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

Abstract

Herbicide-resistant weeds threaten contemporary agriculture by reducing crop yields and quality. Monitoring of herbicide-resistant 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.