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As waterhemp and ragweed shows herbicide resistance, Minnesota researchers want farm samples

University of Minnesota Extension researchers are seeing signs of herbicide resistance in southern Minnesota.

waterhemp seed head ndsu.png
A waterhemp seed head. (NDSU Extension)
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Minnesota farmers are seeing signs of herbicide resistance in waterhemp and ragweed, prompting University of Minnesota researchers to seek out more samples of weed seed.

This is especially worrisome with waterhemp, where one plant can produce 500,000 to 1 million seeds, said Debalin Sarangi, University of Minnesota Extension weed scientist. If there is even one plant that is herbicide resistant, "it can take over a field in a few years."

In 2020, U of M Extension received about 30 waterhemp samples — mostly from southern Minnesota — and the greenhouse resistance screening results showed that the majority of waterhemp samples were resistant to ALS-inhibiting herbicides such as Pursuit, Raptor, and Classic, as well as glyphosate, the active ingredient in Roundup.

Two populations from Carver and Olmsted counties showed resistance to four herbicides, significantly reducing herbicide choices for farmers These populations have survived applications at three times the recommended rate of glyphosate, Raptor, Atrazine, and Flexstar or Callisto, an HPPD inhibitor.

Those results are "already concerning news for Minnesotans," Sarangi said.

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While the samples came from southern Minnesota, that doesn't mean the resistance is not present in other parts of the state, Sarangi said.
So this year, Navjot Singh, a graduate student working with Sarangi, has collected more than 100 weed seed samples from Crookston in the northwest to the Wisconsin border in the southeast.

But researchers also are asking growers to collect samples and submit them. Here's how:

  • Select the fields with lower than expected weed control from herbicide applications (avoid areas with a lack of spray coverage).
  • Collect the seedheads from at least 10 mature (black color seeds) female waterhemp plants, or 20 mature (brown seeds) ragweed plants and place the samples in paper bags. Do not mix the seeds of different species — bag them separately.
  • Fill out a submitter’s form and send it along with the samples. A link to the form can be found here: https://blog-crop-news.extension.umn.edu/2021/10/herbicide-resistant-waterhemp-and.html .
  • Mail samples to the U of M’s St. Paul campus:

Attn: Debalin Sarangi 411 Borlaug Hall
1991 Upper Buford Circle

St. Paul, MN 55108

Samples can also be submitted to local Extension educators in crops: https://extension.umn.edu/crop-production/contacts-crop-production .

From there, researchers will take the samples, clean the seed, refrigerate it to replicate dormancy, then grow in a greenhouse where herbicides can be applied, and results measured, a process that takes about six to nine months.

Management tools

While waiting for more data, Sarangi said farmers should look at a range of management tools, such as crop rotation to provide more herbicide options, using cover crops, and tillage.

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"A cover crop is a great tool for weed suppression," Sarangi said.

For farms that have been moving away from tillage, returning to it, even for one year, can be worthwhile.

Extension educator Ryan Miller at Rochester, Minnesota, said Enlist, soybeans "provided growers with some success and likely played a role with helping to manage waterhemp."

Waterhemp, a member of the pigweed family, is native to the Midwest. According to an Iowa State University study , it was not considered a significant agricultural weed until the early 1990s.

Miller said waterhemp has overtaken giant ragweed as the most problematic weed in southeast Minnesota, though giant ragweed has more potential to dramatically impact yields, especially in corn, with corn and ragweed emerging about the same time.

He said later spring tillage on fields planned for soybeans can be effective on ragweed.

"Every weed that you don’t control is going to add to the seed bank," Miller said, which leads to "that much more weed pressure in future years."

Reach Jeff Beach at jbeach@agweek.com or call 701-451-5651 (work) or 859-420-1177.
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