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 U.S. 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. 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 see the contact information at the end of each factsheet.
Earlier this year glyposate-resistant marestail was confirmed in Ionia and Gratiot countries. For more information on glyphosate-resistant marestail please view: Horseweed Confirmed Resistant to Glyphosate in Michigan Field Crops.