This photo by the Minnesota Department of Agriculture shows Palmer amaranth plant discovered in a soybean field in Redwood County last fall. (The Minnesota Star Tribune)

Public enemy No. 1 for corn and soybean farmers, the Palmer amaranth weed, has made new incursions into Minnesota by way of livestock feed.

The Minnesota Department of Agriculture said this week that cows have eaten remnants of the weed in feed, then eliminated it in manure that farmers spread on fields.

This is bad news for corn and soybean farmers, both because the weed grows and proliferates quickly and because it is resistant to multiple herbicides. It can grow up to 8 feet tall with a woody stem thick enough to damage farm equipment that tries to mow it down.

Officials at North Dakota State University, who are also gearing up to fight the weed, say the spread of Palmer amaranth can reduce yields by up to 91% for corn and 79% for soybeans.

The weed has now been confirmed in six Minnesota counties. The Department of Agriculture is encouraging farmers, especially those who spread manure from feedlots on their land, to look for Palmer amaranth, take pictures of suspicious plants, contact the department and save samples of the weed in plastic bags in the refrigerator.

The weed, which grows up to 3 inches a day and can produce a half-million seeds per plant, was first observed in Minnesota in conservation land in 2016. Thirteen landowners planted it unknowingly in a mix of native grass and flower seeds sold by a small company in Cottonwood, Minn.

This past fall, however, the weed was discovered in a soybean field in Redwood County, and then along a road in Jackson County. The findings were announced this week.

"Up until that point, all the finds had been in conservation plantings," said Allen Sommerfeld, a spokesman for the Minnesota Department of Agriculture.

Investigators figured out that the weeds in Redwood County had come from cow manure that had been spread on the field, after cows had been fed "seed screenings" from contaminated sunflower seed. Screenings are the leftovers after seeds are cleaned and can be good food for livestock.

The Department of Agriculture said it has sampled screenings being fed to livestock throughout the state and found Palmer amaranth seed in "numerous" places, though researchers are still working to figure out exactly which fields have been spread with infested manure.

"Now we think that it could turn up in many more places," Sommerfeld said.

Sales of the weed's seeds are prohibited in Minnesota. The company in Cottonwood that sold the contaminated seed to landowners in 2016 was fined $4,000.

Palmer amaranth is native to the southwestern United States and northern Mexico, but it has now spread to more than half of the states, including Iowa, North and South Dakota and Wisconsin. Some of the trouble with the weed is that its seeds are impossible to identify visually and the weed is highly adaptable.

"They are extremely tiny. They look like poppy seeds," said Anthony Brusa, a researcher at the College of Food, Agricultural & Natural Resource Sciences at the University of Minnesota.

Brusa is working on a genetic test for Palmer amaranth that will make it easier for farmers to identify the seeds and the plants. The tests are "performing very well," he said, but won't be fully ready until the end of 2019.

The adaptability of the weed complicates the fight against it for farmers. It's been able to migrate as far north as North Dakota, despite being native to a much warmer climate, and different varieties of the weed are resistant to different herbicides in different parts of the country.

"In Kansas and in Illinois, there are some Palmer amaranths that are resistant to four or five classes of herbicides," said Jeff Gunsolus, a weed scientist at the University of Minnesota Extension. "We only have about nine classes of herbicides, so you're really reducing your options. You are dealing with uncertainty, because you don't know which herbicides it is resistant to."

The weed will start emerging from fields in the second week of May and should keep sprouting into July, Gunsolus said. If farmers can catch it early and even go through and manually uproot and destroy the weeds that emerge as late as July, they should be able to, after a few years, eliminate the species from their fields.

"This is about diligence," Gunsolus said, "and following through for the whole growing season."