ONTARIO GRAIN FARMER CROP SIDE 13 With support from the Grain Farmers of Ontario, an Ontario Ministry of Agriculture, Food and Agribusiness (OMAFA)-led project tested 95 fields for herbicide-resistant weeds in 2025. KEY FINDINGS FROM THE PROJECT Group 5 resistance: • Most pigweed species tested were resistant to atrazine (a triazine), but still controlled by metribuzin (e.g., Sencor — a triazinone). Waterhemp: • 47 new field populations were tested. • All were herbicide resistant. • Most had multiple group resistance. • Since 2020, Ontario has found 20+ new resistant waterhemp fields every year. Pigweed (Group 14 resistance): • 15 new populations resistant to Group 14 herbicides (e.g., Reflex) were found in: Brant, Chatham-Kent, Hamilton-Wentworth, Huron, Oxford, Perth, Prescott, and Russell United counties. Common ragweed: • Five populations resistant to Group 14 herbicides were found in: Middlesex, Oxford, and Prince Edward counties. • Two populations in Middlesex and Prince Edward counties were resistant to Groups 2, 5, and 14. Ryegrass (Lolium): • Two glyphosate-resistant populations were confirmed in the Region of Waterloo. Palmer amaranth: • Two new detections: Lambton and Stormont, Dundas and Glengarry United counties. • The Stormont, Dundas and Glengarry population was resistant to Groups 2, 5, 9, 14, and 27 (Pursuit, Aatrex 480, glyphosate, Reflex/ Blazer, Callisto). The Lambton population has not yet been tested. Herbicide-resistant weeds found in 2025 Mike Cowbrough Crop side with... Mike Cowbrough, Weed Specialist – Field Crops with OMAFA MANAGEMENT RESOURCES Integrated weed management strategies and herbicide options are available on OMAFA’s Crop IPM and Crop Protection Hub websites. Search the resistance database yourself A searchable database with 30 years of Ontario resistance testing is available at the Crop Protection Hub (scroll to the bottom of the home page). You’ll also find instructions on: • What resistance tests are available • How they differ, and • Where to send samples. The majority of bioassay testing in pigweed species, such as Palmer amaranth shown here, was resistant to atrazine, but susceptible to metribuzin. Photo: supplied.
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