2019 status of herbicide-resistant weeds in Michigan

PPO inhibitor (Group 14) herbicide resistance in Palmer amaranth was confirmed in 2018 as other herbicide resistance issues continue to spread.

Figure 1. Distribution of glyphosate-resistant horseweed in Michigan by county.
Figure 1. Distribution of glyphosate-resistant horseweed in Michigan by county.

In 2018, 39 samples were submitted for herbicide resistance screening to Michigan State University Diagnostic Services. This is a 26% drop from the previous growing season, which may be attributed to delayed crop harvests due to inclement weather last fall. Thirty of the 2018 samples were sponsored by the Michigan Soybean Promotion Committee Weed Resistance Sampling Program. Though most of the submissions were collected in major field crops, there were also submissions this year from the vegetable and nursery industries.

The 2018 samples included Palmer amaranth (three), Powell amaranth (six), common waterhemp (three), common ragweed (one), giant ragweed (two), common lambsquarters (14) and horseweed/marestail (five) among other species (four). Each sample received was screened for resistance to five to seven different herbicide sites of action.

The most notable new case of resistance was the confirmation of low-level Group 14 resistance (i.e., PPO inhibitors, e.g., fomesafen/Flexstar) in Palmer amaranth in combination with glyphosate resistance in Mason County. Previously resistance in Palmer amaranth in Michigan had been confirmed to the following:

  • ALS inhibitors (G2, e.g., thifensulfuron/Harmony)
  • Glyphosate (G9)
  • Multiple resistance
    • Glyphosate + ALS
    • Glyphosate + ALS + photosystem II inhibitors (G5, e.g., atrazine)

There were a record high number of common lambsquarters submissions as part of a targeted screening from Montcalm County collected by MSU Extension educator Fred Springborn. These submissions showed no herbicide resistance to the six herbicide site of action groups tested (Groups 2, 4, 5, 9, 10 and 27). Previously, common lambsquarters has been found to be resistant to ALS inhibitors and photosystem II inhibitors (atrazine) in a few counties within Michigan. No resistance to glyphosate has been reported in common lambsquarters worldwide; however, escapes are common due to advanced plant size or age, continued emergence and plant stress due to environmental conditions at the time of application.

This season also saw a record low for horseweed submissions. Samples numbers peaked in 2016 at 36 and have been in a steady decline since that time. It is hopeful that this is a reaction to the extensive outreach efforts on this weed. Glyphosate-resistant horseweed populations have been confirmed in nearly half the counties in the Lower Peninsula (Fig. 1), with many of these also being resistant to the ALS-inhibiting herbicides. Perhaps the widespread nature of herbicide resistance in this wind-dispersed weed has changed the mindset from seeking confirmation to taking action (preventative and/or reactionary) with alternative means.

Number of herbicide-resistant weed species in Michigan
Figure 2. Number of herbicide-resistant weed species in Michigan by county. For more details, visit the clickable map at Diagnostic Services- Herbicide Resistant Weeds.

The cumulative map and listing of confirmed herbicide-resistant weeds by county has been updated on the Diagnostic Services website to include the 2018 findings screenings (Fig. 2).

Herbicide resistance testing will again be available in the fall of 2019, with all submissions due by mid-November. Information on collecting and submitting samples will be reviewed during the Aug. 29 Virtual Breakfast session held by the MSU Extension field crops team. Information is also available in “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 2019 Weed Control Guide for Field Crops (E-434).

If you have any questions regarding the 2018 results or future sample submissions, please contact Erin Hill (hiller12@msu.edu) for more information.

Thank you to Christy Sprague for reviewing this article.

New county locations and/or incidents of resistance confirmed from 2018

*G# refers to the herbicide site of action group

Common ragweed

  • ALS-resistant (G2): Gratiot Co.

Common waterhemp

  • Multiple-resistant (glyphosate-G9 + ALS-G2): Clinton, Washtenaw Co.
  • ALS-resistant (G2): Sanilac Co.

Horseweed/marestail

  • Multiple-resistant (glyphosate-G9/+ALS-G2): Kalamazoo Co.

Palmer amaranth

  • Multiple-resistant (glyphosate-G9 + PPO-G14): Mason Co.
  • Multiple-resistant (glyphosate-G9 + ALS-G2 + photosystem II/triazine-G5): Ottawa Co.
  • Glyphosate-resistant (G9): Bay Co.

Powell amaranth

  • ALS-resistant (acetolactate synthase inhibitor, G2): Clinton Co.

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