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

Herbicide-resistant weeds threaten contemporary agriculture by reducing crop yields and quality. Monitoring of herbicideresistant weeds is essential to the development of informed integrated weed management (IWM) strategies. In 2019 and 2020, a randomized-stratified preharvest survey of 419 fields in Saskatchewan, Canada, was conducted to determine the distribution, frequency of occurrence, and impact of herbicide-resistant weeds. Mature seeds were collected from uncontrolled weeds in each field. The samples were tested for resistance to acetyl-CoA carboxylase (ACCase) and/or acetolactate synthase (ALS)inhibiting herbicides using whole-plant bioassays under a controlled-environment. In 2019/2020, herbicide-resistant weeds occupied 72% of the surveyed fields, corresponding to an estimated 6.2 million ha of annual cropland and a total field area of 11.4 million ha. Herbicide-resistant weeds cost Saskatchewan farmers an estimated $343 million CAD annually in reduced crop yields and quality, and increased weed control expenditures. Wild oat (Avena fatua L.) had the greatest impact among grass weeds, with ACCase inhibitor resistance documented in 77% and ALS inhibitor resistance in 30% of fields where the weed seeds were collected and tested (47% and 18% of all fields surveyed, respectively). Multiple herbicide (ACCase and ALS inhibitor)-resistant wild oat were documented in 26% of the tested fields. Kochia (Bassia scoparia (L.) A.J. Scott) had the greatest impact among broadleaf weeds, where 100% of the samples tested were ALS inhibitor-resistant (39% of all fields surveyed). The growing prevalence of herbicide-resistant weeds in Saskatchewan warrants further adoption of IWM where non-chemical tactics play an important role in stewardship of the remaining effective herbicides.