Vol.4, No.10, 570-576 (2013) Agricultural Sciences
http://dx.doi.org/10.4236/as.2013.410077
Distribution of glyphosate and cloransulam-methyl
resistant giant ragweed (Ambrosia trifida L.)
populations in southern Ontario
Joanna Follings1, Nader Soltani1*, Darren E. Robinson1, François J. Tardif2,
Mark B. Lawton3, Peter H. Sikkema1
1University of Guelph Ridgetown Campus, Ridgetown, Canada;
*Corresponding Author:
2University of Guelph, Guelph, Canada
3Monsanto Canada, Guelph, Canada
Received 2 August 2013; revised 2 September 2013; accepted 2 October 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
Giant ragw eed is a very competitive weed in row
crop production and has been found to drasti-
cally reduce soybean yield. In 2008, giant rag-
weed was the first w eed species with confirmed
resistance to glyphosate in Canada. As of 2010
there were 48 locations with confirmed glypho-
sate resistant giant ragweed in Essex, Kent and
Lambton counties. In addition, there was sus-
pected resistance to cloransulam-methyl. The
objectives of thi s research were 1) to conduct an
expanded field survey on the distribution of gly-
phosate resistant giant ragweed in Ontario, 2) to
determine the distribution of cloransulam-me-
thyl resistant giant ragweed in Ontario, and 3) to
determine the distribution of multiple resistant
(glyphosate and cloransulam-methyl) giant rag-
weed in Ontario. In 2011 and 2012 giant ragw eed
seed was collected from 85 field sites in Essex
(16), Kent (34), Lambton (23), Elgin (3), Middle-
sex (6), Lennox & Addington (1), Huron (1) and
Brant (1) cou nties. In total there are 34 addi tional
locations confirmed with glyphosate resistant
giant ragweed in Ontario. There are 11 locations
confirmed with cloransulam-methyl resistant
giant ragweed and 5 locations with multiple re-
sistance to both glyphosate and cloransulam-me-
thyl. Glyphosate resistant giant ragweed has
been found in 4 additional counties.
Keywords: Giant Ragweed; Glyphosate; No-Tillage;
Resistance; Soybean; Survey
1. INTRODUCTION
Giant ragweed (Ambrosia trifida L.) is an erect annual
broadleaf weed [1]. It has a long emergence period last-
ing from early March to late July [2,3]. Giant ragweed
also has the ability to grow rapidly and can reach heights
of up to 6 m [4]. Historically, this species was found in
orchards and non-cropped areas such as ditches and river
banks in southern areas of Canada and the Midwestern
and Eastern United States; however, it has recently adap-
ted to current corn and soybean cropping systems [1,5,6].
Giant ragweed is a very competitive weed in row crop
production. In soybean, losses of up to 92% have been
reported [7]. In southwestern Ontario, the most effective
herbicide for the control of giant ragweed is glyphosate
[8].
However, in 2008 a location in southwestern Ontario
was confirmed to have glyphosate resistant giant rag-
weed [9]. As a result, a survey was conducted on the oc-
currence and distribution of glyphosate resistant giant
ragweed [10]. In 2009 and 2010 glyphosate resistant
giant ragweed was confirmed at 18 and 29 additional lo-
cations, respectively [10]. All locations in 2008 and 2009
were located in Essex County in the most southwestern
portion of Ontario; however, in 2010 glyphosate resistant
giant ragweed was found for the first time in Kent and
Lambton counties [10]. Research is required to determine
if glyphosate resistant giant ragweed has spread further
north and east into other counties where giant ragweed is
a problem in glyphosate cropping systems. Therefore, the
first objective of this study was to conduct an expanded
field survey on the distribution of glyphosate resistant
giant ragweed in Ontario.
In addition to glyphosate, cloransulam-methyl is re-
Copyright © 2013 SciRes. Openly accessible at http://www.scirp.org/journal/as/

J. Follings et al. / Agricultural Sciences 4 (2013) 57 0-576 571
commended for the control of giant ragweed in soybean
in Ontario (OMAFRA, 2011). Cloransulam-methyl (17.5
g a.i.h−1) + Agral 90 (0.25% v/v) + UAN 28% (2.5% v/v)
applied post emergence provided 93% to 96% control of
glyphosate resistant giant ragweed 8 weeks after appli-
cation (WAA) [11]. However, resistance to cloransulam-
methyl in some glyphosate resistant giant ragweed po-
pulations is suspected [11]. Research is required to de-
termine if populations of giant ragweed in Ontario are
resistant to cloransulam-methyl and furthermore if there
is multiple resistance to both glyphosate and cloransu-
lam-methyl. Thus, the second objective of this study was
to determine if there is cloransulam resistant giant rag-
weed in the province and furthermore if there is multi-
ple-resistant giant ragweed.
2. MATERIALS AND METHODS
2.1. Seed Collection
In 2011 and 2012 giant ragweed seed was collected
from 85 field sites in Essex (16), Kent (34), Lambton (23),
Elgin (3), Middlesex (6), Lennox & Addington (1), Hu-
ron (1) and Brant (1) counties in Ontario. Survey me-
thods were consistent with a previous survey conducted
on the occurrence and distribution of glyphosate resistant
giant ragweed in Ontario [10]. Field sites were located
by farmers and agriculture retailers contacting us regard-
ing suspicious fields with poor control of giant ragweed
with glyphosate or by driving down country roads and
observing giant ragweed plants in fields from the road.
Beckie et al. [12] reported heavy infestations of a
single weed species surviving herbicide treatment is the
best indication of resistance so seed was collected from
82 soybean and 3 corn fields where giant ragweed was
often the only weed present. Giant ragweed plants in
these fields were found at the field entrance, along the
edges of the field, in dense patches located randomly in
the field, scattered throughout the field, or sparse through-
out the field. Seeds were collected when they reached
maturity (brown in colour) in the fall from September to
October.
When seed was collected, the date, road name, nearest
intersection, grower name/field name, approximate field
acreage, approximate percent of field infested, other weed
species present and distribution pattern throughout field
was recorded. In addition, GPS coordinates were taken at
each site.
To remain consistent with the previous survey [10],
seed was collected from at least 20 plants per site. Ap-
proximately the same amount of seed was collected from
each plant. Seed heads were clipped off the plants just
below the flower heads, stored in paper bags, and dried
under room conditions of 23˚C for approximately two
weeks. Seed was manually removed from the seed heads,
cleaned of any debris then temporarily stored in labelled
zipper storage bags.
2.2. Glyphosate Resistance Testing
Seed dormancy was broken using methods consistent
with Stachler [13] and Vink et al. [10]. Potting soil
(PRO-MIX PGX, Premier Tech Horticulture, Rivière-du-
Loup, QC, Canada) was used to fill 18 cell greenhouse
transplant trays half full. Fifty seeds from each popu-
lation were placed in a single cell and covered with ad-
ditional potting soil. Each cell was labeled with the po-
pulation name and date of seeding. The cells were wa-
tered and placed in a refrigerator for at least 10 weeks at
a temperature between 3˚C and 5˚C. After 8 to 10 weeks,
seed was checked for germination.
Once seeds had germinated, potting mix (PRO-MIX
PGX) was used to fill 10 cm pots and individual seedlings
were transplanted into each pot cell. Seedlings were wa-
tered and placed in a growth room with a photoperiod of
16 hours, a daytime temperature of 25˚C and a nighttime
temperature of 20˚C. All seedlings were watered daily
with a 20-20-20 fertilizer solution.
A population collected from Kent County, Ontario was
initially screened for glyphosate resistance to confirm su-
sceptibility to glyphosate and was used as the susceptible
control. Twenty plants from each population were screen-
ed for glyphosate resistance. Plants were grown to the
two to four node (four to six leaf) stage and glyphosate
was applied at 1800 g a.e. ha−1. Glyphosate was applied
using a chamber sprayer with a single 8002 even flat fan
nozzle (TeeJet, Wheaton, IL, USA) calibrated to deliver
210 L.ha−1 of water at 276 kPa.
Control ratings were taken 1 day after application
(DAA) and 1, 2, and 4 WAA. In addition to visible con-
trol ratings, at 4 WAA plants were identified as dead or
alive. Plants exhibiting symptoms similar to the suscep-
tible check (necrotic growing point) were identified as
being dead and plants exhibiting a healthy growing point
were identified as being alive. A population was deter-
mined to be resistant if at least one plant survived the
herbicide application 4 WAA [12].
2.3. Cloransulam-Methyl Resistance Testing
Ten plants from each population were screened for
cloransulam-methyl resistance, since it is currently re-
commended for control of giant ragweed in non-GMO
soybean [8]. The population from Kent County was ini-
tially treated with cloransulam-methyl to confirm its sus-
ceptibility and was used as the susceptible control. Plants
were grown to the two to four node (four to six leaf)
stage and cloransulam-methylwas applied at a rate of
17.5 g a.i. ha−1 + Agral 90 (0.25% v/v) + UAN 28%
(2.5% v/v). Cloransulam-methyl was applied using the
same procedure as above.
Copyright © 2013 SciRes. Openly accessi ble at http:// www.scirp.org/journal/as/

J. Follings et al. / Agricultural Sciences 4 (2013) 57 0-576
572
Control ratings were taken 1 DAA and 1, 2, and 4
WAA. In addition to visible control ratings, at 4 WAA
plants were identified as dead or alive. Plants exhibiting
symptoms similar to the susceptible check (necrotic grow-
ing point) were identified as being dead and plants ex-
hibiting a healthy growing point were identified as being
alive. If a single plant from a population had survived the
herbicide application 4 WAA, a population was deter-
mined to be resistant [12].
3. RESULTS AND DISCUSSION
3.1. Glyphosate Resistance
In 2011 giant ragweed seed was collected from 50
field sites in Essex (16), Kent (19), Lambton (10), Elgin
(2), Middlesex (2) and Lennox & Addington (1) counties
in Ontario. Out of the 50 populations collected, 23 were
confirmed with glyphosate resistant giant ragweed (Fig-
ures 1 and 2). Resistant populations were found in Essex
(10), Kent (7), Lambton (4), Middlesex (1) and Lennox &
Addington (1) counties (Figure 1). This study confirms
the presence glyphosate resistant giant ragweed in two ad-
ditional counties (Middlesex and Lennox & Addington)
outside of the counties surveyed by Vink et al. [10]. The
percent of glyphosate resistant giant ragweed plants in 19
populations was less than 50% (Table 1 ). There were 2 po-
pulations with 51% to 60% and 2 populations with 81 to
90% glyphosate resistant giant ragweed plants (Table 1).
In 2012 giant ragweed seed was collected from 35
sites in Kent (15), Lambton (13), Elgin (1), Middlesex
(4), Huron (1) and Brant (1) counties in Ontario. Of the
35 sites surveyed, 11 sites were confirmed with glypho-
sate resistant giant ragweed (Figure 1). Glyphosate resis-
tant giant ragweed populations were found in Kent (5),
Lambton (2), Middlesex (2), Elgin (1) and Huron (1)
counties (Figure 1). In 2012, two additional counties (El-
gin and Huron) have been confirmed with glyphosate
resistant giant ragweed in addition to the counties surve-
yed by Vink et al. [10] and those counties surveyed in
2011. The percent of glyphosate resistant giant ragweed
plants at nine sites was less than 20% (Ta b l e 1). At two
sites 31% to 40% of the giant ragweed population was
resistant to glyphosate (Table 1).
Overall, there is a low level of resistance at the sites
surveyed for glyphosate resistant giant ragweed in 2011
and 2012. Of the 34 sites confirmed with glyphosate re-
sistant giant ragweed, 30 sites had less than 50% of the
giant ragweed population resistant to glyphosate (Table
1). The percent of glyphosate resistant giant ragweed at 4
sites was greater than 50% (Table 1). It is suspected that
growers with giant ragweed populations that have a lo-
wer level of resistance may be using herbicides with
multiple modes of action as well as a diverse crop rota-
tion. Growers with giant ragweed populations that have a
higher level of resistance may have used glyphosate more
frequently in the past.
3.2. Cloransulam-Methyl
In 2009 and 2010 giant ragweed seed was collected
from 102 field sites in Essex (70), Kent (21), Lambton
(10), and Waterloo (1) counties in Ontario [10]. Of the
102 populations, 1 site was confirmed with resistance to
cloransulam-methyl (Ta bl e 2 ). This site confirmed with
cloransulam-methyl resistance was also resistant to gly-
phosate (Tab le 3). This multiple resistant site was found
in Essex County (Figure 3). The percent of cloransulam-
methyl resistant giant ragweed plants in this population
was less than 50% (Table 3).
In 2011 giant ragweed seed was collected from 50
field sites in Essex (16), Kent (19), Lambton (10), Elgin
(2), Middlesex (2), Lennox & Addington (1) counties in
Ontario. Of the 50 populations collected, 8 sites were
confirmed with resistance to cloransulam-methyl (Table
2). Resistant populations were found in Essex (2), Kent
(3), and Lambton (3) counties (Figure 2). Three of the po-
pulations with cloransulam-methyl resistance were also
resistant to glyphosate (Table 3). These multiple resistant
sites were found in Essex (1), Kent (1), and Lambton (1)
counties (Figure 3). The percent of cloransulam-methyl
resistant giant ragweed plants at all populations in 2011
was less than 30% (Table 3).
In 2012 giant ragweed seed was collected from 35
sites in Kent (15), Lambton (13), Elgin (1), Middlesex
(4), Huron (1) and Brant (1) counties in Ontario. Of the
35 sites surveyed in 2012, 2 sites were confirmed with
resistance to cloransulam-methyl (Table 2). Both resis-
tant populations were found in Kent County (Figure 2).
Of the two sites confirmed with cloransulam-resistant
giant ragweed, one site was also resistant to glyphosate
(Figure 3). The percent of cloransulam-methyl resistant
giant ragweed plants in this population was less than
10% (Tabl e 3). Overall, the percent of giant ragweed re-
sistant to cloransulam-methyl across all sites from 2009
to 2012 was less than 50% (Tabl e 2 ). The low level of
giant ragweed resistant to cloransulam-methyl may be
due to growers frequently using glyphosate for control of
giant ragweed rather than cloransulam-methyl.
4. CONCLUSION
In summary, this survey demonstrates that glyphosate
resistant giant ragweed is no longer confined to the sou-
thwestern portion of Ontario. Since the initial assessment
of its distribution in Ontario [10], the presence of gly-
phosate-resistant giant ragweed has been confirmed in
four additional counties. This is the first survey in Cana-
da to document cloransulam-methyl resistance and mul-
tiple resistance in giant ragweed in Ontario.
Copyright © 2013 SciRes. Openly accessible at http://www.scirp.org/journal/as/

J. Follings et al. / Agricultural Sciences 4 (2013) 57 0-576
Copyright © 2013 SciRes. http://www.scirp.org/journal/as/
573
Figure 1. Distribution of glyphosate resistant giant ragweed in Ontario in 2009, 2010, 2011 and 2012. Z2009 and 2010 data taken
from previous survey conducted (Vink et al., 2010b).
Table 1. Percent of giant ragweed samples resistant to glyphosate in study populations collected from soybean production fields
across southwestern Ontario in 2011 and 2012.
Number of Sites
Percent of Sample Resistant 2011 2012 Total
0 27 24 51
1 - 10 9 7 16
11 - 20 3 2 5
21 - 30 3 0 3
31 - 40 4 2 6
41 - 50 0 0 0
51 - 60 2 0 2
61 - 70 0 0 0
71 - 80 0 0 0
81 - 90 2 0 2
91 - 100 0 0 0
All Classes 50 35 85
Openly accessible at

J. Follings et al. / Agricultural Sciences 4 (2013) 57 0-576
574
Figure 2. Distribution of cloransulam-methyl resistant giant ragweed in Ontario in 2009, 2010, 2011 and 2012.
Tab le 2 . Percent of giant ragweed samples resistant to cloransulam-methyl in study populations across Ontario in 2009, 2010, 2011
and 2012.
Number of Sites
Percent of Sample
Resistant 2009 2010 2011 2012 Total
0 53 48 42 33 176
1 - 10 0 0 5 2 7
11 - 20 0 0 2 0 2
21 - 30 0 0 1 0 1
31 - 40 0 0 0 0 0
41 - 50 0 1 0 0 1
51 - 60 0 0 0 0 0
61 - 70 0 0 0 0 0
71 - 80 0 0 0 0 0
81 - 90 0 0 0 0 0
91 - 100 0 0 0 0 0
All Classes 53 49 50 35 187
Copyright © 2013 SciRes. Openly accessible at http://www.scirp.org/journal/as/

J. Follings et al. / Agricultural Sciences 4 (2013) 57 0-576 575
Figure 3. Distribution of multiple resistant (glyphosate and cloransulam-methyl) giant ragweed in Ontario in 2009, 2010, 2011
and 2012.
Table 3. Percent of giant ragweed samples with multiple resistance to cloransulam-methyl and glyphosate in study populations across
Ontario in 2009, 2010, 2011 and 2012.
Number of Sites
Percent of Sample
Resistant to
Cloransulam-Methyl 2009 2010 2011 2012 Total
0 53 48 47 34 182
1 - 10 0 0 2 1 3
11 - 20 0 0 0 0 0
21 - 30 0 0 1 0 1
31 - 40 0 0 0 0 0
41 - 50 0 1 0 0 1
51 - 60 0 0 0 0 0
61 - 70 0 0 0 0 0
71 - 80 0 0 0 0 0
81 - 90 0 0 0 0 0
91 - 100 0 0 0 0 0
All Classes 53 49 50 35 187
Copyright © 2013 SciRes. Openly accessi ble at http:// www.scirp.org/journal/as/

J. Follings et al. / Agricultural Sciences 4 (2013) 57 0-576
Copyright © 2013 SciRes. http://www.scirp.org/journal/as/
576
5. ACKNOWLEDGEMENTS
Openly accessible at
The authors acknowledge Chris Kramer for his expertise and techni-
cal 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.
REFERENCES
[1] Bassett, I.J. and Crompton, C.W. (1982) The biology of
Canadian weeds. 55. Ambrosia trifida L. Canadian Jour-
nal of Plant Science, 62, 1003-1010.
[2] Harrison, S.K., Regnier, E.E., Schmoll, J.T. and Webb,
J.E. (2001) Competition and fecundity of giant ragweed
in corn. Weed Science, 49, 224-229.
http://dx.doi.org/10.1614/0043-1745(2001)049[0224:CA
FOGR]2.0.CO;2
[3] Schutte, B.J., Regnier, E.E. and Harrison, S.K. (2004) Pri-
mary seed dormancy in Ambrosia trifida L. (giant rag-
weed). Proceedings of the North Central Weed Science
Society, 59, 119.
[4] Abul-Fatih, H.A. and Bazzaz, F.A. (1979) The biology of
Ambrosia trifida L. II. Germination, emergence, growth,
and survival. New Phytologist, 83, 817-827.
http://dx.doi.org/10.1111/j.1469-8137.1979.tb02313.x
[5] Abul-Fatih, H.A. and Bazzaz, F.A. (1979) The biology of
Ambrosia trifida L. I. Influence of species removal on the
organization of the plant community. New Phytologist, 83,
813-816.
http://dx.doi.org/10.1111/j.1469-8137.1979.tb02312.x
[6] Johnson, B., Loux, M., Nordby, D., Sprague, C., Nice, G.,
Westhoven, A. and Stachler, J. (2007) Biology and mana-
gement of giant ragweed.
http://www.ces.purdue.edu/extmedia/BP/GWC-12.pdf
[7] Baysinger, J.A. and Sims, B.D. (1991) Giant ragweed (Am-
brosia trifida) interference in soybeans (Glycine max).
Weed Sci ence, 39, 358-362.
[8] OMAFRA, Ontario Ministry of Agriculture, Food, and Ru-
ral Affairs (2011) I. Guide to weed control. Publication 75.
Queen’s Printer for Ontario, Toronto.
[9] Heap, I. (2012) International survey of herbicide resistant
weeds. http://www.weedscience.org/In.asp
[10] Vink, J.P., Soltani, N., Robinson, D.E., Tardif, F.J., Law-
ton, M.B. and Sikkema, P.H. (2012) Occurrence and dis-
tribution of glyphosate-resistant giant ragweed (Ambrosia
trifida L.) in southwestern Ontario. Canadian Journal of
Plant Science, 92, 533-539.
http://dx.doi.org/10.4141/cjps2011-249
[11] Vink, J.P., Soltani, N., Robinson, D.E., Tardif, F.J., Law-
ton, M.B. and Sikkema, P.H. (2012) Glyphosate-resistant
giant ragweed (Ambrosia trifida L.) in Ontario: Dose re-
sponse and control with post emergence herbicides. Ame-
rican Journal of Plant Sciences, 3, 608-617.
http://dx.doi.org/10.4236/ajps.2012.35074
[12] Beckie, H.J., Heap, I.M., Smeda, R.J. and Hall, L.M. (2000)
Screening for herbicide resistance in weeds. Weed Tech-
nology, 14, 428-445.
http://dx.doi.org/10.1614/0890-037X(2000)014[0428:SF
HRIW]2.0.CO;2
[13] Stachler, J.M. (2008) Characterization and management
of glyphosate-resistant giant ragweed (Ambrosia trifida L.)
and horseweed [Conyza canadensis (L.) Cronq.]. Ph.D.
Dissertation, The Ohio State University, Columbus, 60-
107.