Forrest Laws

October 29, 2009

4 Min Read

As reports of herbicide-resistant Palmer amaranth continue to spread throughout the Southeast, farmers are turning to hand labor to try to keep the pesky weed at bay. Are moldboard plows or heavy cover crops next?

Many growers who swore they would never take up the hoe again are finding the tool has been indispensable in preventing glyphosate- or ALS-resistant pigweed from spreading even further. Now weed scientists in Georgia, where the problem began, are saying farmers may want to consider a turning plow next.

Stanley Culpepper, the University of Georgia weed specialist who has become a widely recognized authority on resistant pigweed, says hand-hoeing has definitely helped make a difference in his state.

“For the first time in five years, I’m very optimistic,” said Culpepper, who was interviewed at this year’s Sunbelt Expo in Moultrie, Ga. “Overall, I think we had a very good year. Our growers have been super aggressive. We’ve even had a significant amount of hand-weeding to make sure that Palmer amaranth doesn’t go to seed — to help us out next year.”

Culpepper, a speaker at one of the new technology updates held during this year’s Sunbelt, said it’s not that growers weren’t challenged by Mother Nature in 2009. Still, growers have been able to cope, although many were still harvesting cotton and peanut during the Expo (Oct. 20-22).

“Overall our growers have adopted good programs. They’re using residual herbicides at planting. They’re doing better with their postemergence timeliness, and we’re using residual herbicides throughout the crop. So, I think we’ve had a positive year.”

Growers are beginning to turn to cover crops and deep tillage because of the severity of the problem. One female Palmer amaranth plant can produce 500,000 seed, and one of two resistant plants can turn a field into a disaster with two or three growing seasons.

“We’re learning more and more,” says Culpepper. “Back in 2005 and 2006, we realized that you’re not going to control a glyphosate-resistant Palmer amaranth or glyphosate/ALS-resistant Palmer amaranth with herbicides alone. The sooner you accept that, the better off you’re going to be. So as soon as we accepted that, we started doing research looking closely at both aspects from heavy tillage all the way over to cover crops.”

Palmer amaranth weed, more commonly called pigweed. (Photo: University of Georgia)

Researchers have found that the cover crops that produced the largest amounts of biomass — ryegrass for example — give the best results. “If we go into a ryegrass cover crop that’s 7 or 8 feet tall, we’ll kill it, we’ll roll it, we’ll plant into it,” he said. “They are proving to be extremely effective.

“Now that’s only one component of the system. You still have to use herbicides, you still have to use other management tactics, but you can use that cover crop as a mulching effect to basically prevent the sunlight the pigweed needs to germinate from reaching the soil surface. It can be a very effective tool, and we have research to support that.”

Culpepper says deep tillage is becoming more and more common in Georgia as growers continue to try to rein in resistant pigweed.

“What we have seen — especially for our growers who have done little to no deep tillage in the last 10 to 15 years — is that most of our Palmer amaranth seed is in that top inch or so. We know that about 80 percent of the Palmer amaranth that a grower fights in a given year is emerging from that top inch.”

Growers might get another 10 to 15 percent from inches 1 and 2, but very little germination occurs below 2 inches, he said.

“If you’re able to invert that land and truly take the top 2 inches of your soil and put it 4 inches or deeper in the profile, you will reduce the amount of Palmer amaranth that will emerge in a given area by anywhere from 40 to 60 percent.

“Deep tillage, as long as you truly invert those Palmer amaranth seed and get them at least 4 inches deep, can absolutely be a benefit in managing resistant pigweed.”

e-mail: [email protected]

About the Author(s)

Forrest Laws

Forrest Laws spent 10 years with The Memphis Press-Scimitar before joining Delta Farm Press in 1980. He has written extensively on farm production practices, crop marketing, farm legislation, environmental regulations and alternative energy. He resides in Memphis, Tenn. He served as a missile launch officer in the U.S. Air Force before resuming his career in journalism with The Press-Scimitar.

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