Michigan: Resistant Weed Screening

Just as most of our crops are starting to mature, so are some of the persistent summer annual weeds. If you have weeds that you are considering submitting to Michigan State University Diagnostic Services for herbicide resistance screening, then you will want to monitor those plants in the next few weeks to look for mature seed.

For most species, such as the pigweeds (Amaranth family), late August to mid-September is when seeds begin to mature.

To test for herbicide resistance, weeds are grown from seed in the greenhouse and sprayed with up to six different herbicides based on the species, suspected resistance and quantity of seedlings.

The most common issues that occur with sample submissions include submitting samples before seeds are mature (Photo 1), submitting an insufficient quantity of seeds, and submitting the wrong plant parts or male plants, in the case of Palmer amaranth and waterhemp (Photo 2).

MSU created a new publication this spring to help ensure successful submissions for resistance screening titled “Tips for Collecting Weed Seeds.” Within this publication, you will find photos and information on how to look for weed seeds before submitting a sample, along with directions on how to submit a sample.

Photo 1. Immature (left) and mature (right) horseweed/marestail seeds.
Photo 1. Immature (left) and mature (right) horseweed/marestail seeds.
Male common waterhemp plants.
Photo 2. Male common waterhemp plants will not have seeds (top), therefore only female plants (bottom) need to be submitted once seeds are black in color.

As in years past, the Michigan Soybean Promotion Committee will be sponsoring the testing of select species for Michigan soybean growers. The species included are Palmer amaranth, waterhemp, horseweed/marestail, common lambsquarters, common ragweed and giant ragweed.

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The cost of screening for non-soybean growers is $90 per sample. To keep track of the spread of resistance, MSU Extension asks that you include with your sample a submission form detailing the location the seeds were collected and whom to contact with results.

If you have other weed species you would like tested, please contact me directly at hiller12@msu.edu to see if testing is possible.

MSU Diagnostic Services is a fee-for-service laboratory that assists in determining the cause of a wide variety of plant health and insect pest problems. For more information, visit the MSU Diagnostic Services website.