Southscapes Spring 2008

Extension

Pigweed: Cotton's biggest threat since the boll weevil

By Brad Haire

On a cool December morning, Stanley Culpepper walks into a greenhouse in Tifton, Ga., and points to the trays where a few weeks before he planted seeds collected from Georgia fields. The tiny seedlings that break through now are growing into one of the biggest problems to face Georgia cotton farmers in decades.

"Look. We've already sprayed these. They should all be dead. But they're not. Didn't even faze them. It's crazy," says Culpepper, who sounds like a man who just shot a charging wild boar hog square in the nose and is shocked to see it still headed toward him.

Culpepper is a weed specialist with University of Georgia Cooperative Extension. He is amazed by palmer amaranth weed, more commonly called pigweed. that farmers can no longer kill the pest with glyphosate, a popular herbicide sold under the brand name Roundup®.

"Palmer amaranth is by far the one weed we didn't want to develop resistance to glyphosate," he said. "This is potentially the biggest threat we've seen since the boll weevil." The boll weevil crippled cotton production in the Southeast in the mid-1900s until a multimillion dollar eradication program started in 1987 stopped the pest in 1994.

Palmer amaranth weed, more commonly called pigweed.

Found everywhere across the Southeast, unchecked pigweed can grow several inches per day and be the size of a small tree in a few months. It steals nutrients away from cotton plants. So much so that one pigweed can reduce lint yields by 50 percent within a square yard around it.

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. Farmers eagerly embraced the new technology, Culpepper said. Virtually all Georgia cotton grown now is Roundup Ready®.

But in 2004, rumors of resistant weeds started popping up. In 2005, it was confirmed: Georgia was the first place in the world to have glyphosate-resistant pigweed. Since then, it has been confirmed in North Carolina, South Carolina, Tennessee and Arkansas.

Herbicides don't change a plant genetically to become a resistant mutant, Culpepper said. All it takes is one weed plant in a field to be genetically different — in this case, resistant to glyphosate. It makes seeds. (A female pigweed can produce 500,000 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 Georgia. Eighteen Georgia counties, mostly in the south-central region, have the resistant weed now. And it's spreading.

"This problem isn't going away on its own," he said.

Over the past three years, Culpepper has spoken about the problem at hundreds of county meetings, several national conferences and in scores of TV, print and radio news stories.

"We're pretty much taking a shotgun approach to this right now. But our No. 1 goal is education and getting some management strategies out there so farmers who have it can control it and maybe slow its spread,” Culpepper said.

With Roundup Ready® cotton, a farmer spends around $20 per acre to control weeds annually. But a farmer with the resistant pigweed could spend three times that and still not have control, Culpepper said.

"Developing management strategies to overcome weed resistance is vital not only in Georgia but across the Southeast," said Richey Seaton (BBA - Agricultural Economics, '85), executive director of the Georgia Cotton Commission. "Our growers are working on a razor-thin profit margin, and the Roundup Ready system has proven benefits. We've got to find economical ways to work around this problem for growers."

Culpepper began to look for answers in 2005 with a couple of experiments on two acres. Last year, he conducted a dozen experiments on 102 acres. "I'm spending at least 50 percent of my time on this problem alone," he said.

The key, Culpepper said, is finding how the resistant trait spreads. With support from the cotton industry and the Georgia Cotton Commission, Culpepper hired post doctoral associate Lynn Sosnoskie to help answer that question. She's investigating pigweed pollen.

"There isn't much literature out there on anything like this," Sosnoskie said. "We're pretty much writing it as we go."

Pigweed pollen is unusual in that it's dimpled like a golf ball. The dimples on golf balls help them travel farther. The researchers are surprised at how far pigweed pollen can fly.

A weed scientist doesn't lack for work in the Southeast. The region is full of tough weeds. But to keep cotton production viable, Culpepper said, an answer to this weed problem must be found in the next few years.

"I enjoy helping farmers and finding ways to keep family farms in business," he said. "That's what we do. This is what we need to do right now."

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