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Every year, OPVG invests in local research projects to support continuous improvements for Ontario’s processing vegetables and address issues facing our industry.
OMAFA researchers are tracking the scope and scale of herbicide-resistant weeds in Ontario to give growers better management tools. They are identifying the most problematic weed species and herbicide groups based on grower-submitted samples from across the province.
“Weeds resistant to multiple herbicide modes of action are increasingly common, and particularly important for processing vegetable growers who have limited tools available,” says Cesar Cappa, OMAFA weed management specialist, horticulture and part of the project team.
With support from the University of Guelph Weed Science Lab and industry partners, including Turnkey Genomics, the database includes thousands of tests gathered over three decades. The data is fueling a new surveillance tool to help growers and advisors identify resistant weeds and adjust/adapt weed management plans.
“In 2025, 82% of all common ragweed samples tested were resistant to more than one herbicide group, and 100% were resistant to Group 2 herbicides,” says Cappa. “Waterhemp wasn’t far behind with 65% of all samples resistant to more than one herbicide group.”
Cappa also notes that waterhemp populations have developed resistance to five herbicide groups (2, 5, 9. 14, 27), and they’ve seen multiple resistance in other common weed species relevant to vegetable rotations including common ragweed, green and redroot pigweed, and Canada fleabane.
This project will continue to gather weed samples submitted by and tested for Ontario growers (at no cost) to build a more robust database of herbicide resistance in Ontario.
The new, free interactive tool is now available: Ontario Herbicide Resistance Database.
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