Herbicide-resistant weeds found in 2025
Agronomic information from Ontario's crop specialists
with OMAFA
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.
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.
