Contaminated grain and screenings used as animal feed appear to have caused new infestations of Palmer amaranth in the Midwest, Minnesota, officials say.

The weed, which is native to the southwest but gradually spread north, was eradicated in Minnesota in 2016-2018, said Denise Thiede, who oversees the noxious weed program within the Minnesota Department of Agriculture (MDA). Last fall, a new Palmer amaranth infestation was confirmed in a Redwood County soybean field.

Palmer amaranth (University of Minnesota)

After further investigation, Thiede said, the state department of agriculture determined that the soybean farmer had obtained manure from a local feed lot, where cattle were fed sunflower screenings tainted with low quantities of Palmer amaranth seed. The tiny seeds are indigestible and capable of germinating after passing through the digestive tract.

The MDA went on to collect additional feed samples around the state and found Palmer amaranth in both wheat and sunflower screenings this past February. “So right now we’re trying to figure out what action we should take to limit the risk of this pathway,” Thiede said.

Palmer amaranth has developed a tolerance for most herbicides and is extremely difficult to eradicate once it becomes established, she said. Each plant can grow two to three inches per day and is capable of producing hundreds of thousands of seeds, which are so small they can be difficult to see when mixed with grain or other seeds.

“No farmer wants to bring that weed onto their land,” Thiede said. “The costs to them are going to be significant if it gets established.”

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The weed has traditionally spread in contaminated native seed blends used for conservation. However, Thiede said there are some findings to suggest Palmer amaranth in animal feed may be the culprit in some North Dakota outbreaks as well.

Thiede said the MDA is not yet sure how to prevent the spread of the weed through this new pathway. They’re currently encouraging farmers to immediately report any suspect plants through the Arrest the Pest hotline at +1.888.545.6684 or arrest.the.pest@state.mn.us

Reporting can help determine the origin of the outbreak, so officials from the state are working to raise awareness of the weed, and the potential for it to spread via feed and animal manure, according to Jeff Gunsolus, an extension weed scientist from the University of Minnesota. Producers, Gunsolus said, should question their suppliers about whether screenings have been tested for Palmer amaranth to prevent infestations.

“If they have any idea of where they feed has come from, that is a good place to start,” he said. “A number of people have switched. They may not use sunflower screenings, they might use corn instead.

“This issue is in its infancy right now,” he continued. “There is a lot being worked through.”

Long-term, Thiede said, the MDA would like to require screenings to be tested for Palmer amaranth before they can be sold as feed, but that will require state legislative action. In the meantime, she said, “we have met with all kinds of different livestock producers to see what impact it would have if we restricted screenings from out of state, and didn’t get a lot of negative feedback, so we’re probably going to move in that direction.”