As farmers prepare for fall harvests and make plans for 2022, consider any weeds that have escaped control with herbicides. If weeds are surviving that were once controlled with a herbicide program, herbicide resistance could be an issue.
Michigan State University Plant & Pest Diagnostics offers bioassay screening of weed species for herbicide resistance starting in the fall and winter months of each year. This screen can confirm resistance and rule out the many other factors that could contribute to weed escapes.
To screen for resistance, seeds are cleaned from dried plant material, treated for dormancy, grown in the greenhouse and treated with up to seven different herbicides. (Herbicides screened are based on the species, cropping system, suspected resistance and quantity of seedlings.)
A known susceptible population is tested alongside all samples to verify results. The duration of the process depends on the species, but results are usually available no later than March. New incidence of resistance (i.e., not previously confirmed in Michigan) requires additional testing.
Soybean or vegetable growers likely qualify for free screening. The Michigan Soybean Promotion Committee will again be sponsoring the testing of select species (i.e., pigweeds or amaranths, ragweeds, horseweed, and common lambsquarters) for Michigan soybean growers.
Sushila Chaudhari, MSU Extension edible specialty crop weed scientist, has also received a grant from the Michigan Vegetable Council to sponsor herbicide resistance screening of any weed species in vegetable crops.
The cost of screening for non-soybean, non-vegetable growers, or for species not listed above for soybean growers, is $90 per sample. If you intend to submit a species not listed above, including vegetable growers, consult with MSU ahead of time to ensure it has or can collect a known susceptible population of the same weed species (email Erin Hill at [email protected]).
Most weed seeds from summer annual species are maturing by September to October, depending on the species and time of emergence. If you plan to submit a sample, refer to the factsheet “Tips for Collecting Weed Seeds” to ensure you collect mature seeds, gather a sufficient quantity and package them properly.
If the species you are submitting is not on the sheet, MSU can discuss tips for successful collection of mature seeds on a case-by-case basis. A submission form is needed with the sample, which can be found at the link or on the general MSU Plant & Pest Diagnostics website.
All samples need to be submitted no later than Nov. 19 to ensure testing over the winter months.
Further information on herbicide-resistant weed species confirmed in Michigan can be found on the “History & Map of Herbicide-Resistant Weeds in Michigan.”
Hill is part of the multi-disciplinary team in MSU Plant & Pest Diagnostics.