Scout, avoid Southern cottonseed manure to stop Palmer amaranth
Palmer amaranth, also known as a pigweed on steroids, has been reported in 13 counties across Ohio as of late 2015. That's a marked increase from 2012 when the weed was found in only one county in the Buckeye State, according to a weed scientist in the College of Food, Agricultural, and Environmental Sciences at The Ohio State University.
Palmer amaranth is a glyphosate-resistant weed that has devastated many cotton and soybean fields in Southern states, in many cases causing entire cotton and soybean fields to be mowed down, said Mark Loux, an Ohio State University Extension
OSU Extension is the statewide outreach arm of the college.
Now that Palmer amaranth has been found in multiple fields across Ohio, the concern is that unless farmers work diligently to stop its growth, the weed could have a devastating impact on crops, Loux said.
"Palmer is a difficult weed to manage because it requires multiple applications and has to be treated when it is less than 3 inches tall to stop it," he said. "Palmer amaranth can grow 3 inches a day and can release nearly a half-million seeds per plant.
"Because of its fast growth, herbicide resistance and ability to destroy entire crops, Ohio growers are going to have to be vigilant to prevent Palmer amaranth's further spread across the state."
So far, Palmer amaranth has been found in fields in Williams, Brown, Clark, Putnam, Sandusky, Lorain, Wayne, Mahoning, Madison, Fayette, Ross, Highland and Scioto counties in Ohio, Loux said. The infestations in some of these counties have since been controlled.
Palmer seed entered Ohio through fields spread with contaminated manure from animal operations using cottonseed products from the South as feed, he said.
"That has been the source of our most recent and most severe infestations that occurred in 2015 in northeastern Ohio," Loux said. "Palmer can also be spread by farm equipment previously used on a contaminated field and by water."
Loux described a grower in Mahoning County who purchased a load of cottonseed feed product from a local feed dealer in 2014 and spread manure from animals that ate the feed on his fields in late winter and early spring of that year.
As a result, the grower had dense infestations in his fields in 2015, compared to zero Palmer amaranth in 2014, Loux said.
"This is a weed that can overtake a field if you don't address it early enough," he said. "This weed can overtake a field faster than any other weed that we've dealt with.
"Once Palmer amaranth is found, the most effective way to keep this weed from spreading is for farmers to scout their fields to see that none of the plants go to seed."
While early season residual herbicides should reduce the rate of increase following an initial Palmer amaranth introduction, it's difficult to achieve 100 percent control of an established Palmer amaranth infestation with even the most effective program, Loux said.
"Palmer is a moving target," Loux said. "Infestations within a county can range from one or more fields or other areas with just a few plants or patches of plants, to the presence of one or more fields with dense populations."
Although some animal operations are aware of this problem and have stopped using these types of feed products, it's likely that many other operators and feed dealers have not received information about this issue or modified their practices, he said.
"We're working with dealers, agronomists and farmers in a community effort to keep the Palmer amaranth infestation from getting worse," Loux said. "We are still largely in a preventive mode in the state, with some areas that are probably going to get out of control or at least have fields that are going to be pretty ugly."
Steps Loux recommends to keep Palmer amaranth out of your field include:
- Know what it looks like.
- Continue to use residual herbicide in your soybeans to keep that first flush under control.Scout for Palmer late in the season to try to try to make sure you take it out before it produces seed.
- If you find it, look for mature seeds, which are small and black. If you don't have any, just cut the weed out just below the soil line. Remove it from the field and burn it.
- If you are harvesting a field and see something that you suspect is Palmer amaranth, go around that patch or stop and get help with identification to figure out if it is Palmer before you drive your combine through it and blow it out farther into the field.
More information on Palmer amaranth, including how to identify it and manage it, can be found on Ohio State's weed science website at