American Journal of Plant Sciences, 2012, 3, 1193-1204
http://dx.doi.org/10.4236/ajps.2012.39145 Published Online September 2012 (http://www.SciRP.org/journal/ajps)
1193
Precipitation Influences Pre- and Post-Emergence
Herbicide Efficacy in Corn
Christie L. Stewart1, Nader Soltani1*, Robert E. Nurse2, Allan S. Hamill2, Peter H. Sikkema1
1University of Guelph Ridgetown Campus, Ridgetown, Canada; 2Agriculture and Agri-Food Canada, Harrow, Canada.
Email: *nsoltani@ridgetownc.uoguelph.ca
Received June 6th, 2012; revised July 4th, 2012; accepted July 12th, 2012
ABSTRACT
Selecting a preemergence (PRE) and postemergence (POST) herbicide program that has the greatest efficacy can be
difficult for corn producers and is highly dependent on weed spectrum. Weather conditions before and after herbicide
application can further complicate decisions because they influence herbicide efficacy. Eleven field trials were con-
ducted at three locations in Southwestern Ontario from 2003 to 2006, to determine the most effective PRE and POST
corn herbicides for weed control. The most abundant weed species across all locations were redroot pigweed (Amaran-
thus retroflexus L.), common ragweed (Ambrosia artemisiifolia L.), common lambsquarters (Chenopodium album L.),
and green foxtail (Setaria viridis (L.) Beauv.). Nine PRE herbicide treatments and eleven POST (applied at the 3 - 4 leaf
stage of corn) herbicide treatments were tested. Results from this study suggest that the timing and amount of precipita-
tion influence herbicide efficacy. For example, precipitation levels 0 - 17 mm within seven days after herbicide applica-
tion (PRE or POST) provided unacceptable weed control in treatments that included atrazine, dimethenamid-p, isoxa-
flutole/atrazine or S-metolachlor/benoxacor. Cumulative precipitation during the 14 days after PRE application that
exceeded the monthly average (by at least 64%) reduced Setaria viridis control with pendimethalin. This study demon-
strates that a better understanding of how environmental conditions, especially precipitation affect herbicide efficacy,
need to be considered by growers when selecting a corn herbicide program to reduce the possibility of weed control
failure.
Keywords: Preemergence Herbicides; Postemergence Herbicides; Tank-Mixture; Precipitation
1. Introduction
Weather conditions, specifically precipitation amount
and timing, can significantly impact the efficacy of indi-
vidual preemergence (PRE) and postemergence (POST)
herbicides [1-5]. When tank-mixed, the conditions under-
which each herbicide has the greatest efficacy can differ
[1]. However, this also means that tank-mixtures have
the potential to provide more consistent weed control
over a broader range of climatic conditions [6]. Tank-
mixing herbicides is also an effective way to combine
several herbicide modes of action and control a broad
spectrum of weeds, while also being cost effective by
reducing the requirement for multiple applications [6-9].
Some growers attempt to implement a PRE only pro-
gram for weed control [10], which can be problematic if
the PRE herbicide fails to control all weeds. However,
the grower always has the opportunity to apply a POST
herbicide should a failure occur [11,12]. The benefit of
including a POST component is that it allows the grower
to make herbicide choices based on the weed spectrum in
each individual field [13]. However, timing POST herbi-
cide application with weed size is critical [8]; if weeds
become too large, control can be reduced [11,14]. Weather
conditions at the optimum weed size prevents accurate
timing of POST application allowing weeds the opportu-
nity to become too large for effective control [11,15,16].
For most PRE herbicides precipitation is required
within 7 - 14 days after application to dissolve the herbi-
cide in soil water solution so that it can be taken up by
the emerging weeds after germination [10,17-20]. In-
adequate or delayed precipitation can reduce herbicide
effectiveness and decrease weed control [1,4,11,21-23].
Depending on soil type, high amounts of precipitation
(i.e. greater than 25 mm), especially immediately after
application, can cause herbicides to leach through the soil
profile and consequently reduce efficacy [6,24,25]. Most
POST herbicides require that there is no precipitation for
several hours after application to ensure that movement
across the leaf membrane can occur.
It is widely known that PRE herbicides such as S-me-
tolachlor/benoxacor/atrazine, isoxaflutole + atrazine, and
*Corresponding author.
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Precipitation Influences Pre- and Post-Emergence Herbicide Efficacy in Corn
1194
dimethenamid-p + atrazine require precipitation within
7-10 days after application for proper movement into the
active zone of weed seed germination [26-29]. However,
when several modes of action are being tank-mixed, the
differential sensitivity of each herbicide to precipitation
can become complicated. For example, S-metolachlor/
benoxacor/atrazine, an acetanilide/s-triazine mixture that
controls a broad-spectrum of annual grasses and broad-
leaved weeds [6,12,17] requires precipitation within 7 -
10 days for proper activation [1]. However, precipitation
greater than 45 mm, over 2 days, beginning within 12
hours of application may result in leaching of the atrazine
component away from the active zone of weed seed ger-
mination [3]. It is possible that if S-metolachlor/benox-
acor/atrazine is tank-mixed with an additional mode of
action such as mesotrione that there would be no de-
crease in broadleaved weed control because mesotrione
may respond differently to precipitation after application.
Pendimethalin is an example of an herbicide that is more
persistent in the soil under dry conditions and can affect
rotational crops, but is easily leached when soil condi-
tions are wet [27,30,31]. Furthermore, pendimethalin’s
weed spectrum is reduced, especially the control of an-
nual grasses, when soil conditions are dry up to 3 weeks
after application [2].
Glyphosate is a non-selective, broad-spectrum herbi-
cide used to control many annual grasses and broad-
leaved weeds post emergently [32]. This herbicide lacks
residual control which allows weeds to emerge after ap-
plication and escape control [33-35]; however, sequential
in-crop applications of glyphosate are effective at con-
trolling late-emerging weeds [36,37]. Glyphosate effi-
cacy can be reduced by precipitation occurring 15 min to
6 hr after application depending on formulation [24,25,
27]. Thus application may have to be delayed and weed
size at the time of application may no longer be opti-
mal.
Field-specific data from diverse environments that de-
scribe the performance of herbicide mixtures registered
for corn are needed. These data will help to identify weed
management strategies that provide season-long weed
control without reducing crop yields under variable pre-
cipitation. Therefore, the specific objectives of this re-
search were to 1) determine the most effective PRE and
POST herbicide mixtures for corn when precipitation
varies within 14 days of application; 2) determine if a
glyphosate only or conventional management strategy is
better in unfavourable environmental conditions.
2. Materials and Methods
Eleven field trials were conducted from 2003 to 2006 at
the Huron Research Station, Exeter, Ontario, and the
University of Guelph, Ridgetown Campus, Ridgetown,
Ontario and from 2004 to 2006 at the Greenhouse and
Processing Crops Research Centre, Agriculture and Agri-
Food Canada, Harrow, Ontario. Soil characteristics are
presented in Table 1 and precipitation in Table 2. Pro-
cedures at all sites were the same unless otherwise noted.
The soil was moldboard plowed in the fall and the seed-
bed was prepared with two passes with an s-tine cultiva-
tor with rolling basket harrows the following spring of
each year. A total of twenty treatments were tested in two
separate trials (Trial 1 = PRE and Trial 2 = POST) at
each location. The first trial consisted of nine PRE treat-
ments: a non-treated weedy control, S-metolachlor/be-
noxacor/atrazine, dimethenamid-p + atrazine, isoxaflutole
+ atrazine, S-metolachlor/beno- xacor/atrazine + dicamba,
dimethenamid-p + dicamba/atrazine, S-metolachlor/be-
noxacor/atrazine + mesotrione, pendimethalin + dicam-
ba/atrazine, pendimethalin + atrazine and rimsulfuron +
S-metolachlor/benoxacor + dicamba. Rates for PRE her-
bicide are listed in Table 3. The second trial consisted of
eleven POST treatments that were applied at the 3 - 4
leaf stage of the corn unless otherwise stated: a non-
treated weedy control, glyphosate (3 - 4 leaf stage), gly-
phosate (3 - 4 leaf stage) followed by glyphosate (7 - 8
leaf stage), rimsulfuron + S-metolachlor/benoxacor +
dicamba + a non-ionic surfactant, nicosulfuron/rimsul-
furon + dicamba/diflufenzopyr + a non-ionic surfactant +
urea ammonium nitrate (UAN), nicosulfuron/rimsulfuron
+ mesotrione + atrazine + a non-ionic surfactant, nico-
sulfuron + dicamba/diflufenzopyr + a non-ionic surfac-
tant + UAN, nicosulfuron + prosulfuron + dicamba + a
non-ionic surfactant, nicosulfuron + mesotrione + atrazine
+ a non-ionic surfactant, nicosulfuron + pendimethalin +
dicamba + a non-ionic surfactant, nicosulfuron + pri-
misulfuron/dicamba + a non-ionic surfactant, and foram-
sulfuron + dicamba/diflufenzopyr + UAN + a MSO.
Rates for POST herbicides are listed in Table 4.
Glyphosate-resistant corn was seeded in rows spaced
76 cm apart at densities of at least 74,000 seeds ha–1 at
each location. Plots were arranged in a randomized com-
plete block design with four replications and plots were 2
m wide × 8 to 11 m long.
At Exeter, herbicides were applied using a CO2-pres-
surized sprayer calibrated to deliver 200 L·ha–1 aqueous
solution at 241 kPa using 8002 VS nozzles (Teejet
Spraying Systems Co., P.O. Box 7900, Wheaton, IL
60,188) spaced 50 cm apart. At Ridgetown, herbicides
were applied using a CO2-pressurized sprayer calibrated
to deliver 200 L·ha–1 aqueous solution at 207 kPa using
8002 Extended Range (2003, 2004) and Ultra Lo-Drift
120-02 (2005, 2006) nozzles spaced 50 cm apart. At
Harrow, herbicides were applied using a CO2-pressu-
rized sprayer calibrated to deliver 247 L·ha–1 aqueous
solution at 210 kPa using flat fan 110-03 XR nozzles
spaced 50 cm apart.
Percent weed control and crop yield were measured at
Copyright © 2012 SciRes. AJPS
Precipitation Influences Pre- and Post-Emergence Herbicide Efficacy in Corn
Copyright © 2012 SciRes. AJPS
1195
Table 1. Soil characteristics for experimental sites at Exeter and Ridgetown, ON in 2003 to 2006 and Harrow, ON in 2004 to
2006.
Location Year Application timinga Soil type pH Organic matter Sand Silt Clay
%
Exeter 2003 PRE Brookston clay loam 7.4 3.9 39 37 24
2003 POST Brookston clay loam 7.9 4.3 38 41 21
2004 PRE Brookston clay loam 8.0 4.2 28 38 34
2004 POST Brookston clay loam 7.9 4.7 39 33 28
2005 PRE Brookston clay loam 7.6 5.3 39 37 24
2005 POST Brookston clay loam 7.6 5.3 39 37 24
2006 PRE Brookston clay loam 7.9 3.4 33 35 32
2006 POST Brookston clay loam 7.9 3.4 33 35 32
Ridgetown 2003 PRE Loam 6.4 5.2 45 29 26
2003 POST Fine sandy loam 7.2 4.1 57 25 18
2004 PRE Loam 7.4 5.0 43 33 24
2004 POST Loam 7.0 6.4 43 30 27
2005 PRE Fine sandy loam 7.2 4.1 57 25 18
2005 POST Fine sandy loam 7.2 3.8 56 34 20
2006 PRE Watford/Brady loam 7.0 6.4 43 30 27
2006 POST Maplewood/Normandale6.7 5.9 35 35 30
Harrow 2004 to
2006 PRE and POST Fox sandy loam 6.0 2.6 83 5 12
aAbbreviations: PRE, preemergence; POST, postemergence.
Table 2. Mean cumulative precipitation (mm) for day of application and 7 and 14 days before and after PRE and POST her-
bicide application at Exeter, Harrow, and Ridgetown, Ontario between 2003 and 2006.a
Cumulative Precipitation
PRE Herbicides POST Herbicides
Location Year 14
DBA 7 DBA Day of
Application 7 DAA14 DAA14 DBA7 DBADay of
Application 7 DAA14 DAA
mm
Exeter 2003 29 9 1 39 68 68 29 0 3 8
2004 89 73 1 5 20 40 25 0 12 24
2005 22 17 7 0 3 3 0 0 24 35
2006 16 0 0 12 35 39 11 0 19 19
Harrow 2004 65 27 14 55 64 45 11 0 42 68
2005 2 0 4 15 22 12 5 0 1 5
2006 12 10 0 54 69 50 28 0 5 29
Ridgtetown 2003 26 15 0 15 37 60 37 1 18 26
2004 64 8 0 19 60 35 32 0 29 36
2005 17 15 0 17 22 18 12 0 4 33
2006 31 27 0 38 49 39 5 4 18 18
aAbbreviations: PRE, preemergence; POST, postemergence; DBA, Days before application; DAA, Days after application.
Precipitation Influences Pre- and Post-Emergence Herbicide Efficacy in Corn
1196
Table 3. Mean percent control of Chenopodium album in response to preemergence (PRE) herbicides 28 days after treatment
(DAT) at Exeter and Ridgetown, Ontario from 2003 to 2006 and Harrow, Ontario from 2005-2006.a
Rate Percent Weed Control
Treatment kg·ai· ha1 Environment 1b Environment 2
s-metolachlor/benoxacor/atrazine 2.88 99a 67b
dimethenamid-p + atrazine 0.75 + 1.28 100a 79ab
isoxaflutole + atrazine 0.079 + 0.8 100a 78ab
s-metolachlor/benoxacor/atrazine + dicamba 2.52 + 0.6 99a 87a
dimethenamid + dicamba/atrazine 1.13 + 1.48 100a 91a
s-metolachlor/benoxacor/atrazine + mesotrione 2.52 + 0.14 100a 90a
pendimethalin + dicamba/atrazine 1.68 + 1.48 99a 90a
pendimethalin + atrazine 1.68 + 1.53 100a 92a
rimsulfuron + s-metolachlor/benoxacor + dicamba 0.015 + 0.684 + 0.36 100a 84a
LSD0.05 1.41 14.9
aData were pooled by environment (location and year) when the interaction between environment and treatment was non-significant. Means are presented on
the back-transformed scale. Means followed by the same letter within a column are not significantly different according to Fisher’s Protected LSD (P < 0.05);
bEnvironment 1: Exeter 2003/2004/2006, Harrow 2005/2006, Ridgetown 2003/2004/2006; Environment 2: Exeter 2005; Ridgetown 2005
Table 4. Mean percent control of Chenopodium album in response to POST herbicides 28 days after treatment (DAT) at Exe-
ter and Ridgetown, Ontario from 2003 to 2006 and Harrow, Ontario from 2005-2006.a
Rate Percent Weed Control
Treatmentb
kg·ai· ha1 Environment 1c Environment 2
glyphosate 0.9 100a 92b
glyphosate fb glyphosate 0.9 fb 0.9 100a 99a
rimsulfuron + s-metolachlor/benoxacor + dicamba 0.0125 + 0.573 + 0.3 100a 99a
nicosulfuron/rimsulfuron + dicamba/diflufenzopyrd 0.025 + 0.2 98a 98a
nicosulfuron/rimsulfuron + mesotrione + atrazine 0.025 + 0.1 + 0.28 99a 100a
nicosulfuron + dicamba/diflufenzopyr 0.025 + 0.2 94a 98a
nicosulfuron + prosulfuron + dicamba 0.025 + 0.01 + 0.14 95a 98a
nicosulfuron + mesotrione + atrazine 0.0025 + 0.1 +0.28 100a 99a
nicosulfuron + pendimethalin + dicamba 0.0125 + 1.0 + 0.3 100a 100a
nicosulfuron + primisulfuron/dicamba 0.0188 + 0.166 100a 98a
foramsulfuron + dicamba/diflufenzopyre 0.07 + 0.2 100a 100a
LSD0.05 6.58 2.6
aData were pooled by environment (location and year) when the interaction between environment and treatment was non-significant; Means are presented on
the back-transformed scale. Means followed by the same letter within a column are not significantly different according to Fisher’s Protected LSD (P < 0.05).
Abbreviations: POST, postemergence; fb, followed by; bAll treatments other than glyphosate and glyphosate fb glyphosate had a non-ionic surfactant added at
0.2 % v/v; cEnvironment 1: Exeter 2003, Harrow 2005; Environment 2: Exeter 2004/2005/2006, Harrow 2006, Ridgetown 2003/2004/2005/2006; dUAN 28%
was added to nicosulfuron/rimsulfuron + dicamba/diflufenzopyr; nicosulfuron + dicamba/diflufenzopyr; and foramsulfuron + dicamba/diflufenzopyr treatments
at 1.25 L·ha–1; eAn MSO at 1.75 L·ha–1 was added to this treatment.
all sites. Percent weed control was visually assessed 28
and 56 days after treatment (DAT) using a scale of 0 to
100 where a rating of 0 was defined as no visible weed
control and a rating of 100 was defined as complete con-
trol. Only data from 28 DAT are presented in this manu-
script. Corn was mechanically harvested at physiological
maturity using a plot combine at all sites. Corn yields
were adjusted to a 15.5% moisture level.
All data were subjected to analysis of variance
(ANOVA) and analyzed using the PROC MIXED pro-
cedure in SAS statistical software [38]. Variances were
partitioned into the fixed effect of herbicide treatment
and into the random effects of year and location, the in-
teraction of year and location by the fixed effect, and
blocks nested within year and location establishing the
environment groupings presented in each of the data ta-
bles. The assumptions of the variance analysis were
tested by ensuring that the residuals were random, ho-
mogeneous, with a normal distribution about a mean of
zero using residual plots and the Shapiro-Wilk normality
Copyright © 2012 SciRes. AJPS
Precipitation Influences Pre- and Post-Emergence Herbicide Efficacy in Corn 1197
test. When the interaction between year, location and
treatment was not significant, data were pooled by envi-
ronment. Percent weed control data required an acrsine
square-root transformation. Crop yield data did not re-
quire transformation. Transformed data were back-trans-
formed for presentation in the tables. Treatment means
were separated at the 5% level of significance using a
Fisher’s Protected LSD test.
3. Results and Discussion
3.1. Chenopodium album
A significant treatment by year by location interaction
resulted in two distinct environments for Chenopodium
album control with PRE herbicides (Table 3) and two
distinct environments with POST herbicides (Table 4).
Preemergence control of Chenopodium album was near
perfect (>99%) in environment 1 for all treatments (Ta-
ble 3); however, control was variable in environment 2.
S-metolachlor/benoxacor/atrazine, dimethenamid-p + atra-
zine, and isoxaflutole + atrazine had lower control of
Chenopodium album in environment 2 compared to all
other treatments (Table 3). S-metolachlor/benoxacor/
atrazine provided only 67% control of Chenopodium
album in environment 2, and dimethenamid-p + atrazine
or isoxaflutole + atrazine provided less than 80% control.
Because PRE herbicides require precipitation to move
into the zone of active seed germination, a decrease in
precipitation for May 2005 of up to 59% of the 30-yr
average may explain the variability among treatments (S-
metolachlor/benoxacor/atrazine, isoxaflutole + atrazine,
and dimethenamid-p + atrazine) in environment 2 [4,17,
26,29,39]. Additionally, Environment 2 (Exeter 2005) re-
ceived only 3 mm of precipitation within 14 DAT (Table
2). The reduced control with isoxaflutole and S-metola-
chlor is supported by Chomas and Kells [10] who found
that when precipitation was limited to 2 mm 14 DAT,
isoxaflutole + atrazine and S-metolachlor + atrazine only
controlled Chenopodium album 33% and 75%, respec-
tively, in comparison to a weed-free control in corn. In
the same study; however, pendimethalin plus atrazine
provided 91% control of Chenopodium album; which is
comparable to our observations of 94% averaged over
both environments.
Postemergence control of Chenopodium album in en-
vironment 1 was 94% to 100% (Table 4). Sequential ap-
plications of glyphosate increased Chenopodium album
control by 7% in environment 2, compared to a single
application of glyphosate. Chenopodium album control
among the other herbicide treatments in environment 2
only varied by 2%. The efficacy of glyphosate can be
reduced if precipitation occurs 15 min to 6 hr after appli-
cation depending on formulation [24,25,27]. The formu-
lation of glyphosate used in this study has a rain fast time
of 15 min. Therefore, precipitation on the day of gly-
phosate application (Ridgetown, 2003 and 2006) (Table
2) is unlikely to have contributed to reduced Cheno-
podium alb um control.
3.2. Ambrosia artemisiifolia
A significant treatment by environment interaction re-
sulted in two distinct environments for Ambrosia ar-
temisiifolia control with PRE herbicides (Table 5) and
two distinct environments with POST herbicides (Table
6). Ambrosia artemisiifolia control with PRE herbicides
was excellent (>97%) in environment 1, but varied
among treatments in environment 2 (Table 5). S-metola-
chlor/benoxacor/atrazine had the lowest control of Am-
brosia artemisiifolia compared to any other treatment.
Pendimethalin + atrazine had 7% lower control compared
to the dimethenamid + dicamba/atrazine and pendi-
methalin + dicamba/atrazine treatments, respectively, in
environment 2. Poor control with S-metolachlor/benox-
acor/atrazine and pendimethalin + atrazine alone was
expected as neither herbicide provides adequate control
of this species [27,40]. The addition of dicamba to
S-metolachlor/benoxacor/atrazine or pendimethalin + atra-
zine increased the Ambrosia artemisiifolia control al-
ready being provided by the atrazine. Rimsulfuron +
S-metolachlor/benoxacor + dicamba was a tank-mix that
provided excellent (100%) control of Ambrosia artemisi-
ifolia except in environment 2. Exeter and Ridgetown in
2005 (environment 2) received 66% less precipitation in
May and June compared to a 30-yr average (data not
shown). It is likely that these exceptionally dry condi-
tions contributed to the reduced Ambrosia artemisiifolia
control with dicamba because of reduced uptake of the
herbicide from the soil.
Postemergence herbicide treatments provided greater
than 92% control of Ambrosia artemisiifolia in both en-
vironments (Table 6).
3.3. Amaranthus retroflexus
A significant treatment by environment interaction re-
sulted in two distinct environments for Amaranthus ret-
roflexus control with PRE herbicides (Table 7), and two
distinct environments for control with POST herbicides
(Table 8). Amaranthus retroflexus control did not differ
among treatments in environment 1. In environment 2,
S-metolachlor/benoxacor/atrazine and dimethenamid-p +
atrazine treatments had lower control compared to all
other treatments except pendimethalin + atrazine. Li-
mited precipitation at Ridgetown in 2005 (Table 2) after
herbicide application may have contributed to reduced
control of this species. Both of these treatments require
precipitation to move into the zone of active seed germi-
nation [17,26,39].
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Precipitation Influences Pre- and Post-Emergence Herbicide Efficacy in Corn
Copyright © 2012 SciRes. AJPS
1198
Table 5. Mean percent control of Ambrosia artemisiifolia in response to preemergence (PRE) herbicides 28 days after treat-
ment (DAT) at Exeter, Ontario from 2003 to 2006, Ridgetown, Ontario in 2003 and 2006 and Harrow, Ontario in 2004 and
2006.a
Rate Percent Weed Control
Treatment
kg·ai· ha1 Environment 1y Environment 2
s-metolachlor/benoxacor/atrazine 2.88 100a 77b
dimethenamid-p + atrazine 0.75 + 1.28 100a 93a
isoxaflutole + atrazine 0.079 + 0.8 97a 83ab
s-metolachlor/benoxacor/atrazine + dicamba 2.52 + 0.6 100a 98a
dimethenamid + dicamba/atrazine 1.13 + 1.48 100a 98a
s-metolachlor/benoxacor/atrazine + mesotrione 2.52 + 0.14 100a 94a
pendimethalin + dicamba/atrazine 1.68 + 1.48 100a 98a
pendimethalin + atrazine 1.68 + 1.53 100a 91a
rimsulfuron + s-metolachlor/benoxacor + dicamba 0.015 + 0.684 + 0.36 100a 86ab
LSD0.05 3.11 15.97
aData were pooled by environment (location and year) when the interaction between environment and treatment was non-significant. Means are presented on
the back-transformed scale. Means followed by the same letter within a column are not significantly different according to Fisher’s Protected LSD (P < 0.05);
bEnvironment 1: Exeter 2003/2004/2006, Harrow 2004/2006, Ridgetown 2003/2006; Environment 2: Exeter 2005; Ridgetown 2005.
Table 6. Mean percent control of Ambrosia artemisiifolia in response to POST herbicides 28 days after treatment (DAT) at
Exeter and Ridgetown, Ontario from 2003 to 2006 and Harrow, Ontario from 2005 and 2006.a
Rate Percent Weed Control
Treatmentb
kg·ai· ha1 Environment 1c Environment 2
glyphosate 0.9 100a 92b
glyphosate fb glyphosate 0.9 fb 0.9 100a 100a
rimsulfuron + s-metolachlor/benoxacor + dicamba 0.0125 + 0.573 + 0.3 100a 100a
nicosulfuron/rimsulfuron + dicamba/diflufenzopyrd 0.025 + 0.2 99a 100a
nicosulfuron/rimsulfuron + mesotrione + atrazine 0.025 + 0.1 + 0.28 99a 100a
nicosulfuron + dicamba/diflufenzopyr 0.025 + 0.2 99a 100a
nicosulfuron + prosulfuron + dicamba 0.025 + 0.01 + 0.14 99a 100a
nicosulfuron + mesotrione + atrazine 0.0025 + 0.1 +0.28 99a 100a
nicosulfuron + pendimethalin + dicamba 0.0125 + 1.0 + 0.3 100a 100a
nicosulfuron + primisulfuron/dicamba 0.0188 + 0.166 100a 100a
foramsulfuron + dicamba/diflufenzopyre 0.07 + 0.2 99a 100a
LSD0.05 2.19 1.99
aData were pooled by environment (location and year) when the interaction between environment and treatment was non-significant. Means are presented on
the back-transformed scale. Means followed by the same letter within a column are not significantly different according to Fisher’s Protected LSD (P < 0.05).
Abbreviations: POST, postemergence; fb, followed by; bAll treatments other than glyphosate and glyphosate fb glyphosate had a non-ionic surfactant added at
0.2 % v/v; cEnvironment 1: Exeter 2003, Harrow 2005, Ridgetown 2005/2006; Environment 2: Exeter 2004/2005/2006, Harrow 2004/2006, Ridgetown
2003/2004; dUAN 28% was added to nicosulfuron/rimsulfuron + dicamba/diflufenzopyr; nicosulfuron + dicamba/diflufenzopyr; and foramsulfuron + dicamba/
diflufenzopyr treatments at 1.25 L·ha–1; eAn MSO at 1.75 L·ha–1 was added to this treatment.
Precipitation Influences Pre- and Post-Emergence Herbicide Efficacy in Corn 1199
Table 7. Mean percent control of Amaranthus retroflexus in response to preemergence (PRE) herbicides 28 days after treat-
ment (DAT) at Exeter, Ontario from 2003 to 2006, Ridgetown, Ontario from 2004 to 2005 and Harrow, Ontario in 2006.a
Rate Percent Weed Control
Treatment kg·ai· ha1 Environment 1b Environment 2
s-metolachlor/benoxacor/atrazine 2.88 99a 56c
dimethenamid-p + atrazine 0.75 + 1.28 100a 75bc
isoxaflutole + atrazine 0.079 + 0.8 99a 97a
s-metolachlor/benoxacor/atrazine + dicamba 2.52 + 0.6 100a 100a
dimethenamid + dicamba/atrazine 1.13 + 1.48 100a 100a
s-metolachlor/benoxacor/atrazine + mesotrione 2.52 + 0.14 100a 100a
pendimethalin + dicamba/atrazine 1.68 + 1.48 100a 100a
pendimethalin + atrazine 1.68 + 1.53 100a 84ab
rimsulfuron + s-metolachlor/benoxacor + dicamba 0.015 + 0.684 + 0.36 100a 100a
LSD0.05 1.53 20.87
aData were pooled by environment (location and year) when the interaction between environment and treatment was non-significant. Means are presented on
the back-transformed scale. Means followed by the same letter within a column are not significantly different according to Fisher’s Protected LSD (P < 0.05);
bEnvironment 1: Exeter 2003/2004/2005/2006, Harrow 2006, Ridgetown 2004; Environment 2: Ridgetown 2005.
Table 8. Mean percent control of Amaranthus retroflexus in response to POST herbicides 28 days after treatment (DAT) at
Exeter, Ontario from 2003 to 2006, Ridgetown, Ontario in 2003, 2004 and 2006 and Harrow, Ontario in 2006.a
Rate Percent Weed Control
Treatmentb
kg·ai· ha1 Environment 1c Environment 2
glyphosate 0.9 100a 91b
glyphosate fb glyphosate 0.9 fb 0.9 99a 98ab
rimsulfuron + s-metolachlor/benoxacor + dicamba 0.0125 + 0.573 + 0.3 100a 96ab
nicosulfuron/rimsulfuron + dicamba/diflufenzopyrd 0.025 + 0.2 100a 99a
nicosulfuron/rimsulfuron + mesotrione + atrazine 0.025 + 0.1 + 0.28 99a 96ab
nicosulfuron + dicamba/diflufenzopyr 0.025 + 0.2 100a 99a
nicosulfuron + prosulfuron + dicamba 0.025 + 0.01 + 0.14 100a 98ab
nicosulfuron + mesotrione + atrazine 0.0025 + 0.1 +0.28 99a 93b
nicosulfuron + pedimethalin + dicamba 0.0125 + 1.0 + 0.3 99a 98ab
nicosulfuron + primisulfuron/dicamba 0.0188 + 0.166 100a 99a
foramsulfuron + dicamba/diflufenzopyre 0.07 + 0.2 100a 100a
LSD0.05 1.10 5.78
aData were pooled by environment (location and year) when the interaction between environment and treatment was non-significant. Means are presented on
the back-transformed scale. Means followed by the same letter within a column are not significantly different according to Fisher’s Protected LSD (P < 0.05).
Abbreviations: POST, postemergence; fb, followed by; bAll treatments other than glyphosate and glyphosate fb glyphosate had a non-ionic surfactant added at
0.2 % v/v; c Environment 1: Exeter 2003/2004/2005; Environment 2: Exeter 2006, Harrow 2006, Ridgetown 2003/2004/2006; dUAN 28% was added to nico-
sulfuron/rimsulfuron + dicamba/diflufenzopyr; nicosulfuron + dicamba/diflufenzopyr; and foramsulfuron + dicamba/diflufenzopyr treatments at 1.25 L·ha–1;
eAn MSO at 1.75 L·ha–1 was added to this treatment.
Control of Amaranthus retroflexus did not differ among
POST treatments in environment 1. Sequential applica-
tions of glyphosate provided an increase in Amaranthus
retroflexus control by up to 7% compared to a single
application in environment 2. Amaranthus retroflexus
control varied by up to 7% among treatments in envi-
ronment 2, except for treatment 1 where control was 9%
lower. Control of Amaranthus retroflexus was reduced
when nicosulfuron + mesotrione + atrazine was applied
compared to nicosulfuron/rimsulfuron + dicamba/diflu-
fenzopyr, nicosulfuron + dicamba/diflufenzopyr and fo-
ramsulfuron + dicamba/diflufenzopyr. Mesotrione plus
atrazine (POST) can be antagonistic when tank-mixed
with sulfonylureas such as nicosulfuron, but to-date this
has only been shown to affect control of annual grass
species [7,41,42].
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Precipitation Influences Pre- and Post-Emergence Herbicide Efficacy in Corn
1200
3.4. Setaria viridis
A significant treatment by environment interaction re-
sulted in three distinct environments for Setaria viridis
control with PRE herbicides (Table 9) and two distinct
environments for POST herbicides (Table 10). For PRE
herbicides, there was little variability among treatments
in environment 3. In environment 1, Setaria viridis con-
trol was the lowest with pendimethalin + dicamba/
atrazine (Table 9). This result can be attributed to the
high amount of precipitation received. Exeter received 68
mm of precipitation in 2003 and Ridgetown received 49
mm in 2006 during the 14 DAT (Table 2), representing
87 and 64% of the 30-year norm for the entire month of
May, respectively (data not shown). Pendimethalin is
more persistent under dry conditions [31] and can leach
with 20 mm of precipitation in turfgrass [30]. Atrazine
also has the potential to leach with relatively low levels
of precipitation [25].
Table 9. Mean percent control of Setaria viridis in response to preemergence (PRE) herbicides 28 days after treatment (DAT)
at Exeter, Ontario from 2003 to 2006, Ridgetown, Ontario from 2004 to 2006 and Harrow, Ontario from 2005 to 2006.a
Rate Percent Weed Control
Treatment kg·ai·ha 1 Environment 1bEnvironment 2 Environment 3
s-metolachlor/benoxacor/atrazine 2.88 96ab 76a 97a
dimethenamid-p + atrazine 0.75 + 1.28 99a 81a 100a
isoxaflutole + atrazine 0.079 + 0.8 99a 46b 100a
s-metolachlor/benoxacor/atrazine + dicamba 2.52 + 0.6 97ab 85a 100a
dimethenamid + dicamba/atrazine 1.13 + 1.48 99a 91a 100a
s-metolachlor/benoxacor/atrazine + mesotrione 2.52 + 0.14 96ab 79a 100a
pendimethalin + dicamba/atrazine 1.68 + 1.48 84b 85a 100a
pendimethalin + atrazine 1.68 + 1.53 95ab 76a 100a
rimsulfuron + s-metolachlor/benoxacor + dicamba 0.015 + 0.684 + 0.36 98a 79a 98a
LSD0.05 13.81 17.24 3.86
aData were pooled by environment (location and year) when the interaction between environment and treatment was non-significant. Means are presented on
the back-transformed scale. Means followed by the same letter within a column are not significantly different according to Fisher’s Protected LSD (P < 0.05);
bEnvironment 1: Exeter 2003, Ridgetown 2006; Environment 2: Exeter 2004/2005, Ridgetown 2005; Environment 3: Exeter 2006, Harrow 2005/2006, Ridge-
town 2004.
Table 10. Mean percent control of Setaria viridis in response to POST herbicides 28 days after treatment (DAT) at Exeter,
Ontario from 2003 to 2006 and Ridgetown, Ontario from 2003 to 2005.a
Rate Percent Weed Control
kg·ai· ha1 Environment 1c
E
nvironment 2
Treatmentb
MT ha1
glyphosate 0.9 97ab 78e
glyphosate fb glyphosate 0.9 fb 0.9 100a 96a
rimsulfuron + s-metolachlor/benoxacor + dicamba 0.0125 + 0.573 + 0.3 93b 83d
nicosulfuron/rimsulfuron + dicamba/diflufenzopyrd 0.025 + 0.2 98a 89b
nicosulfuron/rimsulfuron + mesotrione + atrazine 0.025 + 0.1 + 0.28 96ab 87bc
nicosulfuron + dicamba/diflufenzopyr 0.025 + 0.2 96ab 88bc
nicosulfuron + prosulfuron + dicamba 0.025 + 0.01 + 0.14 97ab 87bc
nicosulfuron + mesotrione + atrazine 0.0025 + 0.1 +0.28 95ab 87bc
nicosulfuron + pedimethalin + dicamba 0.0125 + 1.0 + 0.3 95ab 85cd
nicosulfuron + primisulfuron/dicamba 0.0188 + 0.166 97ab 89b
foramsulfuron + dicamba/diflufenzopyre 0.07 + 0.2 96ab 90b
LSD0.05 5.42 4.25
aData were pooled by environment (location and year) when the interaction between environment and treatment was non-significant. Means are presented on
the back-transformed scale. Means followed by the same letter within a column are not significantly different according to Fisher’s Protected LSD (P < 0.05).
Abbreviations: POST, postemergence; fb, followed by; bAll treatments other than glyphosate and glyphosate fb glyphosate had a non-ionic surfactant added at
0.2 % v/v; cEnvironment 1: Exeter 2003/2004/2006, Ridgetown 2003/2005; Environment 2: Exeter 2005, Ridgetown 2004; dUAN 28% was added to nicosulfu-
ron/rimsulfuron + dicamba/diflufenzopyr; nicosulfuron + dicamba/diflufenzopyr; and foramsulfuron + dicamba/diflufenzopyr treatments at 1.25 L·ha–1; eAn
MSO at 1.75 L·ha–1 was added to this treatment.
Copyright © 2012 SciRes. AJPS
Precipitation Influences Pre- and Post-Emergence Herbicide Efficacy in Corn 1201
In environment 2, isoxaflutole + atrazine provided
46% control of Setaria viridis, while all other treatments
had 76% or better control. Low levels of precipitation 7
DAT at Exeter in 2004 contributed to reduced control,
because isoxaflutole + atrazine requires precipitation for
activation [29].
Control of Setaria viridis with POST herbicides varied
among treatments in both environments (Table 10).
Rimsulfuron + S-metolachlor/benoxacor + dicamba re-
duced Setaria viridis control by 5% compared to nico-
sulfuron/rimsulfuron + dicamba/diflufenzopyr in envi-
ronment 1. The slight decrease in control can be attri-
buted to delayed precipitation for 2 - 4 DAT at Exeter
(2003, 2006) and Ridgetown (2003, 2005). Similar re-
sults were illustrated by Lyon and Wilson [4].
With the exception of sequential applications of gly-
phosate, all treatments in environment 2 provided 90% or
less control of Setaria viridis. Precipitation at Exeter in
May and June 2005 was 41% and 55% of the monthly
norm, respectively (data not shown). Dry conditions at
Exeter in 2005 may have lead to reduced control through
decreased herbicide translocation and uptake. Morton
and Harvey [43] found similar results with primisulfuron
applied POST on quackgrass (Elytrigia repens L. Nevski.)
in dry conditions (no moisture 6 - 8 days before applica-
tion) and attributed the result to reduced translocation.
Bailey et al. [21] attributed reduced control of yellow
nutsedge (Cyperus esculentus L.) with POST applied
metribuzin + rimsulfuron to abnormally low precipitation
amounts prior to POST application. Additionally, 59 mm
of precipitation was received 14DAT, which may have
promoted late emergence of Setaria viridis. Nicosulfuron,
foramsulfuron and glyphosate do not have residual con-
trol, therefore would not have controlled late emerging
weeds [27,34]. Sequential applications of glyphosate had
18% greater control of Setaria viridis than a single ap-
plication of glyphosate in environment 2 (Table 10).
3.5. Corn Yield
A significant treatment by environment interaction re-
sulted in two distinct environments for corn yield with
PRE herbicides (Table 11). Corn yield did not differ
among herbicide treatments in environment 1. Corn yield
was reduced with S-metolachlor/benoxacor/atrazine, di-
methenamid-p + atrazine and pendimethalin + atrazine
compared to isoxaflutole + atrazine and S-metolachlor/
benoxacor/atrazine + mesotrione in environment 2. The
above results are not linked to reduced weed control.
Therefore, the reason for the decrease in yield is unclear.
In environment 2, S-metolachlor/benoxacor/atrazine
and S-metolachlor/benoxacor/atrazine + mesotrione had
the lowest amount of corn yield compared to all other
herbicide treatments. Dimethenamid-p + atrazine and
pendimethalin + atrazine treatments also had reduced
corn yield and this reduced corn yield coincided with re-
duced weed control at each location for these treatments.
A significant treatment by environment interaction re-
sulted in two distinct environments for corn yield with
POST herbicides (Table 12). There was no variation
among herbicide treatments in environment 2 and only a
slight variation among treatments in environment 1. Corn
yield with nicosulfuron + prosulfuron + dicamba was
reduced by 0.5 - 0.8 T·ha–1 compared to six other treat-
ments, which corresponds to 3% and 6% reduction in
Chenopodium album and Setaria viridis control, respect-
tively (Table 12).
Table 11. Mean corn yield in the untreated check and in response to application of preemergence (PRE) herbicides at Exeter
and Ridgetown, Ontario from 2003 to 2006 and Harrow, Ontario from 2004 to 2006.a
Rate Yield
kg·ai· ha1 Environment 1b Environment 2
Treatment
MT ha1
Untreated Check 5.0b 2.5e
s-metolachlor/benoxacor/atrazine 2.88 9.9a 6.8d
dimethenamid-p + atrazine 0.75 + 1.28 9.9a 8.5c
isoxaflutole + atrazine 0.079 + 0.8 10.7a 8.9abc
s-metolachlor/benoxacor/atrazine + dicamba 2.52 + 0.6 10.5a 9.9ab
dimethenamid + dicamba/atrazine 1.13 + 1.48 10.6a 10.2a
s-metolachlor/benoxacor/atrazine + mesotrione 2.52 + 0.14 10.9a 7.2d
pendimethalin + dicamba/atrazine 1.68 + 1.48 10.7a 10.1a
pendimethalin + atrazine 1.68 + 1.53 10.2a 8.6bc
rimsulfuron + s-metolachlor/benoxacor + dicamba 0.015 + 0.684 + 0.36 10.3a 9.4abc
LSD0.05 1.39 1.28
aData were pooled by environment (location and year) when the interaction between environment and treatment was non-significant. Means followed by the
same letter within a column are not significantly different according to Fisher’s Protected LSD (P < 0.05); bEnvironment 1: Exeter 2003/2004/2006, Ridgetown
2003/2005/2006, Harrow 2004/2005/2006, Ridgetown 2004; Environment 2: Exeter 2005.
Copyright © 2012 SciRes. AJPS
Precipitation Influences Pre- and Post-Emergence Herbicide Efficacy in Corn
1202
Table 12. Mean corn yield in the untreated check and in response to application of POST herbicides at Exeter and Ridgetown,
Ontario from 2003 to 2006 and Harrow, Ontario from 2004 to 2006.a
Treatmentb Rate Yield
kg·ai· ha1 Environment 1c Environment 2
MT ha–1
Untreated Check 4.7c 5.2b
glyphosate 0.9 10.0ab 12.2a
glyphosate fb glyphosate 0.9 fb 0.9 10.2a 12.3a
rimsulfuron + s-metolachlor/benoxacor + dicamba 0.0125 + 0.573 + 0.3 10.1ab 11.9a
nicosulfuron/rimsulfuron + dicamba/diflufenzopyrd 0.025 + 0.2 10.5a 11.5a
nicosulfuron/rimsulfuron + mesotrione + atrazine 0.025 + 0.1 + 0.28 10.5a 11.7a
nicosulfuron + dicamba/diflufenzopyr 0.025 + 0.2 10.2a 12.1a
nicosulfuron + prosulfuron + dicamba 0.025 + 0.01 + 0.14 9.7b 12.1a
nicosulfuron + mesotrione + atrazine 0.0025 + 0.1 + 0.28 10.4a 11.9a
nicosulfuron + pedimethalin + dicamba 0.0125 + 1.0 + 0.3 10.1ab 11.8a
nicosulfuron + primisulfuron/dicamba 0.0188 + 0.166 9.9ab 11.7a
foramsulfuron + dicamba/diflufenzopyre 0.07 + 0.2 10.2a 11.1a
LSD0.05 0.47 2.1
aData were pooled by environment (location and year) when the interaction between environment and treatment was non-significant. Means followed by the
same letter within a column are not significantly different according to Fisher’s Protected LSD (P < 0.05). Abbreviations: POST, postemergence; fb, followed
by; bAll treatments other than glyphosate and glyphosate fb glyphosate had a non-ionic surfactant added at 0.2 % v/v; cEnvironment 1: Exeter 2003/2004/2005,
Ridgetown 2005; Environment 2: Exeter 2006, Harrow 2004/2005/2006, Ridgetown 2003/2004/2006; dUAN 28% was added to nicosulfuron/rimsulfuron +
dicamba/diflufenzopyr; nicosulfuron + dicamba/diflufenzopyr; and foramsulfuron + dicamba/diflufenzopyr treatments at 1.25 L·ha–1; eAn MSO at 1.75 L·ha–1
was added to this treatment.
4. Conclusion
Results demonstrate that the efficacy of PRE and POST
herbicides are affected by the timing and amount of pre-
cipitation. Limited precipitation 7 days before or after
herbicide application can reduce efficacy of PRE herbi-
cides that require precipitation for optimal activity and
excessive precipitation may delay POST herbicide ap-
plication allowing weeds to grow beyond an optimal size.
Additionally, precipitation on the day of PRE herbicide
application or precipitation 14 DAT that is greater than
50% of the monthly average can cause some herbicides
to leach (i.e. atrazine and pendimethalin), resulting in
reduced efficacy. Therefore, precipitation amount and
timing should be considered when selecting the most
appropriate weed management strategy in corn. A further
objective of this study was to determine if a glyphosate
only program would be provide more sustainable weed
control than a mixed mode of action program under
variable precipitation. These data suggest that two se-
quential applications of glyphosate was the most consis-
tent weed management program for postemergence con-
trol of weeds. However, with careful monitoring of pre-
cipitation and other climatic conditions the authors rec-
ommend the incorporation of other modes of action to
ensure proper herbicide resistance management.
5. Acknowledgements
The authors wish to acknowledge the technical assistance
of Todd Cowan, Christy Shropshire, and Mac Whaley.
Funding for this research was provided by BASF, Bayer
CropScience, Dow Agrosciences, Dupont, Monsanto,
and Syngenta Crop Protection
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