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. Herbicide-resistant weeds found in 2024 Mike Cowbrough Crop side with... Weed Management Specialist, Field Crops Ontario Ministry of Agriculture, Food and Agribusiness Figure 1. Herbicide-resistant waterhemp populations in Ontario counties, 2024 ONTARIO GRAIN FARMER CROP SIDE 19 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. 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”. Ryegrass seedhead Waterhemp" resistant populations by county in 2024 Population Resistant to: Group 14 Groups 2 & 14 Groups 2,5,9 & 14 Groups 2,9 & 14 Groups 9 & 14 Leeds and Grenville 2 1 Norfolk 3 1 1 Chatham-Kent Essex 2 Ottawa 2 1 Dufferin 1 Huron 1 Lambton 1 Oxford 1 Prescott-Russell 1 Stormont, Dundas and Glengarry # of populations 1 0 2 3 Common ragweed: resistant populations by county in 2024 Population Resistant to: Group 14 Groups 2 Groups 2 & 5 Groups 2,5 & 14 1 Middlesex 2 Huron 1 Lambton 1 Oxford 1 Stormont, Dundas and Glengarry # of populations 1 0 2 3 2 1 Figure 2. Herbicide-resistant common ragweed populations in Ontario counties, 2024. Footnotes (Figure 1 and 2): Group 2 = Pursuit, Group 5 = atrazine, Group 9 = glyphosate, Group 14 = Reflex
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