Chemical-resistant weeds highlight LSU AgCenter field day

Bruce Schultz, Leonard, Billy R., Stephenson, Daniel O., Williams, Billy James, Miller, Donnie K., Boquet, Donald J., Stevens, Jr., J. Cheston, Padgett, Guy B.  |  8/25/2009 1:34:10 AM

News Release Distributed 08/24/09

ALEXANDRIA, La. – LSU AgCenter weed scientists warned farmers that weeds will develop resistance, as they have in Arkansas and surrounding states, if resistance-management strategies are not adopted.

Speaking at a field at the LSU AgCenter’s Dean Lee Research and Extension Station Aug. 20, the scientists listed several strategies that include tank-mixed herbicides, herbicide rotation and the use of residual herbicides, as well as making sure that herbicides are applied to small weeds.

“If we continue to spray glyphosate, glyphosate, glyphosate, we’ll end up like Arkansas,” Dr. Daniel Stephenson, LSU AgCenter weed scientist, said, referring to a broad-spectrum herbicide frequently used for weed control throughout Louisiana.

Pigweed in Arkansas, Georgia, Mississippi and Tennessee has resistance, and Johnson grass in Louisiana may have resistance, Stephenson said. He said a Louisiana farmer applied glyphosate at the rate of 96 ounces per acre, and Johnson grass survived.

Stephenson said the herbicide Ignite is a possible option for controlling Johnson grass not controlled by glyphosate.

Palmer amaranth is not a widespread weed in Louisiana, Stephenson said, but it is a bad problem in Arkansas and Georgia, where it has developed herbicide resistance.

Farmers in Georgia are spending $50-$80 per acre to control the weed, he said, adding that pollen from Palmer amaranth can travel 1 mile, and one plant can produce 400,000 seeds.

Horseweed from Arkansas, where the weed has become resistant to glyphosate, is moving into northeast Louisiana, he said.

Louisiana farmers also could have a problem controlling ryegrass, said LSU AgCenter weed scientist Dr. Bill Williams.

“We’re on the cusp of being able to say we have glyphosate-resistant ryegrass,” Williams warned.

Williams said many fields in Louisiana had heavy weed infestations this year, making it difficult to tell what crop was being grown. “In some cases, I couldn’t tell there was a crop in a field,” he said.

Stephenson showed test plots of Roundup Ready adjacent to Liberty Link soybeans. He said Roundup, a glyphosate herbicide, is better-suited for controlling grasses and the herbicide Ignite for Liberty Link soybeans is superior on broadleaf weeds.

Stephenson said spraying should be done when weeds are 3 inches tall. Waiting to spray allows weeds to grow enough to be minimally affected by a herbicide, he said. “You’re going to get burned.”

Research conducted by the LSU AgCenter is extremely important, said Dr. Bill Richardson, LSU AgCenter chancellor.

Richardson said sweet potato research being done by the LSU AgCenter was a factor in the decision by the company ConAgra Foods Lamb Weston to make plans to build a sweet potato processing plant near Delhi. The plant could hire up to 500 people.

“Because of budget cuts, the number of research faculty at Dean Lee has been reduced, and scientists from other AgCenter research stations have conducted experiments at this site,” said Dr. David Boethel, LSU AgCenter vice chancellor for research. “We have obtained approval to hire an agronomist and an animal scientist at Dean Lee and hope to have the positions filled before next summer.”

Dr. Paul Coreil, LSU AgCenter vice chancellor for extension, said research and extension personnel work seamlessly. “As a taxpayer, I think that’s the best way to serve your needs,” he said.

Coreil said several county agents will be retiring, and replacing them will require approval by the LSU Board of Supervisors.

Several LSU AgCenter researchers reviewed the research they’re conducting at the research station.

J. Cheston Stevens, an LSU AgCenter soil specialist, said potassium deficiency has been common this year. “We saw potassium deficiency all across the state,” he said.

Stevens said sulfur deficiency stunts plants’ growth, and fertility problems often go undetected because soil testing is not conducted often or early enough.

Asian soybean rust has been detected throughout Louisiana, but it only has been found on plants that are too mature to be affected by the disease, said LSU AgCenter plant pathologist Dr. Boyd Padgett. “I’m not real concerned about rust.”

Padgett said other disease, such as aerial blight, pose a bigger threat, but Asian soybean rust could occur in earlier growth stages of soybeans with cooler weather.

Dr. Don Boquet, LSU AgCenter agronomist, said the U.S. Department of Agriculture has estimated cotton yields in Louisiana will be 2 bales per acre. But he said he believes that is too high. He said only 230,000 acres of cotton is being grown this year, but he expects an increase during the next two years. “Maybe cotton is on the way back.”

Dr. Donnie Miller, LSU AgCenter weed scientist, said cotton growers should consider applying defoliants when 60 percent of the bolls are open, but the material could be applied when they are open anywhere from 40-80 percent open. He said a brown ring around cotton seeds is a good indicator that the plant has matured sufficiently for defoliation.

Dr. Roger Leonard, LSU AgCenter entomologist, said farmers planning to grow wheat this winter should consider varieties with resistance to the Hessian fly. He said once the insects are found, no insecticide is available to control the pest. The insect can overwinter in an unplowed field, he said.

Bruce Schultz

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