More and more weed samples are pouring into Michigan State University’s Diagnostic Services for herbicide resistance screening and the result has been an increase in incidents. In 2015 there were an uptick of 46 samples submitted, a 40% increase over 2014.

The samples included three Palmer amaranth, three Powell amaranth, 10 common waterhemp, nine common ragweed, three common lambsquarters and 18 horseweed/marestail. Each sample received was tested for resistance to five or six herbicide Sites of Action, with all testing being sponsored by the Michigan Soybean Promotion Committee.

Palmer amaranth distribution in Michigan. Most samples are confirmed multiple-resistant to both glyphosate and ALS, Barry County has three-way resistance to glyphosate, ALS and atrazine.

The listing below supplies the new incidents and locations of resistance that were confirmed. A cumulative listing of confirmed herbicide resistant weeds by county is currently being compiled and will be made available through the MSU Diagnostic Services website soon.

There will be another herbicide resistance testing again in the fall of 2016. Please view “How to submit a sample to MSU Diagnostic Services” in the back of “2016 Weed Control Guide for Field Crops,” MSU Extension bulletin E0434, for more information on submitting a sample. Also, MSU has produced a new publication, “Tips for collecting weed seeds: Ensure your resistance sample gets tested,” to reduce requests to re-collect seed. Samples for testing in 2016-17 will need to be submitted by early November to ensure enough time for our team to break seed dormancy and conduct the screenings. For questions regarding sample submissions, please contact [email protected] for more information.

New county locations and incidents of resistance confirmed from 2014

*G# refers to the herbicide site of action group.

Powell amaranth
ALS-resistant (acetolactate synthase inhibitor, G2): Saginaw, Sanilac, Newago counties
Palmer amaranth
Multiple-resistant (ALS-G2/atrazine-G5): Allegan County

Common waterhemp
Multiple-resistant (glyphosate-G9/ALS-G2): Isabella, Sanilac, Wayne counties
Multiple-resistant (ALS-G2/atrazine-G5): Saginaw, St. Clair counties
ALS-resistant (G2): Clinton County

Common ragweed
Multiple-resistant (glyphosate-G9/ALS-G2): Sanilac County
ALS-resistant (G2): Allegan, Arenac, Saginaw counties

Common lambsquarters
ALS-resistant (G2): Saginaw, Sanilac counties

Horseweed/marestail
Multiple-resistant (glyphosate-G9/ALS-G2): Branch, Hillsdale, Macomb, Monroe, Oakland, Saginaw, Shiawassee, St. Clair counties

Source: MSU Extension