In 2016, yet another record number of plant samples were submitted to Michigan State University Diagnostic Services for herbicide-resistance screening. Seventy samples were submitted in total, a 52% increase over 2015. The 2016 samples included two Palmer amaranth, three Powell amaranth, three redroot pigweed, 11 common waterhemp, six common ragweed, three giant ragweed, five common lambsquarters, 36 horseweed/marestail and one eastern black nightshade.

Each sample received was tested for resistance to five or six herbicide sites of action. Fifty of the samples were sponsored by the Michigan Soybean Promotion Committee Weed Resistance Sampling Program. Non-sponsored samples came from field-crop rotations without soybeans, vegetable rotations, orchards or were species not covered by the Michigan Soybean Promotion Committee program.

Though many of the screenings revealed that weeds previously known to be resistant in Michigan had spread to new counties, there were three new incidents of resistance.

• Common ragweed. PPO (Group 14)-resistant, Flexstar/Reflex (fomesafen) and ALS (Group 2)-resistant, and FirstRate (cloransulam); locations: Huron and Saginaw counties

• Giant ragweed. Glyphosate (Group 9)-resistant; location: Macomb County

• Eastern black nightshade. ALS (Group 2)-resistant, Raptor (imazamox) and Pursuit (imazethapyr); location: Huron County

A full listing is below for all new incidents and locations of resistance that were confirmed. A cumulative listing of confirmed herbicide-resistant weeds by county was released in 2016 on the MSU Diagnostic Services website and has been updated to include these latest screenings.

Herbicide-resistance testing will again resume in fall 2017. Please view “Tips for collecting weed seeds: Ensure your resistance sample gets tested” or “How to submit a sample to MSU Diagnostic Services” in the back of the “2017 Weed Control Guide for Field Crops” (MSU Extension bulletin E0434) for more information on submitting a sample.

The sample submission deadline is Nov. 10, to ensure enough time for the MSU team to break seed dormancy and conduct the screenings. Questions regarding sample submissions should be directed to Erin Hill at [email protected] for more information.

Here are the new county locations and incidents of resistance confirmed from 2016. (G# refers to the herbicide site-of-action group.)

• Common waterhemp. Multiple-resistant (glyphosate-G9/ALS-G2): Clare, Gratiot, Kent, Ottawa, Tuscola counties

• Powell amaranth. ALS-resistant (acetolactate synthase inhibitor, G2): Gratiot, Ingham, Tuscola counties

• Redroot pigweed. ALS-resistant (G2): Benzie, Ottawa counties

• Common ragweed. Multiple-resistant (glyphosate- G9/ALS-G2): Antrim County; multiple-resistant (ALS-G2/PPO-G14): Huron, Saginaw counties; ALS-resistant (G2): Eaton County

• Giant ragweed. Glyphosate-resistant (G9): Macomb County

• Common lambsquarters. ALS-resistant (G2): Ottawa County

• Horseweed/marestail. Multiple-resistant (glyphosate-G9/ALS-G2): Allegan, Ingham, Kent, Tuscola counties; glyphosate-resistant (G9): Antrim, Benzie, Grand Traverse, Kent, Leelanau, Ottawa counties; ALS-resistant (G2): Gratiot, Kent counties

• Eastern black nightshade. ALS-resistant (G2): Huron County

Source: Erin Hill, Michigan State University Extension