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.