Young Palmer amaranth plant. Photo courtesy of Nebraska Extension.

ST. PAUL, Minn. – The Minnesota Department of Agriculture (MDA) has identified a previously unrealized mode of travel for Palmer amaranth weed seed to enter the state and get into the fields. Last fall’s find of Palmer weeds in a soybean field in Redwood county have been traced back to grain screenings that were fed to cattle.

“As we were trying to identify the source in that soybean field in Redwood county, we learned that manure had been applied to the field, so we tracked back where the manure had come from,” said Denise Thiede, head of the Seed and Noxious Weed Program at the Minnesota Department of Agriculture, during a recent phone interview.

The manure was sourced from a cattle feedlot in the area that sells manure to local farmers. The MDA visited the lot and decided to collect samples of the grain screenings that were being fed to the cattle.

“We did not detect Palmer in every feed lot, we found it at one,” said Thiede. “It was found at such a low level that we really wanted to duplicate the results, so we went back in February and collected another sample. That's really what raised the alarm.”

The second round of samples had a much higher level of Palmer amaranth seeds present. The rough estimate from the MDA is around 250 Palmer amaranth seeds per pound of grain screenings.

“That is higher than what we saw in the seed that was sold into the state,” she said.

Given the grain screening and Palmer amaranth seeds are being consumed by cattle and going through the rumination process, not all the seed will survive, but some will. It is very much unknown as to what percent of the seeds can survive digestion and still be viable once the manure is spread onto the field.

“The populations that we found in the field were very small, about three or four plants,” she said. “We do not understand enough yet about how this whole pathway is going to work and what the attrition rate will be from the screenings to the manure to the field, in terms of potential plant growth and establishment.”

The MDA already has a plan to move forward and better understand this new pathway.

They are planning to work with the University of Minnesota to test manure samples coming out of the feedlot were the cattle were fed contaminated grain screenings. The goal will be to determine how many Palmer seeds survived digestion and will germinate.

“We have obtained the records for where that feedlot supplied manure and we are going to be doing field inspections of all of those fields throughout the growing season this year to identify establishment of plants and manage in those fields where we find Palmer.”

They also plan to increase the amount of sampling and testing done of grain screenings to be used as feed or bedding coming into the state. The goal will be to identify which type of grain is more problematic and from what area of the country it is coming from.

The contaminated screening on the feedlot in Redwood county were sunflower screenings likely coming from southern states.

“We are a little bit concerned about the scale of the problem. We really are uncertain right now about the scale,” said Thiede. “We really want farmers scouting and letting us know if they see something that they're concerned about.”

Palmer amaranth has already developed herbicide resistance to several different modes of action in the states where it is prolific. The seeds coming into the state have the potential to already be herbicide tolerant and that is why identification and eradication are vital to controlling this particular weed.

“Our goal is to help foster the notification process so we can get out in the field when the plants are small, they're harder to identify and we might need to do some genetic testing to confirm if we have Palmer not,” she said. “All of that takes time, so the sooner we can get out, look at the plants and figure out what we're dealing with, the better able we are going to be to react to the situation.”