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- and acetolactate synthase (ALS)-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 sampled 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 non-chemical tactics augment contemporary herbicidal weed control.