Glyphosate-resistant Canada Fleabane Research Articles

The Addition of Saflufenacil to Glyphosate plus Dicamba Improves Glyphosate-Resistant Canada Fleabane (Erigeron canadensis L.) Control in Soybean

Glyphosate + dicamba has provided variable glyphosate-resistant Canada fleabane (GRCF) control in glyphosate/dicamba-resistant (GDR) soybean. Previous research has indicated improved GRCF control when a third herbicide was added to glyphosate + dicamba, though research is limited. The objective of this research was to ascertain if the level and consistency of GRCF control can be improved when adding tiafenacil, metribuzin, bromoxynil, pyraflufen-ethyl/2,4-D, 2,4-D ester, halauxifen-methyl or saflufenacil to glyphosate + dicamba applied preplant (PP) in GDR soybean. Four field trials were conducted in 2020 and 2021 in commercial fields in southwestern Ontario, Canada. Glyphosate + dicamba controlled GRCF 57, 93 and 94% at 2, 4 and 8 WAA, respectively. Adding bromoxynil to glyphosate + dicamba improved GRCF control from 57 to 77% at 2 WAA; adding saflufenacil to glyphosate + dicamba improved GRCF control from 57 to 92, 93 to 99, and 94 to 99% at 2, 4 and 8 WAA, respectively. All three-way tank-mixtures improved the consistency of GRCF control, except for glyphosate + dicamba + 2,4-D ester at 2 WAA, glyphosate + dicamba + 2,4-D ester, tiafenacil or metribuzin at 4 WAA, and glyphosate + dicamba + tiafenacil or bromoxynil at 8 WAA. This study concludes that the level and consistency of GRCF control was improved when saflufenacil was added to a PP application of glyphosate + dicamba in soybean.

Tolerance of azuki bean to herbicides applied preplant for weed control in a strip‐tillage cropping system

AbstractFive field experiments were conducted at Huron Research Station near Exeter, Ontario, Canada, during 2018 to 2020 to assess the tolerance of strip‐till‐grown azuki bean to various preplant (PP) herbicides. The herbicides selected have activity on glyphosate‐resistant (GR) Canada fleabane, an emerging weed biotype in strip‐till azuki bean production. Saflufenacil, metribuzin, 2,4‐D ester, saflufenacil + metribuzin, saflufenacil + 2,4‐D ester, metribuzin + 2,4‐D ester, and saflufenacil + metribuzin + 2,4‐D ester, applied PP 1 week before seeding, at the proposed label rate (1X) and twice that rate (2X) caused as much as 6%, 5%, 6%, 7%, 8%, 10%, and 13% visible azuki bean injury. The herbicide‐induced azuki bean injury was transient and had no effect on plant density, aboveground dry biomass, height, maturity, and yield except for the dry biomass which was reduced by 28% with metribuzin + 2,4‐D ester and 36% with saflufenacil + metribuzin + 2,4‐D ester at the 2X rate and azuki bean height which was reduced 9% at the 2X rate with saflufenacil + metribuzin + 2,4‐D ester. Based on these results, saflufenacil, metribuzin, 2,4‐D ester, saflufenacil + metribuzin, saflufenacil + 2,4‐D ester, and metribuzin + 2,4‐D ester applied PP have potential for GR Canada fleabane control in strip‐till azuki beans. However, there is not enough crop safety for using a three‐way tankmix of saflufenacil + metribuzin + 2,4‐D ester, applied PP, in azuki bean production.

Improving the Level and Consistency of Glyphosate-Resistant Canada Fleabane (Erigeron canadensis L.) Control With Bromoxynil-Based Tankmixes Applied Preplant to Soybean

Glyphosate-resistant (GR) Canada fleabane (Erigeron canadensis L.) is a problematic weed in soybean. Bromoxynil-based tankmixes provide control of GR Canada fleabane in monocot crops; however, there is limited research on preplant (PP) applications in soybean. The objective of this study was to determine the best third tankmix partner with glyphosate plus bromoxynil applied PP to improve the level and consistency of GR Canada fleabane control in soybean. Four field trials were conducted over a two-year (2020, 2021) period. Glyphosate plus bromoxynil controlled GR Canada fleabane 52, 58, and 69% at 2, 4, and 8 weeks after application (WAA) respectively. The addition of saflufenacil to glyphosate plus bromoxynil improved GR Canada fleabane control 39% at 2 WAA and the addition of dicamba to glyphosate plus bromoxynil improved GR Canada fleabane control 39 and 29% at 4 and 8 WAA, respectively. The addition of bromoxynil to glyphosate plus 2,4-D ester or dicamba improved GR Canada fleabane control by 36 and 35%, respectively, at 2 WAA. The addition of bromoxynil to glyphosate plus tiafenacil or pyraflufen-ethyl/2,4-D improved GR Canada fleabane control by 19% at 8 WAA. The addition of bromoxynil to glyphosate plus metribuzin improved GR Canada fleabane control 31, 32, and 36% at 2, 4, and 8 WAA, respectively. Antagonism was observed between bromoxynil plus glyphosate plus tiafenacil or halauxifen-methyl 8 WAA, while the interaction between bromoxynil plus glyphosate plus pyraflufen-ethyl/2,4-D was considered synergistic. The addition of metribuzin, tiafenacil, pyraflufen-ethyl/2,4-D, 2,4-D ester, halauxifen-methyl, saflufenacil or dicamba to glyphosate plus bromoxynil improved the consistency of GR Canada fleabane at 2, 4, and 8 WAA. This study concludes that the addition of saflufenacil or dicamba to glyphosate plus bromoxynil can improve the level and consistency of GR Canada fleabane control applied PP to soybean.

Does the Inclusion of Atrazine in S-Metolachlor/Mesotrione/Bicyclopyrone Improve Glyphosate-Resistant Canada Fleabane Control in Corn?

A total of seven field studies were conducted in southwestern Ontario from 2018 to 2020 to evaluate S-metolachlor/mesotrione/bicyclopyrone (Acuron Flexi®) and S-metolachlor/atrazine/mesotrione/bicyclopyrone (Acuron®) for the control of glyphosate-resistant (GR) Canada fleabane in corn. Acuron Flexi® and Acuron® applied preplant (PP) controlled GR Canada fleabane 90-97% and 99-100%, respectively. Commonly used PP herbicides in Ontario, dicamba/atrazine + glyphosate and tolpyralate + atrazine + glyphosate controlled GR Canada fleabane 97-99%. Acuron Flexi® and Acuron® applied postemergence (POST) provided 75-92% and 92-99% control of GR Canada fleabane, respectively. Dicamba/atrazine + glyphosate and tolpyralate + atrazine + glyphosate applied POST provided 84-97% and 94-98% control of GR Canada fleabane, respectively. Glyphosate applied POST at 900 ae ha-1 provided only 35-38% control of Canada fleabane in corn. Orthogonal contrasts indicated that herbicides applied PP provided up to 10% greater control of GR Canada fleabane than POST. There was no significant difference between the low and high rates of Acuron® or Acuron Flexi® in controlling GR Canada fleabane. Acuron® controlled GR Canada fleabane 14, 9 and 7% greater than Acuron Flexi® at 4, 8, and 12 WAA, respectively. GR Canada fleabane interference with corn reduced yield up to 42%. However, there was no difference in corn yield between herbicides treatments evaluated. Orthogonal contrasts indicated that reduced GR Canada fleabane interference with PP herbicides evaluated provided 0.9 t ha-1 greater corn yield than the POST herbicides evaluated. There was no significant difference in corn yield with Acuron® or Acuron Flexi® at the lower rate or higher rates. Additionally, Acuron Flexi® applied PP provided a similar corn yield as when applied POST. Based on these results, the use of Acuron® results in improved GR Canada fleabane control than Acuron Flexi® but this did not result in an increase in corn yield. The study concludes that there is potential for Acuron Flexi® for GR Canada fleabane control in corn especially in areas where atrazine use is restricted, however additional weed control tactics may be required for acceptable control.

Glyphosate-Resistant Canada Fleabane Control in Winter Wheat with Postemergence Herbicides

In recent years, there has been a rapid increase in the number of herbicide-resistant weeds, including glyphosate-resistant (GR) biotypes in Ontario, Canada. A total of six field experiments were conducted over a two-year period (2018 and 2019) to determine the control of GR Canada fleabane (Conyza canadensis (L.) Cronq.) with currently available herbicides for winter wheat in Ontario. Winter wheat was not injured with any of the herbicides evaluated. Among herbicides evaluated, pyrasulfotole/bromoxynil (preformulated), 2,4-D ester, halauxifen, fluroxypyr/halauxifen (preformulated) + MCPA, pyrasulfotole/bromoxynil/fluroxypyr (preformulated), pyrasulfotole/bromoxynil/thiencarbazone (preformulated), pyrasulfotole/bromoxynil/thiencarbazone + MCPA, and fluroxypyr/halauxifen + pyroxsulam + MCPA controlled GR Canada fleabane 94–100% at 8 weeks after application (WAA) and reduced density 95–100% and biomass 97–100%. Reduced GR Canada fleabane interference with pyrasulfotole/bromoxynil/thiencarbazone + MCPA increased winter wheat yield 27% compared to the weedy control. GR Canada fleabane interference had no adverse effect on winter wheat yield with all other treatments. Based on these results, herbicide treatments that include 2,4-D, pyrasulfotole, or halauxifen can adequately control GR Canada fleabane in winter wheat.

Tolpyralate + Atrazine Applied Preemergence Provides Residual GR Canada Fleabane [<i>Conyza canadensis</i> (L.) Cronq.] Control Similar to Current Industry Standards

Tolpyralate is a benzoylpyrazole, 4-hydroxyphenyl-pyruvate dioxygenase inhibitor, and a herbicide registered for use in corn. The efficacy of tolpyralate plus atrazine to provide full-season residual control of glyphosate-resistant (GR) Canada fleabane in corn is not known under Ontario environmental conditions. Five field trials were completed over a two-year period (2018-19) in south-western Ontario on farms with confirmed GR Canada fleabane [Conyza canadensis (L.) Cronq.] populations to determine if tolpyralate + atrazine provides full-season residual control of GR Canada fleabane in corn. Corn injury was less than 10% with all treatments. At 4 weeks after application (WAA), tolpyralate (30 g·ai·ha−1), tolpyralate (40 g·ai·ha−1), and atrazine (560 g·ai·ha−1) controlled GR Canada fleabane 64, 78 and 72%, respectively. A tank mix of tolpyralate + atrazine at both rates improved GR Canada fleabane control to 94%. Saflufenacil/dimethenamid-p, mesotrione + atrazine, and dicamba/atrazine controlled GR Canada fleabane 99, 95 and 92%, respectively. At 8 WAA, tolpyralate (30 g·ai·ha−1), tolpyralate (40 g·ai·ha−1) and atrazine (560 g·ai·ha−1) controlled GR Canada fleabane 83, 88, and 83%, respectively (Table 2). The tank mixes of tolpyralate (30 g·ai·ha−1) + atrazine (560 g·ai·ha−1) and tolpyralate (40 g·ai·ha−1) + atrazine (560 g·ai·ha−1) controlled GR Canada fleabane 94, and 97%, respectively, 8 WAA which was similar to saflufenacil/dimethenamid-p, mesotrione + atrazine and dicamba/atrazine. There was no treatment difference for corn yield. Based on these results, tolpyralate (40 g·ha−1), tolpyralate (30 g·ha−1) + atrazine and tolpyralate (40 g·ha−1) + atrazine, applied PRE, provided similar control of GR Canada fleabane as current industry standards at 8 WAA.

Influence of Adjuvants on the Control of Glyphosate-Resistant Canada Fleabane and Waterhemp in Corn with Tolpyralate

Tolpyralate is a new benzoylpyrazole herbicide for weed management in corn. It is recommended to be co-applied with atrazine along with the adjuvants methylated seed oil concentrate (MSO) plus an ammonium nitrogen fertilizer, such as urea ammonium nitrate (UAN). Two studies were conducted on glyphosate-resistant (GR) Canada fleabane and GR waterhemp to determine if an additional adjuvant is still required when tolpyralate plus atrazine are tankmixed with a commercial glyphosate formulation (Roundup WeatherMAX®) in corn. Trials were conducted over a two-year period (2018-19) on farms in south western Ontario with confirmed GR populations. When co-applied with Roundup WeatherMAX®, the addition of MSO to tolpyralate + atrazine increased control of GR waterhemp 9%; however, there was no increase in GR Canada fleabane control from the addition of additional adjuvants. At 8 WAA, all treatments provided > 91% and > 84% control of GR waterhemp and GR Canada fleabane, respectively. This study concludes that the addition of Roundup WeatherMAX® to tolpyralate plus atrazine improves the control of GR waterhemp and GR Canada fleabane in corn.

Control of glyphosate-resistant Canada fleabane in Ontario with multiple effective modes-of-action in glyphosate/dicamba-resistant soybean

Canada fleabane is a winter or summer annual weed that is found throughout North America. Fall-emerged Canada fleabane can fix carbon early in the growing season, giving it a competitive advantage over nearby crop and weed species. Glyphosate-resistant (GR) Canada fleabane was originally found in one county in Ontario, Canada, in 2010 and had spread to at least 29 additional counties within the province by 2016. Previous research with several preplant herbicides resulted in variable control of GR Canada fleabane in soybean. The objective of this study was to evaluate the efficacy of glyphosate/dicamba (1800 g a.e. ha−1) alone or with the addition of a second effective mode-of-action for the control of GR Canada fleabane in glyphosate/dicamba-resistant soybean. At 4 weeks after application, glyphosate/dicamba + saflufenacil, saflufenacil/dimethenamid-P, saflufenacil/imazethapyr, or paraquat controlled GR Canada fleabane 97%, 96%, 97%, and 98%, respectively. All herbicide treatments decreased Canada fleabane density and biomass by 93%–99%. When choosing herbicide programs, it is important to consider the use of multiple modes-of-action to decrease selection pressure for the evolution of herbicide-resistant Canada fleabane. Treatments containing saflufenacil, saflufenacil/dimethenamid-P, or saflufenacil/imazethapyr with the addition of glyphosate/dicamba are recommended for the control of GR Canada fleabane.

Distribution of glyphosate and cloransulam-methyl resistant Canada fleabane (Conyza Canadensis L. Cronq.) in Ontario

From 2013 to 2015, a survey was conducted to document the distribution of glyphosate-resistant (GR) and multiple-resistant Canada fleabane with resistance to glyphosate and cloransulam-methyl across Ontario. This survey shows that GR Canada fleabane is present throughout southern Ontario, from Essex county in the southwest to Glengarry county adjacent to the Quebec border.

Biologically effective rate of metribuzin for glyphosate-resistant Canada fleabane control in soybean

Eight field experiments were conducted during 2013–2015 to determine the dose response of glyphosate-resistant Canada fleabane to preplant metribuzin in a tank mixture with glyphosate in soybean. T...

Control of glyphosate resistant Canada fleabane in soybean with preplant herbicides

Thirteen herbicide tankmixes were evaluated during 2013–2015 for control of glyphosate-resistant (GR) Canada fleabane in soybean. Glyphosate + saflufenacil + s-metolachlor/metribuzin, glyphosate + amitrole, and glyphosate + metribuzin were the most efficacious, controlling 86%–92% of GR Canada fleabane, reducing density by 98%–99% and aboveground biomass by 96%–97%.

Postemergence herbicides for control of glyphosate-resistant Canada fleabane in corn

Post-emergence herbicides were evaluated for glyphosate-resistant Canada fleabane control in corn (Zea mays L.) from 2013 to 2015. By 8 wk after treatment, dicamba/atrazine (96% control), dicamba (...

Herbicide tank mixtures to control co-existing glyphosate-resistant Canada fleabane and giant ragweed in soybean

Populations of glyphosate-resistant (GR) Canada fleabane and GR giant ragweed can be found in several locations in southwestern Ontario. While these species can be managed individually, a scenario has developed where both species are present in GR soybean. Ten separate field experiments (five with Canada fleabane and five with giant ragweed) were conducted over a 2-yr period (2013–2014) in soybean to evaluate preplant (PP) herbicide tank mixtures that could control both weed species if they were present in the same field. Herbicides were rated for soybean injury, weed control, population density, and aboveground biomass. Two- and three-way tank mixtures containing amitrole (i.e., glyphosate + amitrole, glyphosate + amitrole + saflufenacil, and glyphosate + amitrole + 2,4-D) were among the most effective treatments. For example, control of GR Canada fleabane and GR giant ragweed was at least 92% at 4 wk after treatment (WAT) and weed density and biomass were generally similar to the weed-free control. However, without amitrole, the best PP herbicide option was a three-way tank mixture of glyphosate + saflufenacil + 2,4-D which provided improved control and greater reductions in density and biomass compared with the two-way glyphosate tank mixtures containing saflufenacil or 2,4-D.

Control of glyphosate-resistant Canada fleabane in corn with preplant herbicides

Nine preplant herbicides were tested for the control of glyphosate-resistant (GR) Canada fleabane in corn. At eight weeks after treatment (WAT), the herbicides that resulted in the greatest control and least amount of density and biomass of GR Canada fleabane were dicamba/atrazine (99%), mesotrione + atrazine (97%), saflufenacil/dimethenamid-p (97%), and dicamba (94%).

Control of glyphosate-resistant Canada fleabane [Conyza canadensis (L.) Cronq.] with isoxaflutole and metribuzin tank mix

The future release of ‘Balance GT’ soybean, which is resistant to isoxaflutole and glyphosate, opens up the possibility for control of glyphosate-resistant (GR) Canada fleabane using HPPD-inhibiting herbicides (Group 27) in soybean. Field trials were conducted over two years to evaluate the dose response of an isoxaflutole plus metribuzin tank mix, as well as each chemical applied alone, to assess their response using Flint's adaptation of Colby's equation. Factorial experiments were performed in growth room and greenhouse environments to assess isoxaflutole versus glyphosate, isoxaflutole versus metribuzin, and isoxaflutole plus metribuzin versus glyphosate. Tank mixes of isoxaflutole plus metribuzin in a 1:4 ratio provided 80% control of GR Canada fleabane at a dose range between 420 (84 + 336) and 611 (122 + 489) g a.i. ha−1 at 8 WAA (weeks after application). Tank mixes achieved an 80% reduction in biomass at a dose range between 498 and 738 g a.i. ha−1, while 80% reduction in density was obtained with doses from 96 to 423 g a.i. ha−1, 8 WAA. With glyphosate as a constant tank partner, field treatments of isoxaflutole plus metribuzin were mostly synergistic with some analyses showing an additive response. When tested in the growth room, isoxaflutole plus glyphosate tank mixes indicate additivity in the majority of treatments on glyphosate-susceptible (GS) fleabane.

Canada fleabane (Conyza canadensis) control with preplant applied residual herbicides followed by 2,4-D choline/glyphosate DMA applied postemergence in corn

Ford, L., Soltani, N., Robinson, D. E., Nurse, R. E., McFadden, A. and Sikkema, P. H. 2014. Canada fleabane (Conyza canadensis) control with preplant applied residual herbicides followed by 2,4-D choline/glyphosate DMA applied postemergence in corn. Can. J. Plant Sci. 94: 1231–1237. Glyphosate resistant (GR) Canada fleabane (Conyza canadensis) is an extremely problematic weed in no-tillage farming operations. A total of five field trials were conducted over a 2-yr (2012 and 2013) period in Ontario to determine the level of GR Canada fleabane control with a two-pass weed control program of a pre plant (PP) residual herbicide followed by 2,4-D choline/glyphosate dimethylamine (DMA) applied POST. Among residual herbicide treatments evaluated, s-metolachlor (1600 g a.i. ha−1)+flumetsulam (50 g a.i. ha−1)+clopyralid (135 g a.e. ha−1) provided the most consistent (95–99%) control across all sites 8 wk after application (WAA). S-metolachlor/atrazine (1800 g a.i. ha–1) did not provide effective GR Canada fleabane control (21–86%) 8 WAA. The preplant residual herbicides followed by 2,4-D choline/glyphosate DMA (1720 g a.e. ha−1) POST provided 97–100% control. Glyphosate (900 g a.e. ha−1) applied PP followed by 2,4-D choline/glyphosate DMA POST provided 80–93% control 8 WAA. The application of 2,4-D choline/glyphosate DMA POST following any PP residual herbicide resulted in 97% or greater control of GR Canada fleabane. Results from this research demonstrate that residual herbicides applied PP followed by 2,4-D choline/glyphosate DMA POST provides excellent control of GR Canada fleabane, and also incorporates different modes of action thereby limiting the selection of resistant weeds.

Efficacy of 2,4-D Choline/Glyphosate Dimethylamine on Glyphosate Resistant Canada Fleabane (<i>Conyza canadensis</i>) at Different Sizes

Glyphosate resistant (GR) Canada fleabane has spread quickly across southwestern Ontario and new strategies for the control of this competitive weed must be developed especially in no-tillage crops. A premix of 2,4-D choline and glyphosate dimethylamine (DMA) has been developed for application on tolerant corn, soybean and cotton crops that provides an option for the control of this problematic GR weed. The objective of this research was to determine the required dose needed to effectively control GR Canada fleabane at different size categories in field and greenhouse experiments. In the field experiments, nine rates of 2,4-D choline/glyphosate DMA (53.8 to 13,760 g·ae·ha-1) were applied to GR Canada fleabane that were 10 cm in diameter/tall, 20 cm tall or 30 cm tall. Similarly, in the greenhouse, seven rates of 2,4-D choline/glyphosate DMA (0 to 3440 g·ae·ha-1) were applied to 10, 20 and 30 cm tall GR Canada fleabane plants. The three different size classes of GR Canada fleabane responded similarly to 2,4-D choline/glyphosate DMA in the field experiment. In the greenhouse there were some differences in control for the three size classes of GR Canada fleabane with 2,4-D choline/glyphosate DMA; the 20 and 30 cm tall plants required similar rates to provide equivalent control, but the 10 cm plants required a lower rate. In all situations, greater than 1720 g·ae·ha-1 of 2,4-D choline/glyphosate DMA was required to provide 95% control of 10, 20 and 30 cm tall Canada fleabane in greenhouse (35 DAA) and field experiments (8 WAA), respectively.

Control of Canada fleabane (<i>Conyza canadensis</i>) with glyphosate DMA/2,4-D choline applications in corn (<i>Zea mays</i>)

Glyphosate resistant (GR) Canada fleabane (horseweed) has quickly spread across southwestern Ontario and is a difficult weed to control in GR crops. Glyphosate dimethylamine (DMA)/2,4-D choline (Enlist Duo?TM Dow AgroSciences LLC), a new herbicide premix developed by Dow Agro Sciences, provides control of GR and other problematic weeds. The objective of this study was to compare single and sequential applications of glyphosate DMA/2,4-D choline for the control of GR Canada fleabane in GR corn. Three single applications of glyphosate (DMA)/2,4-D choline (1720 g·ae·ha-1) were evaluated: 1) preplant (PP) applied to Canada fleabane up to 10 cm diameter/height, 2) postplant 1 (POST 1) applied when Canada fleabane was up to 20 cm tall and 3) postplant 2 (POST 2) applied up to 30 cm tall Canada fleabane. Four sequential applications were also examined:1) PP followed by (fb) POST 1, 2) PP fb POST 2, 3) POST 1 fb POST 2 and 4) PP fb POST 1 fb POST 2. The single applications provided 69%-86% control of the GR Canada fleabane while the sequential applications increased control to 92%-100%. Three applications did not provide an increase in control over a sequential two-pass application at 8 weeks after the application (WAA). Results from this research indicate that a sequential 2-pass application of glyphosate DMA/2,4-D choline provided acceptable control of GR Canada fleabane in corn.

Evaluation of 2,4-D Amine, Glyphosate, 2,4-D Amine plus Glyphosate DMA and 2,4-D Choline/Glyphosate DMA for Their Efficacy on Glyphosate Susceptible and Resistant Canada Fleabane Populations

The 2,4-D choline/glyphosate DMA formulation has reduced drift and volatility compared to the amine or ester formulation of 2,4-D and therefore is advantageous compared to a tank mix of 2,4-D amine or ester with glyphosate. The objective of this research was to compare the control of glyphosate susceptible and glyphosate resistant Canada fleabane with 2,4-D choline/glyphosate DMA with 2,4-D amine, glyphosate, and a tank mix of 2,4-D amine and glyphosate. Ten rates of 2,4-D amine (0 - 6708 g∙ae∙ha⁻¹), glyphosate (0 - 7052 g∙ae∙ha⁻¹), a tank mix of glyphosate plus 2,4- D amine (0 - 7052 g∙ae∙ha⁻¹ + 0 - 6708), and 2,4-D choline/glyphosate DMA (0 - 13760 g∙ae∙ha⁻¹) were examined in the greenhouse for the control of two susceptible (GS) and two resistant to gly- phosate (GR) Canada fleabane biotypes. The tank mix of 2,4-D amine plus glyphosate and 2,4-D choline/glyphosate DMA provided equivalent control of the GR Canada fleabane biotypes at 35 days after the application (DAA). The 2,4-D choline/glyphosate DMA treatment was more effica- cious than the tank mix on the GS biotypes. Glyphosate (880 g∙ae∙ha⁻¹) provided 50% and 100% control of the resistant and susceptible biotypes, respectively. The 2,4-D choline/glyphosate DMA formulation and the tankmix of 2,4-D amine and glyphosate provided similar control of GR Canada fleabane. * Corresponding author.

Glyphosate-resistant Canada fleabane [Conyza canadensis (L). Cronq.]: Dose response to glyphosate and control with postemergence herbicides in soybean in Ontario

Byker, H. P., Soltani, N., Robinson, D. E., Tardif, F. J., Lawton, M. B. and Sikkema, P. H. 2013. Glyphosate-resistant Canada fleabane [ Conyza canadensis (L). Cronq.]: Dose response to glyphosate and control with postemergence herbicides in soybean in Ontario. Can. J. Plant Sci. 93: 1187–1193. Glyphosate-resistant (GR) Canada fleabane was first reported in Ontario in 2010. Twelve field studies were conducted over a 2-yr period (2011, 2012) to determine the biologically effective dose of glyphosate in the field and to determine the efficacy of postemergence herbicides for control of GR Canada fleabane. A dose between 1271 and 5652 g a.e. ha−1 was required for a 50% reduction in Canada fleabane biomass depending on site and year. To achieve acceptable (>95%) control with glyphosate, 18 840 to 43 200 g a.e. ha−1 of glyphosate was required across all sites. Therefore application of glyphosate at these doses for acceptable (>95%) control is not economical. None of the postemergence herbicides labeled for use in soybean provided acceptable (>95%) control, with cloransulam providing the greatest control (67%) 4 wk after application. Therefore, GR Canada fleabane must be controlled prior to soybean emergence as current herbicides registered in soybean for postemergence use do not provide sufficient control to prevent soybean yield loss.