Ohio State Weed Specialist: Preventing the Spread of Costly Herbicide-Resistant Weed Calls for Zero Tolerance

Palmer amaranth

COLUMBUS, Ohio – If you see even one of this notoriously damaging weed in your field, pull it up – fast!

Otherwise it could be the worst mistake you’ve ever made in your farming career, according to a researcher from Ohio State University’s College of Food, Agricultural, and Environmental Sciences.

Palmer amaranth, a glyphosate-resistant weed also known to many cotton and soybean farmers in the South as a “pigweed on steroids,” has already begun showing up in Ohio fields, said Mark Loux, an Ohio State University Extension weed specialist. OSU Extension is the statewide outreach arm of the college.

Because of its fast growth, herbicide resistance and ability to destroy entire crops, Ohio growers are going to have to take a zero tolerance approach to prevent Palmer amaranth’s further spread across the state, according to new research, Loux said.

The concern is that this weed could become a significant problem in Ohio if it continues to spread in the state, he said. It has already had a substantially negative impact on yields and profitability for cotton and soybean growers in Southern states.

If it takes hold, Palmer amaranth could become even harder to control than the glyphosate-resistant weeds already in Ohio, Loux said. The weed has caused entire cotton and soybean fields to be mowed down in some states.

“We’re on the cusp – we’ve already identified Palmer amaranth in roughly nine sites in Ohio, ranging from a few plants to one site that had multiple plants on several fields,” Loux said. “This weed has more potential to impact the profitability of our corn and soybean production than any of our other resistance problems.”

At this point, even though Palmer amaranth has been found in southern Ohio near Portsmouth and in central Ohio in Madison County, Ohio is still in a prevention mode, meaning that if growers take aggressive action now, the weed’s spread statewide can be lessened, he said.

The need for aggression is clear, Loux said, based on results released last week of a study in Arkansas cotton fields to determine the effect over time of releasing 20,000 glyphosate-resistant Palmer amaranth seeds in a patch within a one-square-mile area. 

According to the study, the first growing season resulted in a separate patch of Palmer amaranth emerging 375 feet from the original location. Resistant plants expanded to reach field boundaries and infested 20 percent of the entire area in the second growing season, resulting in decreased cotton yield and interference with harvest, Loux said. 

By the third growing season, Palmer amaranth had completely colonized the fields, making cotton harvest impossible, he said.

“The study results show the need for a zero tolerance threshold on Palmer amaranth,” Loux said. “Prevention requires that not even a single plant be allowed to go to seed.” 

Palmer amaranth, which can grow 3 inches a day, can release nearly a half-million seeds per plant. And because the weed is glyphosate-resistant, many growers in Southern states, in addition to spraying, have had to hire workers to go into their fields to chop down the weeds with hoes and pull them by hand, Loux said. 

“Farmers generally have a philosophy that says if they see only one or two weeds, then they’ve got good weed control,” he said. “But Palmer amaranth is so aggressive that we need to get people to understand that they can’t afford to have even one of these plants.

“The only way to keep this weed from spreading is for farmers to scout to see that none of the plants go to seed. If you see Palmer amaranth out there before it produces seeds and you don’t pull it out, it may be the worst mistake you make in farming.”

Growers with fields that have been spread with manure from animal operations using cottonseed products as a feed need to be particularly aware, as this manure may contain Palmer amaranth seed, Loux said.

While residual herbicides in addition to glyphosate 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, he said.

The potential financial impact for growers is a resistance problem with yield loss because the weed couldn’t be controlled, Loux said, along with permanently higher weed management costs. Growers could find themselves in a position of having to pay an additional $15 to $30 an acre to control Palmer amaranth, and in some cases even higher costs have been incurred, ranging from $20 to $40 per acre, he said.

In fact, cotton growers in Georgia spend more than $100 million annually to manage Palmer amaranth, according to Chris Chammoun, director of public affairs for the Georgia Cotton Commission, a Perry, Georgia-based industry association. Growers are spending anywhere between $80 and $100 per acre for chemical and hand weeding to control Palmer amaranth, he said.

“We’re trying to get people’s awareness up about these weeds,” Loux said. “If we’ve seen Palmer amaranth in some sites, then it’s most likely that we have it in more areas.

“We still have a chance here in Ohio against this weed because we are still in a prevention mode. Palmer amaranth scares the heck out of farmers because they know what can happen when this weed takes hold.”

The result of not stopping this weed in its tracks could be a substantial loss of income in future years, he said.

“Letting even one plant go to seed is a big mistake.”

Images of the weeds and more information on them can be found at the OSU weed management website, agcrops.osu.edu/specialists/weeds.

Tracy Turner
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Mark Loux