Mike Hamilton figures an uncontrolled outbreak of glyphosate-resistant horseweed has the potential to cause Arkansas cotton and soybean producers nearly $500 million in losses. Hamilton, Crittenden County Extension agent, said his figures reflect what Arkansas farmers could lose in inputs if they had a 50 percent loss of yields in a 900,000-acre cotton crop and a 25 percent yield loss in 3 million-plus acres of soybeans.
Ken Smith, Extension weed specialist, said, “If you're in the Arkansas Delta, and you don't have a resistant horseweed problem now, you soon will.” Since the herbicide-resistant weed first showed up in Arkansas in 2003, it has infested about a half-million acres, he estimated.
“In 2003, we found that glyphosate no longer worked on some horseweed in Arkansas. The weed had become resistant to the herbicide. This threw a wrench into our weed control programs. All of a sudden what we've always told farmers and what we knew would work, no longer worked.”
Horseweed, sometimes called marestail, germinates in the fall and into the spring. A mature plant can grow almost as tall as a person and can produce 50,000 seeds, which are dispersed by the wind, Smith said.
“We can't stop the spread of this weed. It will spread over the entire Arkansas Delta. We have to live with it and adjust to it.”
Smith said farmers have traditionally managed it well in fields with glyphosate plus a hormone herbicide in the spring. While a normal rate of glyphosate will kill a susceptible horseweed, eight times the recommended rate may not kill a resistant weed.
Smith is also concerned about how horseweed will affect conservation tillage efforts, which the University of Arkansas promotes and many farmers have adopted.
“In Arkansas, we've steadily increased our conservation tillage acres. Roundup Ready technology has allowed us to farm more acreage in a conservation tillage program. Our goal is to set up a situation where we would not lose that amount of conservation tillage and take that step backward. The university has been promoting conservation tillage, and that's something we want to maintain.
“Tennessee, which has had the resistant weed longer than we have, lost 50 percent of its no-till acreage in one year. And it's mainly caused by horseweed.”
Smith said the University of Arkansas initiated a small plot research project to identify chemicals that can give reliable control of the weed.
Susan Matthews, a Mississippi County, Ark., Extension agent, said one of the plot tests was in her county. “Our best results were in tank mixes with Clarity and glyphosate (for the other weeds). That was our most effective performer over two years. That's our recommendation. Clarity is a phenoxy herbicide that interferes with plant growth. With Clarity, farmers have to work around a plant-back restriction. Rain is also a consideration.”
Hamilton said one producer in his county had a severe infestation in a 120-acre field that traditionally yielded 1,000 to 1,100 pounds of cotton per acre.
“Last year, all the fields around it had phenomenal yields, but it yielded 400 pounds per acre because of horseweed. The farmer lost more than $35,000 because he didn't get his money back for what he spent on inputs.”
Lamar James is an Extension communications specialist with the University of Arkansas.