Glyphosate-resistant Palmer amaranth confirmed in Southwest Michigan

Michigan now has a second weed that can be added to the list of glyphosate-resistant weeds.

In 2007, horseweed (marestail) was the first weed confirmed resistant to glyphosate (Roundup) in Michigan.  This past season, a grower in Southwest Michigan reported that he was not able to control “pigweed” in his soybean field with glyphosate.  We identified this pigweed species as Palmer amaranth, a weed not common to the northern United States. 

Through greenhouse testing we have confirmed that this population of Palmer amaranth is resistant to glyphosate. The combination of this being the first report of Palmer amaranth in Michigan and that this weed is resistant to glyphosate should be a concern for Michigan growers.  To help with the identification and possible management strategies for this weed, we have developed two fact sheets: Palmer amaranth in Michigan: Keys to Identification and Glyphosate-resistant Palmer amaranth in Southwest Michigan: Confirmation and management options.These fact sheets outline what we know about this weed and can be found on our MSU weeds website.

We are also trying to track the spread of Palmer amaranth in Southwest Michigan. If you believe that you have glyphosate-resistant Palmer amaranth, please contact Dan Rajzer, MSU Extension Educator at the Cass County office by email or phone 269-445-4438. Or contact Christy Sprague, MSU Weed Extension specialist by email or phone 517-355-0271 x. 1224.

View interview with Dr. Christy Sprague on the subject via YouTube.

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