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Weed specialists: Palmer amaranth control requires identification, action and teamwork

In Benson County, North Dakota, where Palmer amaranth was first found in 2017, Scott Knocke, North Dakota State University agricultural extension agent, continues to teach farmers how to identify and control the weed.

PalmerAmaranthWithinCornRowsUnaffectedByHerbicide.jpeg
A Palmer amaranth plant found in a Goodhue County, Minnesota, corn field in September 2021 was not killed by a herbicide application.
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As the noxious weed Palmer amaranth continues to spread across the United States, weed experts are encouraging farmers to be vigilant about its control.

The pigweed species has been found in 28 states, including North Dakota, South Dakota and Minnesota. As recently as November 2021, the North Dakota Agriculture Department confirmed that Palmer amaranth was found in Ward County, bringing the total number of counties where it has been identified to 14.

The weed poses a major threat to crops because it grows quickly, produces as many as 1 million seeds per plant and is prone to herbicide resistance. Palmer amaranth has been known to reduce corn yields as much as 91% and soybean yields as much as 79%.

Bill Johnson, Purdue University Extension weed specialist, has called the weed the only one he’s seen that can drive a farmer out of business, and retired NDSU weed specialist Rich Zollinger said that the weed’s characteristics make it the most pernicious, noxious, and serious weed threat that North Dakota farmers have ever faced.”

In Benson County, North Dakota, where Palmer amaranth was first found in 2017, Scott Knocke, North Dakota State University agricultural extension agent continues to teach farmers how to identify and control the weed.

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He wants to be sure farmers know what Palmer amaranth looks like so they can take immediate action to control it.

“I’ll go to the farm with it very controlled in a bag,” Knocke said.

“The key is to identify that you have it, and you have to take steps so you don’t spread it around,” Knocke said.

If farmers find the plant in a field they should report and be sure to let let their neighbors know about it, he said.

“This shouldn’t be a secret. It should be a team effort to control,” Knocke said. “We need to work together.”

If farmers suspect that they have Palmer amaranth in their fields, they should take quality photographs and show them to an agricultural weed expert for identification, he said.

Farmers whose fields only have a few weeds in their field can pull them, and, ideally, burn them and bury them at least 6 inches deep or more in the same field, said Joe Ikley, NDSU Extension weed specialist.

Another way to control Palmer amaranth is to rotate crops so varied herbicides are used on them, he said.

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Herbicides used to control Palmer amaranth in corn have the most effective post-emergency control and there also are post-emergence herbicide options for control of the weed in soybean fields. However, sunflowers and pulse crops don’t have post-emergence herbicide options for control, so if possible, those should be rotated with corn and soybeans, Ikley said.

Herbicides should be applied when the weed is no higher than 3- to 4-inches tall — it can grow to as high as 6- to 8-feet — because it becomes more difficult to control as it matures, Ikley said.

South Dakota State University has more information on Palmer amaranth weed control is available online at https://extension.sdstate.edu/identification-and-management-palmer-amaranth-south-dakota . To report a suspect plant in North Dakota, go to https://www.nd.gov/ndda/pa or contact a local county weed officer.

In Minnesota possible infestations should be reported to the Minnesota Department of Agriculture at 1-888-545-6684 or arrest.the.pest@state.mn.us .

Ann is a journalism veteran with nearly 40 years of reporting and editing experiences on a variety of topics including agriculture and business. Story ideas or questions can be sent to Ann by email at: abailey@agweek.com or phone at: 218-779-8093.
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