Paul L. Hollis

December 17, 2008

5 Min Read

Glyphosate-resistant Palmer amaranth pigweed continues to spread at a rapid rate in Georgia, says Stanley Culpepper, Extension weed scientist. And — while it's scant consolation — those who have fought the problem for a few years now are seeing better results from control programs

The glyphosate-resistance situation in his state this year has been a “mixed bag,” says Culpepper. “Farmers who originally had resistance, or who have had it for three to four years and are using recommended programs, have learned to do a better job of controlling it. For other growers, who are in their first, second or third year of resistance, it has been a very devastating year,” he says.

Through 2007, glyphosate-resistant Palmer amaranth pigweed had been confirmed in 20 Georgia counties, says Culpepper, and 20 other counties are being screened for resistance during the 2008 production season. Resistance was first confirmed in three counties in 2005, growing to 10 in 2006 and 20 in 2007. “Resistance is still spreading extremely rapidly — it's unbelievable how much it has increased since 2005,” he says.

Multiple resistance — to both glyphosate and the ALS herbicides — has been confirmed in Georgia, says Culpepper.

Residual herbicide programs developed to battle glyphosate resistance have been fairly effective, especially in irrigated situations, says Culpepper. These programs also are effective in dryland fields if growers receive rainfall in a timely manner, he adds. But in either case, the pigweed has to be very small before it can be controlled.

“We did a lot of cultivating, hand-weeding and irrigating this year to fight this weed. All of our programs for battling this weed are based on the residual herbicides, so it's definitely a plus if you have irrigation to water it in,” he says.

Culpepper calls 2007 a “transition” year as far as growers recognizing the severity of the glyphosate resistance problem in Georgia. “In 2007, our growers really started paying attention to the fact this was a serious problem and a threat to cotton production and to conservation-tillage in the state. This year, producers have been very aggressive in addressing the problem,” he says.

Specific programs for controlling pigweed in 2009 will be presented during winter cotton meetings, says Culpepper. They also will be published in Southeast Farm Press as they become available.

Mike Patterson, Auburn University Extension weed scientist, calls glyphosate resistance the “800-pound gorilla in the weed science business.” “If you go to south Georgia, they hand pulled more than 15,000 acres of this weed out of cotton fields this year — it grows 8 or 9 feet tall, and it has a stem as big as your arm. As farmers have discovered through experience, mere chopping isn't enough. Only root-and-shoot removal will work — otherwise weeds return with a vengeance as new plants sprout from the roots,” he says.

Palmer pigweed can grow 2 to 4 inches a day and each plant can produce as many as half a million seeds, he adds.

Historically, Alabama growers haven't had a problem with glyphosate-resistant pigweed, but it is coming into the state, says Patterson. “About 90 percent of our cotton acreage is planted in Roundup Ready varieties, virtually all of our soybeans are Roundup Ready, and a good portion of our corn is Roundup Ready, so the problem is bound to occur here,” he says.

Ninety-nine percent control is not good enough for this weed, says Patterson. “If you pick up a shovel full of soil, you may have a million seeds in that shovel full of soil. It sounds unbelievable but it's true. So, if you've got a million plants on an acre of ground and you control 99 percent of them, you've still got 10,000 plants left per acre. So you've got to maintain complete control, because this weed is so competitive and produces so much seed. And with our current technology, we'll never exhaust the seed supply in the soil in these fields. This is going to be a long-term problem.”

One reason for that, Patterson says, is because it would take as much as 10 years for chemical companies to bring any new “miracle” product to market.

“They may slow it down, but I don't think farmers can save their farms waiting for that to happen,” he says. “Too bad we can't find a use for it, maybe grow it for biomass and make fuel out of it.”

If a residual herbicide program doesn't work, the pigweed ultimately emerges and out-competes the cotton, says Patterson. “By the time the cotton reaches 3 or 4 inches, you've got pigweed that is 7 or 8 inches. And by then, it's too late,” he says. “Beyond that, there's nothing available in our cotton herbicide arsenal to kill the weed postemergence.”

One of the only remaining options is rotating cotton with corn. As a grass crop, corn will tolerate applications of herbicides that broad-leaf crops such as cotton won't, he says.

For example, pigweed still can be controlled using corn herbicides such as atrazine, which farmers have used for 40 years.

“Almost every acre of planted corn is treated with atrazine,” says Patterson, adding that a couple of other corn-related herbicides are also still effective in treating resistant pigweed.

Researchers have discovered that pigweed seed may typically degrade after only three years in the soil. “That may be one of the weak points of this weed, which is a good thing,” Patterson says, adding that other weed seed, such as sicklepod and morningglory, may survive for decades in the soil.

Researchers are searching for ways to keep these fields clean — managing them so germination doesn't occur, leaving pigweed seed to decay in the soil.

“That's the only way we're going to beat this thing, unless chemical companies come up with another weed control system similar to the Roundup approach,” says Patterson.

“But don't hold your breath on that one, because Roundup represented a once-in-a-lifetime discovery that is not likely to be repeated.”

About the Author(s)

Paul L. Hollis

Auburn University College of Agriculture

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