Distribution, frequency, and impact of herbicide-resistant weeds in Manitoba

Herbicide-resistant weeds are a growing concern for Manitoba farmers. Continued monitoring is essential to understand how to mitigate and manage this increasing threat to crop production. A randomized-stratified preharvest survey of 155 annual-cropped fields was conducted in 2022 to determine the distribution, frequency, and impact of herbicide-resistant weeds in Manitoba. Mature seeds were collected from all visible uncontrolled weed patches and tested for resistance to acetyl-CoA carboxylase (ACCase)-, acetolactate synthase (ALS), and 5-enolpyruvylshikimate-3-phosphate synthase (EPSPS)-inhibiting herbicides using whole-plant bioassays. Overall, 584 weed seed samples were collected, representing 44 different species. Uncontrolled herbicide-resistant weeds occupied 72% of the fields surveyed and about 1.5 million ha of cropland across the province. Herbicide-resistant weeds were estimated to cost Manitoba farmers $77 million annually in reduced crop yields, quality, and increased weed management expenditures. Compared to the previous 2016 survey, a trend toward increasing ALS inhibitor-resistant broadleaf weeds and decreasing herbicide-resistant grasses was observed, with some exceptions. Eleven herbicide-resistant weed species were documented, four of which were not observed in the 2016 survey; including ALS inhibitor-resistant common chickweed ( Stellaria media (L.) Vill.), pale smartweed ( Persicaria lapathifolia (L.) Delarbre), Powell amaranth ( Amaranthus powellii S. Watson), and spiny sowthistle ( Sonchus asper (L.) Hill). Manitoba farms are increasingly affected by wild oat ( Avena fatua L.) and kochia ( Bassia scoparia (L.) A.J. Scott) with resistance to multiple herbicide sites-of-action, leaving few alternatives for chemical management. The increasing impact of herbicide-resistant weeds in Manitoba emphasizes the critical need for adoption of integrated weed management where nonchemical tactics augment contemporary herbicidal weed control.