Palmer amaranth resistance growing
MINNEAPOLIS – It’s a beast of a weed, creeping north into the Midwest from cotton country.
Palmer amaranth can shoot up as high as 7 feet, and just one plant can produce up to a million seeds. Herbicide is increasingly futile against it, and the weed’s thick stems and deep roots make it hard work to clear by hand. It can slash yields and profits when it gets out of control.
Midwestern weed scientists are sounding the alarm because the plant recently turned up in Iowa and can cause deep losses in corn and soybean yields.
“This is not just a nuisance. This is a game-changer,” warned Purdue University weed scientist Bill Johnson, whose state has well-established pockets of the plant.
Cotton growers in the South already spend about $100 million a year to try to keep it out of their fields, University of Georgia scientist Stanley Culpepper said.
“This is a crop robber,” said W.C. Grimes, who farms 1,600 acres of cotton, peanuts and corn near Twin City in eastern Georgia. “It will steal your profit. It will choke your cotton out, and anything else you’re trying to grow.”
Grimes said he was losing up to 200 pounds of cotton per acre until farmers learned the key to overcoming Palmer amaranth’s resistance to glyphosate – sold under brand names like Roundup – was to continuously change herbicides.
His advice to Midwesterner farmers: Keep your eyes open and do whatever it takes to kill the weed as soon as it turns up.
One thing that makes Palmer amaranth so much tougher than other weeds is that one plant can produce 500,000 to 1 million seeds. A combine can scatter seeds from a couple plants across an entire field, Johnson said, and the untrained eye can’t tell the difference between Palmer amaranth and more common but less aggressive Corn Belt weeds, such as waterhemp and other kinds of pigweed.
Palmer amaranth probably took root in Kendell Culp’s fields near Rensselaer in northwestern Indiana last year, but he wasn’t aware of it until a seed salesman spotted it this summer. Culp pulled it up by hand – filling a pickup truck bed from one spot and a half load from another.
“Unfortunately I think it’s going to be a pretty difficult weed to control for us,” Culp said. He’s working with a consultant on strategies for deploying herbicides on his 1,750 acres of corn, soybean and wheat.
Palmer amaranth often hitches a ride on dirt stuck to farm machinery. It may also hide in grass seeds planted as cover for conservation programs, experts say. But they disagree on whether the seeds spread through animal feed containing cottonseeds or hulls, which are commonly added to dairy cattle rations.
Johnson said the weed is often seen near dairy farms, and the presumption is that when manure from those cattle is spread on fields, the seeds can spread with it. But Culpepper said the research he’s seen doesn’t back up that theory, adding that spreading the idea without proof could hurt demand for cottonseeds – and the entire cotton industry.
The infestation found this August in two western Iowa soybean fields probably got there by truck, Iowa State University weed scientist Bob Hartzler said.
Despite those fields being adjacent to a stretch of flood plain with poor soil where sludge from a Nebraska company has been spread as fertilizer, he said there’s no reason to think the sludge contained Palmer amaranth seeds. His suspicion is that the seeds were stuck in mud on trucks that hauled the sludge.
But Hartzler’s not convinced the weed will be as difficult to manage as many fear. Farmers who already take a proactive approach to common waterhemp should be able to control Palmer amaranth, as long as they try new strategies, he said.
Given the weed’s resistance to glyphosate, which is typically applied after weeds sprout, farmers need pre-emergent herbicides to kill the weed earlier in its growing cycle. Those have a much narrower window of time when they can be applied.
Palmer amaranth likes long growing seasons and hot, sunny weather, Culpepper said, so it may not be quite as aggressive in colder states. However, he said it’s still going to be “the baddest boy on the block.”
The weed isn’t known to have a beachhead as far north as Minnesota, but University of Minnesota Extension researchers have already advised their farmers to be vigilant.
“I’d like to say we’re not going to have the problem, but we’re not going to say that,” weed scientist Jeff Gunsolus said.
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