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Resistant pigweed plagues central Georgia cotton

Earlier this year Georgia confirmed the world's first population of Palmer amaranth resistant to glyphosate, a herbicide commonly sold under the brand name Roundup. This will cause problems for cotton farmers, says a University of Georgia weed specialist.

Right now, glyphosate-resistant Palmer amaranth is known to infest about 500 acres of cotton in central Georgia. Stanley Culpepper, a UGA Cooperative Extension weed scientist studying the outbreak, said seeds from at least 100 fields in the area have been harvested to determine any further distribution.

“This could be a real threat to future cotton production in our region,” he says. “It's the one weed cotton farmers didn't want resistant to Roundup.”

Palmer amaranth, also called pigweed, is found throughout the state. The troublesome weed can quickly grow more than 8 feet tall with a thick stalk and suck valuable nutrients from nearby plants. It can clog a cotton picker, too, making it hard to harvest the crop.

In 1997, farmers started planting cotton that was developed to stay healthy when sprayed with Roundup. They could spray the herbicide over-the-top of this cotton, killing weeds, but not the cotton. This saved farmers time and money because they didn't have to repeatedly plow between rows to kill weeds.

Roundup Ready varieties cost more than conventional cottons. But farmers gladly embraced the new technology, Culpepper says.

About 94 percent of Georgia's 1.21 million acres of cotton this year is Roundup Ready. “Roundup has been our most effective tool to manage this weed in Roundup Ready crops,” he says. “Most alternative control options are much less effective than Roundup in controlling a normal population of Palmer amaranth.”

Each year, some Georgia farmers have to deal with some Palmer amaranth plants that continue to grow after a spray with Roundup. This usually happens due to weather conditions or improper spraying.

Specialists with the UGA College of Agricultural and Environmental Sciences last fall suspected some Palmer amaranth weeds in central Georgia had resistance to Roundup. Many field and greenhouse trials and heritability studies now show that the Palmer amaranth population in central Georgia has true resistance, he says.

Scientists with Monsanto, registrant of Roundup Ready, are providing technical expertise and other help to address the problem, Culpepper says.

Farmers need to watch their fields carefully this year and remove any Palmer amaranth not hurt after a spray with Roundup, he says. This could help keep resistant plants from spreading.

It's too early to say what long-term effect this will have on cotton production in Georgia. But if farmers are no longer able to control this weed with Roundup, things will have to change.

Farmers may once again have to plow fields to manage pigweed, Culpepper says. This will cost them time and money. The resistant weed could keep farmers from using conservation-tillage, too.

Farmers have relied too heavily on Roundup to control weeds in cotton, Culpepper says. This has given nature the upper hand.

Herbicides don't cause a plant like Palmer amaranth to change genetically or become a resistant mutant, he says.

All it takes is one weed plant in a field to be genetically different — in this case, resistant to glyphosate. All the other weeds are killed when sprayed, but not the resistant one. It makes seeds. The next year, a few more resistant plants grow from those seeds. If the process is allowed to continue, the offspring of that one resistant weed could eventually cover the field.

This is what has happened in central Georgia. But it could happen anywhere, Culpepper says.

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