American Journal of Plant Sciences, 2013, 4, 1790-1798
http://dx.doi.org/10.4236/ajps.2013.49220 Published Online September 2013 (http://www.scirp.org/journal/ajps)
Glyphosate-Resistant Giant Ragweed (Ambrosia trifida L.):
2,4-D Dose Response and Control with Postemergence
Herbicides in Soybean
Joanna Follings1, Nader Soltani1*, Darren E. Robinson1, François J. Tardif2, Mark B. Lawton3,
Peter H. Sikkema1
1University of Guelph Ridgetown Campus, Ridgetown, Canada; 2University of Guelph, Guelph, Canada; 3Monsanto Canada, Guelph,
Canada.
Received June 25th, 2013; revised July 25th, 2013; accepted August 15th, 2013
Copyright © 2013 Joanna Follings et al. This is an open access article distributed under the Creative Commons Attribution License,
which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.
ABSTRACT
Glyphosate resistant giant ragweed is an increasing problem in glyphosate resistant cropping systems in southwestern
Ontario. The postemergence herbicides registered for use in soybean in Ontario do not provide consistent control of
glyphosate resistant giant ragweed. There is limited research on the lowest effective rate of 2,4-D for the control of
glyphosate resistant giant ragweed. Consequently, the objectives of this study were a) to determine the efficacy of her-
bicides applied postemergence for the control of glyphosate resistant giant ragweed in glyphosate resistant soybean, and
b) to determine the lowest effective rate of 2,4-D for the control of glyphosate-resistant giant ragweed. Ten postemer-
gence herbicide combinations and seven rates of 2,4-D were evaluated in field studies conducted in 2011 and 2012 at
six locations confirmed with glyphosate-resistant giant ragweed. The post emergence herbicides evaluated did not pro-
vide acceptable/consistent control. Of the herbicides evaluated, glyphosate plus cloransulam-methyl provided 26% to
70% control 8 WAA of glyphosate resistant giant ragweed, which was the best of the herbicides combinations evaluated.
The doses of 2,4-D required to reduce giant ragweed shoot dry weight by 50, 80 and 95% were 142, 310 and 1048 g a.e.
ha–1, respectively
Keywords: Glyphosate Resistance; Multiple Herbicide-Resistant Crops; Preplant Herbicides; Postemergence
Herbicides
1. Introduction
Glyphosate is the most widely used postemergence, non-
selective herbicide in the world [1] and is used in row
crops, orchards, fallow lands and pastures [2]. Since the
introduction of glyphosate resistant soybean in 1996,
there has been a rapid increase in the use of glyphosate
resistant crops [3]. In large soybean growing countries
such as Argentina and the United States, more than 90%
of soybeans grown are glyphosate resistant [3,4]. The use
of glyphosate resistant crops has changed weed man-
agement practices causing intense selection pressure for
glyphosate resistant weeds [5]. There is a widespread gly-
phosate resistance in weed species around the world. The
first glyphosate resistant weed reported was a population
of rigid ryegrass (Lolium rigidum L. Gaud) in Australia
in 1996 [6]. Since then, additional glyphosate resistant
weeds were reported. Currently there are 24 weed species
resistant to glyphosate worldwide [7].
Giant ragweed (Ambrosia trifida L.) is an erect broad-
leaf weed that can be found in southern areas of Mani-
toba, Ontario, Quebec, New Brunswick, Nova Scotia and
Prince Edward Island in Canada [8]. In Ontario it is
commonly found in crop production fields in the south-
western part of the province [9]. Giant ragweed is diffi-
cult to control due to its long emergence period. Giant
ragweed seedlings begin to emerge in early March [10]
and continue to emerge until late July [11]. Historically,
growers in Ontario would control this problematic weed
with glyphosate; however, in 2008 giant ragweed was
confirmed to be resistant to glyphosate [12].
Glyphosate resistant giant ragweed is an increasing
problem in glyphosate resistant cropping systems in On-
*Corresponding author.

Glyphosate-Resistant Giant Ragweed (Ambrosia trifida L.): 2,4-D Dose Response and Control
with Postemergence Herbicides in Soybean
1791
tario. As of 2010 there were 48 locations confirmed with
glyphosate resistant giant ragweed in Ontario (Vink et.
al., 2012). There are few herbicides applied postemer-
gence that provide acceptable control of glyphosate re-
sistant giant ragweed control in soybean. Vink et al. [13]
reported that glyphosate (900 g a.e. ha–1) plus cloransu-
lam-methyl (17.5 g a.i h–1) provided 80% to 81% control
of glyphosate resistant giant ragweed 8 weeks after ap-
plication (WAA). Research is required to identify addi-
tional postemergence herbicides for the control of gly-
phosate resistant giant ragweed in soybean. The first ob-
jective of this study was to determine the efficacy of all
the currently registered postemergence broadleaf herbi-
cides registered for use in Ontario in soybean.
2,4-D is a herbicide commonly used for the control of
broadleaf weeds and has been reported to control giant
ragweed with one application [8]. Similarly Vink et al.
[14] reported that glyphosate (900 g a.e. ha–1) plus 2,4-D
(500 g a.e. ha–1) applied as a preplant burndown provided
97 to 99% control of glyphosate resistant giant ragweed.
Research is required to identify the lowest effective rate
of 2,4-D for the control of glyphosate resistant giant rag-
weed. The second objective of this study was to deter-
mine the lowest effective rate of 2,4-D tank mixed with
glyphosate and applied as a preplant burndown for the
control of glyphosate-resistant giant ragweed in soybean.
2. Materials and Methods
Field studies were conducted in 2011 and 2012 at six
locations for the postemergence broadleaf herbicide ex-
periment and five locations for the 2,4-D dose response
experiment with confirmed glyphosate resistant giant
ragweed. The field sites were located near Windsor (L2
and L5), La Salle (L1, L4 and L6) and Amherstburg (L3),
Ontario. The first series of experiments evaluated the
effectiveness of postemergence broadleaf herbicides. The
second series of experiments, evaluated the biologically
effective rate of 2,4-D, is referred to as “dose response”.
Soil texture, soil organic matter content, soil pH, soybean
cultivar, seeding date, seeding rate, row spacing, herbi-
cide application date and giant ragweed height are pre-
sented in Table 1.
Experiments were set up in a randomized complete
block design with four replications. Each plot was 8 m
long and 2.5 m wide. Herbicides in the postemergence
study included glyphosate (900 g a.e. ha–1) applied alone,
and acifluorfen (600 g a.i. ha–1), fomesafen (240 g a.i.
ha–1) + Turbocharge (0.50% v/v), bentazon (1080 g a.i.
ha–1), thifensulfuron-methyl (6 g a.i. ha–1) + Agral 90
(0.010% v/v) + UAN 28% (8.0 L·ha–1), chlorimuron-
ethyl (9 g a.i. ha–1) + Agral 90 (0.20% v/v) + UAN 28%
(2.0 L·ha–1), cloransulam-methyl (17.5 g a.i. ha–1) +
Agral 90 (0.25% v/v) + UAN 28% (2.5% v/v), imaz-
ethapyr (100 g a.i. ha–1) + Agral 90 (0.25% v/v) + UAN
28% (2.0 L· ha–1), or imazethapyr (75 g a.i. ha–1) plus
bentazon (840 g a.i. ha–1)+ UAN 28% (2.0 L·ha–1) ap-
plied with glyphosate (900 g a.e. ha–1) and glyphosate/
fomesafen (1200 g a.i. ha–1). The herbicide rates used
were the maximum labeled rate registered for use in On-
tario. The dose response experiment evaluated glypho-
sate (900 a.e. ha–1) applied with 2,4-D at 31.25, 62.5, 125,
250, 500, 1000 or 2000 g a.e. ha–1. A weedy and weed-
free check was included in each experiment. All weed-
free check plots were maintained with 2,4-D ester (500 g
a.e. ha–1) and glyphosate (900 g a.e. ha–1) applied pre-
plant (PP) and subsequent hand hoeing as required.
Herbicide treatments were applied with a CO2-ressur-
ized backpack sprayer equipped with ULD 120 - 02 noz-
zles (Hypro, New Brighton, MN) calibrated to deliver
200 L·ha–1 of water at 210 kPa. Herbicide treatments
were applied with a 1.5 meter boom with four nozzles
spaced 50 cm apart over the centre of the plot. Herbicide
treatments were applied when giant ragweed reached 15
cm in height (Table 1).
Table 1. Location and soil characteristics, soybean cultivar, seeding date, soybean population, herbicide application date, and
giant ragweed height at time of application for a post herbicide and 2,4-D dose experiments conducted in Ontario in 2011 and
2012.
Location Year Soil
texture Soil OM Soil pH Soybean cultivar Seeding date Soybean population Herbicide application date Giant ragweed height
(%) (seeds·ha–1) (cm)
1-LaSalle 2011 Loam 2.6 7.5 Dekalb 31 - 10June 13467,029 May 21 0 - 7
2-Windsor 2011 Loam 2.8 6.9 Pioneer 92Y80June 15420,079 June 2 0 - 12
3-Amherstburg 2012 Clay loam 3.7 7.9 Pioneer 92Y53May 22568,100 May 1 0 - 7
4-LaSalle 2012 Loam 3.1 7.3 Dekalb 21 - 11May 16444,780 May 8 0 - 10
5-Windsor 2012 Clay loam 4.6 6.6 Pioneer 93Y05June 8 432,250 May 8 0 - 10
6-LaSalle 2012 Loam 3.1 7.3 Dekalb 21 - 11May 16444,780 May 8 0 - 11

Glyphosate-Resistant Giant Ragweed (Ambrosia trifida L.): 2,4-D Dose Response and Control
with Postemergence Herbicides in Soybean
1792
Weed control was rated visually 1, 2, 4 and 8 WAA on
a scale of 0% to 100%, where 0% was no control of giant
ragweed compared to the weedy check and 100% was
complete control of giant ragweed. At each control rating
giant ragweed height and density (plants per two 0.25 m2
quadrats) were recorded. At 4 WAA, giant ragweed den-
sity and biomass was determined in each plot by count-
ing giant ragweed plants in two 0.25 m2 quadrats. Giant
ragweed plants were cut off at the soil surface from the
two quadrats, placed in bags, dried at 60˚C to a constant
moisture content and the dry weights were recorded.
Soybean injury was rated 1, 2, 4 and 8 WAA. Soybean
injury was rated visually on a scale of 0% to 100%,
where 0% was no soybean injury and 100% was soybean
death. At crop maturity, soybeans were hand harvested
from 2 m of row from each plot at all locations. Soy-
beans were threshed in a stationary thresher and the
weight and moisture were recorded. Yields were adjusted
to 13.5% moisture.
2.1. Statistical Analysis
2.1.1. Postemergence Herbicides
An analysis of variance was conducted on all data using
the PROC MIXED procedure in SAS (Ver. 9.2, SAS
Institute Inc., Cary, NC). Variances were separated into
the random effects of location (year and location), repli-
cation (at each location) and location by treatment. Her-
bicide treatment was considered the fixed effect. The
significance of the random effects (location, replication
and location by treatment) and their interaction with
fixed effects was tested using the Z-test of the variance
estimate. The significance of the fixed effects was tested
using the F-test. Significant location by treatment inter-
actions were found for all variables; therefore, locations
were analyzed according to their interaction and pre-
sented accordingly. To ensure the assumptions (errors are
independent, homogenous and normally distributed) of
the variance analysis were met; residual plots were ex-
amined. Data were tested for normality using the Sha-
piro-Wilk statistic as generated by the UNIVARIATE
procedure in SAS. If necessary, a transformation of the
data (natural log, square root or arcsine square root) was
applied and chosen based on the highest Shapiro-Wilk
statistic generated. Control data 1 WAA were arcsine
square root transformed at L2, L4, L5 and L6 and data at
L1 and L3 were log transformed. Control data 2 WAA
were arcsine square root transformed at L1, L2, L5 and
L6 and data at L3 and L4 were log transformed. Control
data 4 WAA were arcsine transformed at L1 and L2 and
data at L3, L4, L5 and L6 were log transformed. Control
data 8 WAA were square root transformed at L1, L2 and
L3 and data at L4, L5, and L6 were log transformed. All
giant ragweed shoot dry weight data was square root
transformed. Soybean yield data were square root trans-
formed at L1 and L2 and data at L3, L4, L5 and L6 were
arcsine square root transformed. The means between
treatments were separated using Fisher’s protected LSD
at P < 0.05.
2.1.2. Field Dose Response
An analysis of variance was conducted on all data using
the PROC MIXED procedure in SAS (Ver. 9.2, SAS
Institute Inc., Cary, NC). Variances were separated into
the random effects of location (year and location), repli-
cation (at each location) and location by treatment. Her-
bicide treatment was considered the fixed effect. The sig-
nificance of the random effects (location, replication and
location by treatment) and their interaction with fixed ef-
fects was tested using the Z-test of the variance estimate.
The significance of the fixed effects was tested using the
F-test. Significant location by treatment interactions were
found for all variables; therefore, locations were ana-
lyzed according to their interaction and presented accord-
ingly. To ensure the assumptions (errors are independent,
homogenous and normally distributed) of the variance
analysis were met; residual plots were examined. Data
were tested for normality using the Shapiro-Wilk statistic
as generated by the UNIVARIATE procedure in SAS. If
necessary, a transformation of the data (natural log,
square root or arcsine square root) was applied and cho-
sen based on the highest Shapiro-Wilk statistic generated.
Control data 1 WAA were arcsine square root trans-
formed at L1, L2, L3 and L5 and data at L4 was log
transformed. Control data 2 WAA were arcsine square
root transformed at L1, L2, L3, and L5 and data at L4
were not transformed. Control data 4 WAA were arcsine
square root transformed at L1, L4, L3 and L5 and data at
L2 was square root transformed. Control data 8 WAA
were arcsine square root transformed at all locations.
Giant ragweed shoot dry weight was presented as a per-
cent of the weedy control and was log transformed. Soy-
bean yield was presented as a percent of the weed-free
control and was not transformed.
A non-linear regression analysis was conducted on all
data using the PROC NLIN procedure in SAS (Ver. 9.2,
SAS Institute Inc., Cary, NC). A sigmoidal log-logistic
curve was used:
 





50
Y C DC 1expBlndoselnI

 

where Y is percent giant ragweed control or percent
soybean yield, C is the lower limit, D is the upper limit,
B is the slope, and I50 is the dose where there is a 50%
response [15]. The effective dose (ED) of 2,4-D was also
calculated using this equation. Where possible, the ED50,

Glyphosate-Resistant Giant Ragweed (Ambrosia trifida L.): 2,4-D Dose Response and Control
with Postemergence Herbicides in Soybean
1793
ED80 and ED95 were calculated and represent the dose
required to achieve 50%, 80%, and 95% control of gly-
phosate resistant giant ragweed compared to the weed-
free control. The ED50, ED80 and ED95 also represent
50%, 80%, and 95% of the soybean yield compared to
the weed-free control. For giant ragweed shoot dry
weight, the ED50, ED80, and ED95 represent the dose
needed to reduced giant ragweed shoot dry weight by
50%, 80% and 95%.
3. Results and Discussion
3.1. Postemergence Herbicides
Control of glyphosate resistant giant ragweed with post-
emergence herbicides is extremely difficult as no herbi-
cide consistently provided greater than 75% control at 1
WAA (Table 2). At 1 WAA, control data at L4, L5 and
L6 could be combined and L1, L2 and L3 were analyzed
separately (Table 2). Control with glyphosate ranged
from 30% to 56% confirming the resistance status of the
sites. Glyphosate plus fomesafen as a tank mix and gly-
phosate/fomesafen as a premix were the most effective
treatments providing 75% to 88% and 69% to 83% con-
trol, respectively. The highest level of control 1 WAA
with the addition of acifluorfen or bentazon to glyphosate
was 81% and 82% control, respectively (Table 2). Gly-
phosate plus thifensulfuron, chlorimuron-ethyl, cloran-
sulam-methyl, imazethapyr, or imazethapyr plus benta-
zon provided less than 80% control.
At 2 WAA, L1 and L2, L5 and L6 could be combined
while L3 and L4 were analyzed separately (Table 3).
Two weeks after application, all treatments had a level of
control that was declining compared to the 1 WAA as-
sessment. Glyphosate provided less than 40% control of
glyphosate resistant giant ragweed across all locations,
while the addition of fomesafen provided 52% to 74%
control which is similar to the findings of Vink et al. [13]
who reported 50% to 86% control with glyphosate plus
fomesafen applied at the same rate. Glyphosate/fomesa-
fen or glyphosate plus acifluorfen, thifensulfuron, chlori-
muron-ethyl, cloransulam-methyl, bentazon, imazethapyr,
or imazethapyr plus bentazon provided less than 76%
control across all locations (Table 3).
At 4 WAA data could be combined and analyzed in
groups L1 and L2 and L3, L4, L5 and L6 (Table 4).
Glyphosate provided 23% to 32% control, while glypho-
sate plus imazethapyr provided 46% to 82% control. This
is similar to the findings of Vink et al. [13] who reported
69% to 82% control with glyphosate plus imazethapyr
applied at 900 g a.e. ha–1 + 100 g a.i. ha–1. Surprisingly,
glyphosate plus cloransulam-methyl provided 56% to
74% control 4 WAA opposite to what was observed be-
fore with this herbicide combination providing 88% to
92% control [13]. Cloransulam-methyl POST is espe-
Table 2. Percent control of glyphosate resistant giant ragweed 1 WAA with herbicides applied post emergence.
Control 1 WAAa
Treatment Rate L1a L2 L3 L4, L5, and L6
(g a.i. ha–1) _____________________% ________________________
Weedy Check 0 i 0 f 0 h 0 e
Weed Free Check 100 a 100 a 100 a 100 a
Glyphosate 900 56 g 34 e 30 ef 36 d
Glyphosate + Acifluorfen 900 + 600 76 cd 81 b 60 c 72 b
Glyphosate + Fomesafenb 900 + 240 88 b 75 bc 76 b 76 b
Glyphosate + Bentazon 900 + 1080 64 ef 82 b 30 ef 55 c
Glyphosate + Thifensulfuronc 900 + 6 46 h 63 cd 29 fg 51 c
Glyphosate + Chlorimuron-ethyld 900 + 9 70 de 74 bcd 26 g 52 c
Glyphosate + Cloransulam-methyle 900 + 17.5 78 c 63 d 34 de 55 c
Glyphosate + Imazethapyre 900 + 100 66 e 77 b 35 d 55 c
Glyphosate + Imazethapyr + Bentazonf 900 + 75 + 840 60 fg 79 b 30 ef 57 c
Glyphosate/Fomesafen 1200 82 bc 83 b 69 b 75 b
aL1, LaSalle; L2, Windsor; L3, Amherstburg; L4, LaSalle; L5, Windsor; L6, LaSalle, WAA, weeks after herbicide application. bIncluded Turbocharge at 0.50%
vol/vol. cIncluded Agral 90 at 0.10% vol/vol plus UAN 28%. dIncluded Agral 90 at 0.20% vol/vol plus UAN 28%. eIncluded Agral 90 at 0.25% vol/vol plus
UAN 28%. fIncluded UAN 28%. a-iMeans followed by the same letter are not significantly different according to Fisher’s Protected LSD at P < 0.05.

Glyphosate-Resistant Giant Ragweed (Ambrosia trifida L.): 2,4-D Dose Response and Control
with Postemergence Herbicides in Soybean
1794
Table 3. Percent control of glyphosate resistant giant ragweed 2 WAA with herbicides applied post emergence.
Control 2 WAAa
Treatment Rate L1 and L2a L3 L4 L5, and L6
(g a.i. ha–1) ________________________% __________________________
Weedy Check 0 d 0 h 0 h 0 g
Weed Free Check 100 a 100 a 100 a 100 a
Glyphosate 900 38 c 37 ef 32 g 34 f
Glyphosate + Acifluorfen 900 + 600 72 b 70 b 52 f 68 b
Glyphosate + Fomesafenb 900 + 240 74 b 65 b 59 de 52 cde
Glyphosate + Bentazon 900 + 1080 67 b 31 g 57 def 44 def
Glyphosate + Thifensulfuronc 900 + 6 50 bc 34 fg 56 ef 40 ef
Glyphosate + Chlorimuron-ethyld 900 + 9 70 b 40 e 61 cde 50 cde
Glyphosate + Cloransulam-methyle 900 + 17.5 75 b 62 b 66 bc 56 bcd
Glyphosate + Imazethapyre 900 + 100 76 b 47 c 66 bc 63 bc
Glyphosate + Imazethapyr + Bentazonf 900 + 75 + 840 75 b 41 de 69 b 53 bcde
Glyphosate/Fomesafen 1200 72 b 46 cd 62 bcd 65 bc
aL1, LaSalle; L2, Windsor; L3, Amherstburg; L4, LaSalle; L5, Windsor; L6, LaSalle, WAA, weeks after herbicide application. bIncluded Turbocharge at 0.50%
vol/vol. cIncluded Agral 90 at 0.10% vol/vol plus UAN 28%. dIncluded Agral 90 at 0.20% vol/vol plus UAN 28%. eIncluded Agral 90 at 0.25% vol/vol plus
UAN 28%. fIncluded UAN 28%. a-hMeans followed by the same letter are not significantly different according to Fisher’s Protected LSD at P < 0.05.
Table 4. Percent control of glyphosate resistant giant ragweed 4 WAA with herbicides applied post emergence.
Control 4 WAAa
Treatment Rate L1 and L2a L3, L4, L5 and L6
(g a.i. ha–1) _________________% ___________________
Weedy Check 0 e 0 g
Weed Free Check 100 a 100 a
Glyphosate 900 32 d 23 f
Glyphosate + Acifluorfen 900 + 600 65 bcd 35 de
Glyphosate + Fomesafenb 900 + 240 63 bcd 28 ef
Glyphosate + Bentazon 900 + 1080 60 bcd 24 f
Glyphosate + Thifensulfuronc 900 + 6 44 cd 25 f
Glyphosate + Chlorimuron-ethyld 900 + 9 73 bc 29 def
Glyphosate + Cloransulam-methyle 900 + 17.5 74 bc 56 b
Glyphosate + Imazethapyre 900 + 100 82 b 46 bc
Glyphosate + Imazethapyr + Bentazonf 900 + 75 + 840 71 bc 34 de
Glyphosate/Fomesafen 1200 69 bc 37 cd
aL1, LaSalle; L2, Windsor; L3, Amherstburg; L4, LaSalle; L5, Windsor; L6, LaSalle, WAA, weeks after herbicide application. bIncluded Turbocharge at 0.50%
vol/vol. cIncluded Agral 90 at 0.10% vol/vol plus UAN 28%. dIncluded Agral 90 at 0.20% vol/vol plus UAN 28%. eIncluded Agral 90 at 0.25% vol/vol plus
AN 28%. fIncluded UAN 28%. a-gMeans followed by the same letter are not significantly different according to Fisher’s Protected LSD at P < 0.05. U

Glyphosate-Resistant Giant Ragweed (Ambrosia trifida L.): 2,4-D Dose Response and Control
with Postemergence Herbicides in Soybean
1795
cially active on this species with 18 g a.i. ha–1 providing
98% to 99% control of 12 to 15 cm glyphosate-resistant
giant ragweed [16]. Glyphosate plus acifluorfen provided
35% to 65% control and was equivalent to glyphosate ap-
plied alone at L1 and L2. This is in contrast to the find-
ings of Norsworthy et al. [16] who reported 76% to 87%
control of glyphosate resistant giant ragweed with acif-
luorfen applied at 420 g a.i ha–1. Similarly applying fo-
mesafen with glyphosate either as a tank mix or a premix
gave only 69% control (Table 4). Similar results were
found with bentazon mixed with glyphosate. Glyphosate/
fomesafen, glyphosate plus chlorimuron-ethyl or imaze-
thapyr plus bentazon provided up to 69%, 73% and 69%
control, respectively (Table 4). Glyphosate plus fomesa-
fen, bentazon and thifensulfuron provided less than 65%
control and were equivalent to glyphosate applied alone
across all locations. This is in contrast to the findings of
Norsworthy et al. [16] who reported 100% control of
glyphosate resistant giant ragweed with fomesafen ap-
plied alone at 263 g a.i. ha–1 or bentazon applied alone at
840 g a.i. ha–1.
At 8 WAA data could be combined into groups L1 and
L2 and L4, L5 and L6 while L3 was analyzed separately
(Table 5). Control was generally higher with all herbi-
cides evaluated for group L1 and L2 compared to L3 and
L4, L5 and L6 and may be due to higher levels of rainfall
in 2011. The average rainfall for the months of May and
June 2011 were 179.4 mm and 83.4 mm, respectively for
Windsor Ontario [17]. The average rainfall for the months
of May and June 2012 were 88.6 mm and 42.2 mm, re-
spectively for Windsor, Ontario [17]. Control may have
also been higher in 2011 due to a higher proportion of
resistant biotypes at sites L3 and L4, L5 and L6. Gly-
phosate provided 3% to 19% control. Glyphosate plus
cloransulam-methyl was the most effective post emer-
gence treatment providing 26% to 70% control. This is in
contrast to the findings of Vink et al. [13] who reported
80% to 81% control with glyphosate plus cloransulam-
methyl applied at 900 g a.e. ha–1 + 17.5 g a.i. ha–1. Gly-
phosate/fomesafen, glyphosate plus acifluorfen, chlori-
muron-ethyl, or imazethapyr provided up to 45%, 38%,
53% and 60% control, respectively. Glyphosate plus fo-
mesafen, bentazon, thifensulfuron or imazethapyr plus
bentazon were equivalent to glyphosate applied alone.
For giant ragweed shoot dry weight all data were com-
bined and analyzed (Table 6 ). Glyphosate alone and gly-
phosate plus bentazon reduced giant ragweed shoot dry
weight by 24% and 27% respectively and were equiva-
lent to the weedy control. Glyphosate plus cloransulam-
methyl reduced giant ragweed shoot dry weight by 64%.
This is in contrast to Vink et al. [13] who reported a 98%
reduction in giant ragweed shoot dry weight with gly-
phosate plus cloransulam-methyl applied at 900 g a.e.
ha–1 + 17.5 g a.i. ha–1. Glyphosate plus fomesafen or
Table 5. Percent control of glyphosate resistant giant ragweed 8WAA with herbicides applied post emergence.
Control 8 WAAa
Treatment Rate L1 and L2a L3 L4, L5 and L6
(g a.i. ha–1) ____________________% __________________
Weedy Check 0 f 0 f 0 e
Weed Free Check 100 a 100 a 100 a
Glyphosate 900 19 e 3 e 11 d
Glyphosate + Acifluorfen 900 + 600 38 cde 11 cd 14 cd
Glyphosate + Fomesafenb 900 + 240 41 bcde 7 cde 12 cd
Glyphosate + Bentazon 900 + 1080 35 cde 5 de 12 cd
Glyphosate + Thifensulfuronc 900 + 6 29 de 5 de 10 d
Glyphosate + Chlorimuron-ethyld 900 + 9 53 bcd 7 cde 12 cd
Glyphosate + Cloransulam-methyle 900 + 17.5 70 ab 55 b 26 b
Glyphosate + Imazethapyre 900 + 100 60 bc 13 c 18 bc
Glyphosate + Imazethapyr + Bentazonf 900 + 75 + 840 40 bcde 4 e 13 cd
Glyphosate/Fomesafen 1200 45 bcd 5 e 14 cd
aL1, LaSalle; L2, Windsor; L3, Amherstburg; L4, LaSalle; L5, Windsor; L6, LaSalle, WAA, weeks after herbicide application. bIncluded Turbocharge at 0.50%
vol/vol. cIncluded Agral 90 at 0.10% vol/vol plus UAN 28%. d \Included Agral 90 at 0.20% vol/vol plus UAN 28%. eIncluded Agral 90 at 0.25% vol/vol plus
UAN 28%. fIncluded UAN 28%. a-fMeans followed by the same letter are not significantly different according to Fisher’s Protected LSD at P < 0.05.

Glyphosate-Resistant Giant Ragweed (Ambrosia trifida L.): 2,4-D Dose Response and Control
with Postemergence Herbicides in Soybean
1796
Table 6. Glyphosate resistant giant ragweed shoot dry weight and soybean yield for herbicides applied post emergence.
Giant ragweed shoot dry weightSoybean yielda
Treatment Rate All combined L1 and L2a L3 L4, L5, and L6
(g a.i. ha–1) (g·m–2) _______________ (t·a–1) ________________
Weedy Check 43.1 e 0.65 c 0.20 c 0.26 cd
Weed Free Check 0.0 a 2.68 a 4.03 a 1.90 a
Glyphosate 900 32.9 de 1.10 bc 0.13 cd 0.28 cd
Glyphosate + Acifluorfen 900 + 600 22.6 bcd 1.57 b 0.08 cd 0.30 cd
Glyphosate + Fomesafenb 900 + 240 20.2 bc 1.31 bc 0.16 cd 0.33 c
Glyphosate + Bentazon 900 + 1080 31.4 de 1.37 b 0.09 cd 0.18 d
Glyphosate + Thifensulfuronc 900 + 6 28.9 cd 1.17 bc 0.07 cd 0.27 cd
Glyphosate + Chlorimuron-ethyld 900 + 9 24.1 bcd 1.60 b 0.07 d 0.31 c
Glyphosate + Cloransulam-methyle 900 + 17.5 15.7 b 1.70 b 0.45 b 0.50 b
Glyphosate + Imazethapyre 900 + 100 16.6 b 1.63 b 0.18 cd 0.41 bc
Glyphosate + Imazethapyr + Bentazonf 900 + 75 + 84023.0 bcd 1.02 bc 0.09 cd 0.34 bc
Glyphosate/Fomesafen 1200 25.9 cd 1.26 bc 0.09 cd 0.31 c
aL1, LaSalle; L2, Windsor; L3, Amherstburg; L4, LaSalle; L5, Windsor; L6, LaSalle. bIncluded Turbocharge at 0.50% vol/vol. cIncluded Agral 90 at 0.10%
vol/vol plus UAN 28%. dIncluded Agral 90 at 0.20% vol/vol plus UAN 28%. eIncluded Agral 90 at 0.25% vol/vol plus UAN 28%. fIncluded UAN 28%,
a-eMeans followed by the same letter are not significantly different according to Fisher’s Protected LSD at P < 0.05.
imazethapyr reduced giant ragweed shoot dry weight by
53% and 61%, respectively (Table 6). Glyphosate/fome-
safen, glyphosate plus acifluorfen, thifensulfuron, chlori-
muron-ethyl or imazethapyr plus bentazon reduced giant
ragweed shoot dry weight by less than 50% (Table 6).
Soybean yield data L1 and L2 and L4, L5 and L6
could be combined while L3 was analyzed separately
(Table 6). Giant ragweed interference caused a reduction
in soybean yield of 76% to 95% across all sites. Bay-
singer and Sims [18] reported a 92% yield loss in soy-
bean due to giant ragweed interference. Giant ragweed
interference with glyphosate alone caused a 59 to 97%
reduction in soybean yield and was equivalent to the
weedy control across all sites. Giant ragweed interfer-
ence where glyphosate plus cloransulam-methyl was ap-
plied reduced soybean yield by 37% to 89%. In a previ-
ous study, there was no reduction in soybean yield with
cloransulam-methyl applied at 17.5 g a.i. ha–1 and there
was a 32% to 40% reduction in soybean yield with gly-
phosate plus cloransulam-methyl applied at 900 g a.e. h–1
+ 17.5g a.i. ha–1 [13]. Glyphosate plus acifluorfen, benta-
zon, chlorimuron-ethyl or imazethapyr reduced soybean
yield by 41% to 98%, 49% to 98%, 40% to 98%, 39% to
96%, respectively. This is similar to the findings of a
previous study that reported a reduction in soybean yield
equivalent to the weedy control when glyphosate plus
clorimuron-ethyl, fomesafen and imazethapyr plus ben-
tazon was applied [13]. Giant ragweed interference with
glyphosate/fomesafen, glyphosate plus fomesafen, thifen-
sulfuron, or imazethapyr plus bentazon reduced soybean
yield by up to 98%, 96%, 98% and 98%, respectively and
were equivalent to the weedy control. The reduction in
yields with these herbicides is consistent with the control
ratings and giant ragweed shoot dry weight.
3.2. 2,4-D Dose Response
Soybean injury was observed at L4 1 WAE (weeks after
emergence) with 2,4-D at rates of 1000 g a.e. ha–1 or
greater. Soybean injury (delayed emergence) of 10% and
50% was observed at 1000 and 2000 g a.e. ha–1, respec-
tively (data not shown). Soybean injury was not observed
at rates of 500 g a.e. ha–1 or less. Accentuated soybean
injury at this site may be due to higher rainfall before and
after application in 2011. The average rainfall for the
month of May 2011 at this location was 179.4 mm [17].
In contrast the average rainfall for the month of May
2012 at this location was 88.6 mm [17].
Generally there was a high dose required for 80% and
95% control compared to the dose required for 50% con-
trol (Table 7). This may be due to the greater chance of
error as the experiment was not designed specifically for
evaluating the ED80 and ED95 [15]. The 2,4-D dose
needed to achieve 50% control 1 WAA was 19 to 57 g

Glyphosate-Resistant Giant Ragweed (Ambrosia trifida L.): 2,4-D Dose Response and Control
with Postemergence Herbicides in Soybean
1797
Table 7. 2,4-D dose response for glyphosate-resistant giant ragweed control 1, 2, 4, and 8 WAA, shoot dry weight and soybe an
yielda.
Regression parametersb (±SE) 2,4-D Dose (g a.e. ha–1)c
Dose Response Locationd D C B I50 ED50 ED80 ED95
Giant ragweed control
1 WAA L1, L2 88.8 (0.0) 30.6 (0.4) 3.0 (0.8)72.3 (7.5) 57.4 128.5-
L3, L5 84.8 (0.0) 0.0 (0.0) 2.4 (0.2)16.6 (1.1) 19.3 53.6 -
L4 89.6 (0.1) 0.0 (0.0) 1.1 (0.2)24.5 (2.4) 30.3 168.4-
2 WAA L1, L2 91.7 (0.0) 24.5 (0.2) 3.7 (0.8)97.3 (7.7) 85.2 148.2-
L3, L5 92.8 (0.0) 0.0 (0.0) 0.9 (0.1)31.1 (2.0) 37.0 238.3-
L4 91.0 (1.4) 0.0 (0.0) 1.4 (0.1)40.2 (2.1) 46.3 165.9-
4 WAA L1 100.0 (1.3)25.0 (9.4) 1.0 (0.5)118.6 (65.8) 59.3 326.21660.4
L2 95.7 (1.3) 14.4 (-0.4)6.5 (2.7)98.3 (11.2) 94.6 122.5204
L4 100 (0.1) 9.5 (1.4) 1.6 (0.2)84.9 (12.5) 74.4 186.6500.6
L3, L5 100 (0.1) 0.0 (0.0) 1.0 (0.1)68.2 (12.5) 68.2 272.81295.8
8 WAA L1, L2 99.2 (0.2) 4.5 (1.1) 2.4 (0.6)115.2 (13.7) 111.5 203.8414.0
L3, L5 100.0 (0.1)1.5 (2.0) 1.5 (0.3)95.9 (14.7) 94.0 238.6675.6
L4 100.0 (0.1)0.0 (0.0) 1.1 (0.1)84.4 (5.8) 84.4 297.61227.0
Giant ragweed shoot dry weight All combined 97.0 (0.2) 0.0 (0.0) 1.9 (0.2)137.3 (11.7) 141.8 310.21047.5
Soybean yield All combined 100.0 (0.0)20.0 (4.6) 1.5 (0.3)190.9 (28.9) 135.8 397.11161.1
aAbbreviations: WAA, weeks after application. bRegression parameters: D, upper limit; C, lower limit; B, slope at I50; I50, rate needed for 50% response. cED50,
ED80 and ED95: Rates needed to achieve 50%, 80% and 95% control of giant ragweed compared to weed-free control. Rates needed to achieve 50%, 80% and
95% soybean yield compared to the weed-free control. Rates needed to achieve 50%, 80% and 95% reduction in giant ragweed shoot dry weight compared to
the weedy control. dLocation: L1, LaSalle; L2, Windsor; L3, Amherstburg; L4, LaSalle; L5, Windsor.
a.e. ha–1. To achieve 80% control 1 WAA, the 2,4-D dose
required was 53 to 168 g a.e. ha–1. At 2 WAA, the dose
needed to obtain 50% control was 37 to 85 g a.e. ha–1
while the dose needed to achieve 80% control was 148 to
238 g a.e. ha–1.
At 4 WAA, L3 and L5 could be combined and L1, L2
and L4 were analyzed separately (Table 7). At L1 and
L3 and L5, 59 and 68 g a.e. ha–1 of 2,4-D was needed to
achieve 50% control and 1660 and 1296 g a.e. ha–1 was
needed to achieve 95% control, respectively. In contrast
at L2 and L4, 95 and 74 g a.e. ha–1 was needed to achieve
50% control and 204 and 501 g a.e. ha–1 was needed to
achieve 95% control, respectively. This is similar to the
findings of Vink et al. [14] who reported 97% to 98%
control of glyphosate-resistant giant ragweed with gly-
phosate plus 2,4-D ester applied at 900 g a.e. ha–1 + 500
g a.e. ha–1 4 WAA.
At 8 WAA L1 and L2 and L3 and L5 could be com-
bined and L4 was analyzed separately (Table 7). At L1
and L2, 414 g a.e. ha–1 of 2,4-D was needed to achieve
95% control. At L3 and L5 94 g a.e. ha–1 of 2,4-D was
needed to achieve 50% control and 676 g a.e. ha–1 was
needed to achieve 95% control. In contrast, at L4 1227 g
a.e. ha–1 of 2,4-D was needed to achieve 95% control.
For giant ragweed shoot dry weight all data could be
combined (Table 7). The 2,4-D dose required to reduce
giant ragweed shoot dry weight by 50, 80 and 95% was
142, 310 and 1048 g a.e. ha–1, respectively, which was
generally higher than the dose required for control 4
WAA.
For soybean yield all data could be combined (Table
7). The dose of 2,4-D required for 50, 80 and 95% of the
soybean yield in the weed free control was 136, 397 and
1161 g a.e. ha–1, respectively which closely follows the
rate of 2,4-D required to reduce giant ragweed shoot dry
weight by 50, 80 and 95%. These data confirm that weed
shoot dry weight is a good indicator of weed interfer-
ence.
4. Conclusion
In summary, the postemergence broadleaf herbicides re-
gistered for use in Ontario provided variable control of
glyphosate resistant giant ragweed across all sites. In
general, the postemergence broadleaf herbicides did not

Glyphosate-Resistant Giant Ragweed (Ambrosia trifida L.): 2,4-D Dose Response and Control
with Postemergence Herbicides in Soybean
1798
provide commercially acceptable control of glyphosate
resistant giant ragweed. Glyphosate plus cloransulam-
methyl was the best of the herbicides evaluated; however,
it did not provide acceptable control. The reduced control
observed in this study may be due to multiple resistant
giant ragweed. For the 2,4-D dose response experiment,
414 to 1227 g a.e. ha–1 of 2,4-D plus glyphosate applied
at 900 g a.e. ha–1 was needed to achieve 95% control.
This research concludes that growers must control gly-
phosate resistant giant ragweed before soybean emer-
gence since none of the postemergence broadleaf herbi-
cides registered in Ontario provides commercially ac-
ceptable control of glyphosate resistant giant ragweed. In
addition, the lowest effective rate of 2,4-D applied pre-
plant for the control of glyphosate resistant giant rag-
weed is 500 g a.e. ha–1. Future research should study
other herbicide tank mixes coupled with alternative man-
agement strategies such as tillage and crop rotation.
5. Acknowledgements
The authors acknowledge Chris Kramer for his exper-
tise and technical assistance in these studies. Funding for
this project was provided in part by Monsanto Canada
Inc., the Grain Farmers of Ontario and the Agricultural
Adaptation Council through the Canadian Agricultural
Adaptation Program.
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