Glyphosate-Resistant Horseweed Now In 16 States Across Country

Michigan joined the list this year, and no-tillers heavily reliant on glyphosate for weed control are at particular risk of developing resistance in their fields.

Earlier this year, Michigan became the sixteenth state in the country to confirm a case of glyphosate-resistant horseweed when it was discovered in a Christmas tree plantation.

Horseweed, also called marestail, became the first of the seven known glyphosate-resistant weed species in the United States when it was found in 2000 in Delaware. It has been spreading ever since.

Steve Gower of Michigan State University’s Diagnostic Services has been screening weed samples for glyphosate-resistance for the past 5 years. He says repeated use of glyphosate to control weeds in the plantation was the main cause in the development of resistance.

Gower warns that resistant horseweed might be present in other counties in Michigan. He says the risk is particularly high in counties where no-till crop production is common and glyphosate is used exclusively for weed control.

Diversity Is Critical

While resistance might not be present yet, continuous glyphosate use without other weed control strategies will most likely lead to the development of glyphosate-resistance, he adds.

Diversity is the key to avoiding herbicide resistance, Gower says. He points to several steps no-tillers should use to manage resistant weeds and reduce the spread of glyphosate-resistance:

Rotate glyphosate with herbicides that have different modes of action.

Apply a residual herbicide before glyphosate, or tank mix another herbicide with glyphosate.

If glyphosate is used as a burndown treatment and in-crop in the same year, tank mix the burndown glyphosate treatment with an herbicide that has a different mode of action.

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