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. Email: *soltanin@uoguelph.ca 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. Copyright © 2013 SciRes. AJPS
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 Copyright © 2013 SciRes. AJPS
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, Copyright © 2013 SciRes. AJPS
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. Copyright © 2013 SciRes. AJPS
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 Copyright © 2013 SciRes. AJPS
Glyphosate-Resistant Giant Ragweed (Ambrosia trifida L.): 2,4-D Dose Response and Control with Postemergence Herbicides in Soybean Copyright © 2013 SciRes. AJPS 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 Copyright © 2013 SciRes. AJPS
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 Copyright © 2013 SciRes. AJPS
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. REFERENCES [1] J. E. Franz, M. K. Mao and J. A. Sikorski, “Glyphosate: A Unique Global Herbicide,” American Chemical Society, Washington, 1997. [2] G. M. Dill, R. D. Sammons, P. C. C. Feng, F. Kohn, K. Kretzmer, A. Mehrsheikh, M. Bleeke, J. L. Honegger, D. Farmer, D. Wright and E. A. Haupfear, “Glyphosate: Discovery, Development, Applications, and Properties,” In: V. K. 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Heap, “International Survey of Herbicide Resistant Weeds,” 2012. http://www.weedscience.org/In.asp [8] I. J. Bassett and C. W. Crompton, “The Biology of Cana- dian Weeds. 55. Ambrosia trifida L.,” Canadian Journal of Plant Science, Vol. 62, No. 4, 1982, pp. 1003-1010. doi:10.4141/cjps82-148 [9] Ontario Ministry of Agriculture, Food and Rural Affairs (OMAFRA), “Ontario Weeds,” Publication 505, Guelph, 2001, pp. 214-215. [10] S. K. Harrison, E. E. Regnier, J. T. Schmoll and J. E. Webb, “Competition and Fecundity of Giant Ragweed in Corn,” Weed Science, Vol. 49, No. 2, 2001, pp. 224-229. doi:10.1614/0043-1745(2001)049[0224:CAFOGR]2.0.C O;2 [11] B. J. Schutte, E. E. Regnier and S. K. Harrison, “The Association between Seed Size and Seed Longevity among Maternal Families in Ambrosia trifida L. Popula- tions,” Seed Science Research, Vol. 18, No. 4, 2008, pp. 201-211. doi:10.1017/S0960258508082974 [12] J. P. Vink, N. Soltani, D. E. Robinson, F. J. Tardif, M. B. Lawton and P. H. 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Fuerst, “Log-Logis- tic Analysis of Herbicide Dose-Response Relationships,” Weed Technology, Vol. 9, No. 2, 1995, pp. 218-227. [16] J. K. Norsworthy, D. Riar, P. Jha and R. C. Scott, “Con- firmation, Control, and Physiology of Glyphosate-Re- sistant Giant Ragweed (Ambrosia trifida) in Arkansas,” Weed Technology, Vol. 25, No. 3, 2011, pp. 430-435. doi:10.1614/WT-D-10-00155.1 [17] Environment Canada, “Climate Normals,” 2013. http://www.climate.weatheroffice.gc.ca/climateData/daily data_e.html [18] J. A. Baysinger and B. D. Sims, “Giant Ragweed (Am- brosia trifida) Control in Soybean (Glycine max),” Weed Technology, Vol. 6, No. 1, 1992, pp. 13-18. Copyright © 2013 SciRes. AJPS
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