated with F7583 at 896 g·ha1 yielded 2690
kg·ha1 seed and no yield benefit was gained by increas-
ing F7583 rate. Sunflower yields for both rates of tank
mixed sulfentrazone plus S-metolachlor were similar to
the yields of sunflower treated with F7583 at any rate.
Yields of sunflower treated with sulfentrazone at 140
g·ha1 were similar to F7583 treatments, whereas sulfen-
trazone at 90 or 112 g·ha1 yielded 8% less compared to
F7583 averaged across rates. Poor control of green fox-
tail (<70%) might be the reason for reduced seed yields
with sulfentrazone at 90 or 120 g·ha1 (Table 2). Simi-
larly, 15 and 25% lower seed yields were recorded for
sunflower treated with S-metolachlor (across rates) and
pendimethalin, respectively compared to F7583. It was
evident from the weed control data that both S-meto-
lachlor and pendimethalin were not as effective as F7583
in controlling Palmer amaranth and kochia.
This study indicated that F7583 at 1120 or 1400 g·ha1
applied PRE controlled troublesome weeds like Palmer
amaranth and kochia better than some commercial stan-
dard herbicides without causing visible crop injury.
However, in drought prone areas like Kansas, lack of
rainfall or insufficient soil moisture after PRE herbicides
application is common and this may decrease the effi-
cacy of herbicides. Probability of rainfall is greater earlier
in the season when preplan herbicides typically are t
Weed Control and Crop Safety with Premixed S-Metolachlor and Sulfentrazone in Sunflower
Table 2. Effect of F7583 applied preemergence on weed control and sunflower seed yields, Colby and Hays, KS, 2010.
pigweed Kochia Puncturevine Green foxtail Seed yield
Pooled Pooled Colby Pooled Pooled Colby
g·ha1 ------------------------------------------------%------------------------------------------------ kg·ha1
Control - - - - - 960
F7583 896 89 99 94 66 89 2690
F7583 1120 100 100 100 73 94 2500
F7583 1400 100 100 99 74 97 2380
Sulfentrazone + S-metolachlor 90 + 806 97 99 98 64 91 2200
Sulfentrazone + S-metolachlor 112 + 1008 96 99 100 68 93 2760
Sulfentrazone 90 83 98 96 49 54 2320
Sulfentrazone 112 80 93 97 62 68 2330
Sulfentrazone 140 93 99 99 68 76 2960
S-metolachlor 806 78 83 70 45 92 2090
S-metolachlor 1008 93 99 84 50 88 2200
Pendimethalin 1600 84 83 83 81 85 1900
LSD (P = 0.05) 15 13 15 NS 13 630
applied. Hence, additional studies were done in 2011 to
refine F7583 application time and rate.
3.2. Year II
At Colby, rainfall totaling 4.3 cm was received over a
two day period immediately after preplant herbicide ap-
plication and 3.6 cm rainfall was received 7 days after
PRE herbicides were applied (data not shown). At Hays,
nearly 4.3 cm rainfall was received within three days
prior to preplant herbicide application with the next rain-
fall event (3.1 cm) occurring 15 days after preplant her-
bicide application. Rainfall totaling 2.9 cm was received
over a three day period within hours after PRE herbicide
application. At Manhattan, 6.5 cm of rainfall was re-
ceived immediately after preplant application and 3.2 cm
was received within 7 days period after PRE herbicides
application. Kochia and puncturevine were predominate
weed species at Colby. Palmer amaranth was the only
predominate weed species in the Hays trial. Palmer ama-
ranth, large crabgrass, stinkgrass (Eragrostis cilianensis)
and giant foxtail (Setaria faberi) were major weeds at
3.2.1. Palmer Amaranth Control
At Hays, preplant-applied F7583 at 860 g·ha1 controlled
Palmer amaranth 86% and control increased with in-
crease in rate (Table 3). F7583 at 1100 to 1840 g·ha1
controlled Palmer amaranth 96%. Preemergence-ap-
plied F7583, regardless of rate, controlled Palmer ama-
ranth 95%. Preemergence treatments looked slightly
better than preplant treatments, but not significantly dif-
ferent. This trend was more conspicuous at Manhattan.
The maximum palmer amaranth control achieved at
Manhattan with preplant F7583 treatments was 70%. The
lowest rate of F7583 (860 g·ha1) applied PRE controlled
Palmer amaranth greater than any preplant rate (95 vs
70). At both sites F7583 applied PRE controlled Palmer
amaranth similarly, but the control with preplant treat-
ments varied between two sites. Lower control of Palmer
amaranth with F7583 applied preplant at Manhattan
compared to Hays could be due to higher weed density
(data not shown) and lower soil pH (6.8) than at Hays
(7.8). Greater sulfentrazone adsorption occurs in soils
with lower pH [10]. Sulfentrazone is a weak acid and its
concentration in soil solution increases as soil pH in-
creases and is available for root uptake by plants [11].
Both S-metolachlor at 1070 g·ha1 and pendimethalin
at 1600 g·ha1 applied either preplant or PRE were con-
siderably less effective on Palmer amaranth compared to
F7583 treatments. They were more effective when ap-
plied PRE compared to preplant application.
3.2.2. Kochia and Puncture v i n e Control
Regardless of rate or appliation timing, F7583 treat- c
Copyright © 2012 SciRes. AJPS
Weed Control and Crop Safety with Premixed S-Metolachlor and Sulfentrazone in Sunflower 1629
Table 3. Effect of F7583 applied 21 days preplant (DPP) or preemergence (PRE) on Palmer amaranth, kochia and puncture-
vine control in 2011.
Palmer amaranth Kochia Puncturevine
Hays Manhattan Colby Colby
Treatment Time of application
g·ha1 -------------------------------------------------%--------------------------------------------------
F7583 21 DPP 860 86 33 100 33
F7583 21 DPP 1100 96 33 100 40
F7583 21 DPP 1350 96 46 100 48
F7583 21 DPP 1840 99 70 100 50
S-metolachlor 21 DPP 1070 33 0 38 5
Pendimethalin 21 DPP 1600 37 48 63 38
F7583 PRE 860 95 95 100 45
F7583 PRE 1100 96 100 100 48
F7583 PRE 1350 100 100 100 40
S-metolachlor PRE 1070 86 73 58 35
Pendimethalin PRE 1600 73 87 66 35
LSD (P = 0.05) 13 25 15 17
ments maintained complete kochia control at Colby (Ta-
ble 3). In a similar study at Lingle, Wyoming, F7583
applied PRE at 860 to 1350 g·ha1 controlled kochia
completely, but control for preplant treatments were
much less than PRE treatments [12]. At Colby, S-meto-
lachlor and pendimethalin applied preplant or PRE con-
trolled kochia significantly less than F7583. There was
no significant difference between preplant and PRE
treatments of pendimethalin. S-metolachlor was more
effective on kochia when applied PRE compared to pre-
plant application. No treatment controlled puncturevine
more than 50%. Lowest puncturevine control (5%) was
observed with S-metolachlor applied preplant.
3.2.3. Grass Weed Control
None of the preplant treatments including F7583 were
effective against large crabgrass, stinkgrass or giant fox-
tail at Manhattan (Table 4). F7583 applied PRE, regard-
less of rate, provided complete control of large crabgrass,
92% to 96% control of stinkgrass and 85% to 93% con-
trol of giant foxtail. Similar to these results, at Sydney,
Nebraska, F7583 at 860 to 1350 g·ha1 applied PRE con-
trolled witchgrass (Panicumcapillare) 94%, but the
same rate applied preplant (4 weeks before planting) re-
sulted in significantly less weed control [13]. It suggests
the importance of time interval between preplant applica-
tion of herbicide and planting. As the interval widens, the
residual effect of F7583 may decrease and result in poor
weed control. S-metolachlor applied PRE controlled
large crabgrass and stinkgrass similar to F7583, but giant
foxtail control (68%) was lower compared to F7583 at
any rate. Pendimetalin applied PRE controlled all grasses
similar to F7583.
3.2.4. Crop In jury and Seed Yields
At all three locations, no treatment reduced sunflower
plant population or visibly injured sunflower anytime
during the season (data not shown). At Sidney, NE,
above normal rainfall after preplant application of F7583
and again good amount of rainfall after PRE treatments
resulted in 19% - 26% sunflower injury, but plants re-
covered fully within a few weeks [13]. In a high pH (8.15)
silt loam soil, 64% - 75% injury was reported due to PRE
application of F7583 at 27 DAT; however, injury de-
clined to 9% - 20% at 85 DAT and did not affect yields
[12]. At Manhattan, treatments did not differ signifi-
cantly in seed yields with each other and with untreated
control (data not shown). On average, sunflowers treated
with F7583 preplant and PRE yielded 1802 and 1772
kg·ha1 seed, respectively.
4. Conclusion
Based on this study, premixed product F7583 (S-meto-
lachlor & sulfentrazone) applied preplant (3 weeks be-
fore planting) or PRE can safely be used in sunflower for
weed control. Results indicated that, in neutral pH soils,
F7583 at 1100 g·ha1 or more applied preplant or PRE
provided satisfactory controlof kochia and Palmer ama-
Copyright © 2012 SciRes. AJPS
Weed Control and Crop Safety with Premixed S-Metolachlor and Sulfentrazone in Sunflower
Table 4. Effect of F7583 applied 21 days preplant (DPP) or preemergence (PRE) on large crabgrass, stinkgrass and giant
foxtail control at Manhattan, KS, 2011.
Rate Large crabgrass Stinkgrass Giant foxtail
Treatment Time of application
g·ha1 -------------------------------------------%-------------------------------------------
F7583 21 DPP 860 5 5 5
F7583 21 DPP 1100 8 8 8
F7583 21 DPP 1350 45 35 28
F7583 21 DPP 1840 63 36 33
S-metolachlor 21 DPP 1070 25 0 0
Pendimethalin 21 DPP 1600 68 60 55
F7583 PRE 860 100 94 85
F7583 PRE 1100 100 92 90
F7583 PRE 1350 100 96 93
S-metolachlor PRE 1070 98 80 68
Pendimethalin PRE 1600 100 98 98
LSD (P = 0.05) 26 21 16
ranth. In slightly acidic soils, where greater herbicide
adsorption is expected than high pH soils, PRE applica-
tion of F7583 was more effective on kochia and grass
weeds compared to preplant application. Results also
indicated that F7583 can control troublesome weeds as
effectively as tank mixed S-metolachlor plus sulfentra-
zone and performed much better than commercial stan-
dards pendimethalin and S-metolachlor especially on
broadleaf weeds. However, puncturevine control was not
commercially satisfactory with F7583. No crop injury or
yield reductions were observed with F7583. Overall the
study indicated that F7583 is a promising new herbicide
for use in sunflower.
5. Acknowledgements
The authors thank FMC Corporation, National Sunflower
Association and Kansas Sunflower Commission for their
financial support to this project. Contribution number
13-175-J from the Kansas Agricultural Experiment Sta-
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