Herbicide-resistant weeds found in 2024

Grain Farmers of Ontario provided partial support for an Ontario Ministry of Agriculture, Food and Agribusiness-led project which tested 57 fields for herbicide-resistant weeds.

Significant 2024 findings:

Pigweed: six field populations resistant to Group 14 herbicides (e.g., Reflex) were found in Chatham-Kent, Huron, Oxford, Ottawa, Leeds, and Grenville counties.

Waterhemp: 21 field populations were tested; all were herbicide-resistant, and the majority were resistant to multiple herbicide groups (Figure 1). Since 2020, 20-30 new fields have been found annually.

Ragweed, common: four field populations resistant to Group 14 herbicides (e.g., Reflex) were found in Huron, Middlesex and Lambton counties. Two three-way resistant (Groups 2,5 and 14) populations were found in Oxford and Lambton counties (Figure 2).

Ryegrass (Lolium) – Figure 3: Four glyphosate-resistant populations were found in Bruce, Essex, and Huron counties. Testing for Group 1 herbicide (e.g., Assure II) resistance is underway.

Palmer amaranth: Two new locations were discovered in Bruce and Haldimand counties.

Management: Integrated strategies and herbicide options can be found at OMAFA’s “Crop IPM” website and the “Crop Protection Hub”.