2024 status of herbicide-resistant weeds in Michigan

Erin Hill<hiller12@msu.edu>, Michigan State University Extension, Department of Plant, Soil and Microbial Sciences - April 01, 2024

Each year, new herbicide resistance cases are identified by MSU Plant & Pest Diagnostics.

Testing on plants for herbicide resistance — Figure 1. Purple amaranth screen 16 days after herbicide treatments were applied. Note the survival of the plants treated with the 1x field rates of Lorox (active ingredient linuron) and Caparol (active ingredient prometryn). Photo by MSU Plant & Pest Diagnostics.

In fall 2023, Michigan State University Plant & Pest Diagnostics received 15 samples to screen for herbicide resistance. All but one of the samples were submitted from Michigan, and 12 of these samples were funded by the Michigan Soybean Committee. The Michigan Soybean Committee has provided ongoing support for the screening of select weed species for herbicide resistance that are found growing in soybean rotations (i.e., pigweeds, ragweeds, marestail/horseweed and common lambsquarters). One of the submitted samples was supported by the Michigan Vegetable Council, which covered the screening of any suspected resistant weeds found in vegetable rotations from 2021-2023.

Of the 14 Michigan samples, 13 were confirmed to be resistant to various herbicides and eight were cases that had not previously been confirmed in the counties in which they were sampled. Glyphosate plus atrazine (triazine) resistance in Palmer amaranth was documented for the first time in Allegan and St. Joesph counties, with St. Joesph County also having a confirmed case of three-way resistance [i.e., Acetolactate synthase (ALS) inhibitors plus glyphosate plus atrazine] in this species. Waterhemp resistant to glyphosate plus ALS inhibiting herbicides was confirmed for the first time in Montcalm County.

Purple amaranth was confirmed resistant to photosystem II inhibiting herbicides (i.e., linuron and prometryn) in Kent County (Figure 1). Multiple resistant common ragweed (i.e., ALS inhibitor plus protoporphyrinogen oxidase inhibitor) was confirmed in Eaton County (Figure 2) and glyphosate resistant giant ragweed was confirmed in Branch County (Figure 3). Finally, ALS resistance was confirmed in common lambsquarters in Clinton County.

2024 HerbResistance FIG 2.jpg — Figure 2. Common ragweed regrowth 26 days after treatment with the PPO inhibiting herbicide Flexstar. Photo by MSU Plant & Pest Diagnostics.

2024 HerbResistance FIG 3.jpge.jpeg — Figure 3. The giant ragweed population screened this year showed a rapid necrotic response, sometimes also referred to as the phoenix phenomenon, to treatment with glyphosate (i.e., Roundup PowerMax); a rather uncommon way for resistance to appear. Pictured here is an untreated plant (left) next to a plant treated with glyphosate 48 hours earlier. The plant on the right was able to resume growth from the meristem. Photo by MSU Plant & Pest Diagnostics.

In addition to these confirmations, we saw for the first time a waterhemp sample that showed reduced control with mesotrione, a 4-hydroxyphenylpyruvate dioxygenase (HPPD) inhibiting herbicide. While the 1x and 4x rates tested in the screen controlled all but one of the eight plants treated, this survivor paired with outside dose response data suggests that this population is developing resistance. Continued reliance on this sole active ingredient for postemergence waterhemp management would be expected to favor advanced resistance. Resistance to the HPPD inhibiting herbicides by waterhemp has been confirmed in other states in the Midwest since at least 2009, according to the Herbicide Resistance Action Committee, but should this shift continue, it would be the first confirmed case in Michigan.

Herbicide resistance testing will again be available in the fall of 2024. Fully mature seeds are needed for these screens, with most annual species expected to have mature seeds starting in mid-September through frost. All submissions are due by mid-November 2024. Information on these supported programs, fees for other types of samples and the submission process can be found on the Plant & Pest Diagnostics website or in the back of the 2024 Weed Control Guide for Field Crops (E-434) from Michigan State University Extension. Information and photos showing how to collect seeds for the bioassays is available in “Tips for collecting weed seeds: Ensure your resistance sample gets tested.”

If you have any questions regarding the 2023 results or future sample submissions, please contact Erin Hill at hiller12@msu.edu for more information.

New county locations and incidents of resistance confirmed from 2023:

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

Palmer amaranth

Multiple resistant (glyphosate-G#9 + triazine-G#5): Allegan and St. Joesph
Multiple resistant (ALS-G#2 + glyphosate-G#9 + triazine-G#5): St. Joesph

Waterhemp

Multiple resistant (ALS-G#2 + glyphosate-G#9): Montcalm

Purple amaranth

PSII-resistant (G#5): Kent

Common ragweed

Multiple resistant (ALS-G#2 + PPO-G#14): Eaton

Giant ragweed

Glyphosate-resistant (G#9): Branch

Common lambsquarters

ALS-resistant (G#2): Clinton

Thank you to Christy Sprague for reviewing this article.

This article was published by Michigan State University Extension. For more information, visit https://extension.msu.edu. To have a digest of information delivered straight to your email inbox, visit https://extension.msu.edu/newsletters. To contact an expert in your area, visit https://extension.msu.edu/experts, or call 888-MSUE4MI (888-678-3464).