Posted 7 September 2005. Crop Management.

Soybean Growers May Send Horseweed Samples for Free Herbicide Resistance Analysis

Michigan State University. www.canr.msu.edu

East Lansing, Michigan (September 1, 2005) - Glyphosate and acetolactate synthase (ALS) inhibitors are important herbicides for Michigan soybean growers, though there is concern that continued use might encourage herbicide resistance in horseweed (Conzyza canadensis), also known as marestail.

Glyphosate-resistant horseweed has not been reported in Michigan, but it has been confirmed in at least 11 other states. ALS-resistant horseweed has been confirmed in 29 fields spanning 11 Michigan counties.

Michigan State University (MSU) researchers are working to address this issue. MSU Diagnostic Services has been screening horseweed for glyphosate, ALS and triazine resistance since 2001.

"Confirming herbicide-resistant horseweed in Michigan will provide producers with the knowledge to implement the best possible management strategies. The goal of any resistance management program is to prevent or limit the spread of resistant weeds," says Steven Gower, weed science diagnostician with MSU Diagnostic Services.

Farmers suspecting that they may have herbicide-resistant horseweed in their soybean fields should collect seedheads from mature plants in the next few weeks prior to soybean harvest and send them to MSU Diagnostic Services for a free analysis.

For more information, contact Gower at 517-432-9693 or e-mail sgower@msu.edu.

The herbicide resistance analysis is supported by check-off dollars through the Michigan Soybean Promotion Committee.