Herbicide-resistant weeds were estimated to cost prairie farmers about $658 million each year in reduced crop yields, quality, and increased weed management expendituresThe 2019-2023 herbicide-resistant weed survey across the Canadian Prairies identified 21 unique herbicide-resistant weed biotypes representing 17 different herbicide-resistant weed species in 3,232 weed samples collected from 827 randomly-selected fields. Herbicide-resistant weeds were estimated to occupy 24.6 million ac (36%) of annual cropland before crop harvest, equivalent to 49.3 million ac (71%) based on total field area. When the total field area infested by herbicide-resistant weeds is combined with farmer-reported costs of herbicide resistance in reduced crop yield, quality, and increased weed management expenditures (13 ac^-1 reported in 2014-2017), it is estimated that herbicide-resistant weeds cost prairie farmers about $658 million each year. Group 1-resistant wild oat, Group 2-resistant kochia, and Group 2-resistant wild oat (not accounting for overlap of Group 1+2-resistant wild oat) were the top three most common herbicide-resistant weeds in the region and occurred 46%, 30%, and 27% of all fields surveyed, respectively. Group 1+2-resistant wild oat occurred in 25% of all fields surveyed, while wild oat with Group 1 resistance only and Group 2 resistance only occurred separately in 21% and 2% of the fields surveyed, respectively. Group 1-resistant green foxtail, and Group 2-resistant spiny sowthistle, false cleavers, and pale smartweed were observed in 11%, 10%, 5% and 5% of surveyed fields, respectively. All other herbicide-resistant weed biotypes were observed in <5% of the surveyed fields. Based on patch area, it was estimated that Group 2-resistant kochia, Group 1-resistant wild oat, and Group 1+2-resistant wild oat were the most widely-distributed herbicide-resistant weeds, occupying about 10.6 million, 7.1 million, and 7.0 million ac of annual cropland across the region, respectively. Group 1-resistant green foxtail, and Group 2-resistant spiny sowthistle and false cleavers occupied 3.1 million, 2.9 million, and 2.1 million ac of annual cropland. All other herbicide-resistant weeds occupied <1 million ac each. Establishment of a system for early detection and rapid response of novel herbicide-resistant weeds in the prairie region of Canada successfully identified four new herbicide-resistant weed cases in the region, Group 14-resistant kochia, Group 9-resistant downy brome, Group 1-resistant foxtail barley, and Group 2-resistant Japanese brome. Three of these represent first confirmations globally (one simultaneous with a similar biotype in the United States), while one was a first confirmation in Canada. Mapping of the distribution of herbicide-resistant weeds identified certain issues that were localized to particular ecoregions within the Canadian Prairies. Others, like Group 1-resistant yellow foxtail, appeared to be spreading into new provinces. The information generated in this fourth round of herbicide-resistant weed surveys across the Canadian Prairies and other enhanced detection efforts should be used to inform farmers, agronomists, agriculture industry, government, researchers, and policy makers about the growing prevalence of herbicide-resistant weeds in the region. Increasing impact of herbicide-resistant weeds warrants further adoption of integrated weed management strategies, where non-chemical practices help to augment herbicidal weed management. Diversification of over-simplified weed control programs can go a long way to mitigating further spread of herbicide-resistant weeds and managing the issues that are already widespread across the region.

• Herbicide-resistant weeds were observed in 70% of the fields under annual crop production across the Canadian Prairies.
• Herbicide-resistant weeds were estimated to occupy 24.6 million ac (36%) of annual cropland across the Canadian Prairies, equivalent to 49.3 million ac (71%) based on field area.
• Group 2-resistant kochia, Group 1-resistant wild oat, and Group 1+2-resisitant wild oat were the top three most abundant herbicide-resistant weed biotypes based on infestation area.
• New herbicide-resistant weeds documented include:
  • Group 14-resistant kochia (in the Mixed Grassland ecoregion of west-central Saskatchewan and southern Alberta)
  • Group 9-resistant downy brome (in the Mixed Grassland ecoregion of southern Alberta)
  • Group 1-resistant foxtail barley (in the Peace Lowland ecoregion of northern Alberta)
  • Group 2-resistant Japanese brome (in the Mixed Grassland ecoregion of south-central Saskatchewan)