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Palmer resistance to herbicides means cover, cutting needed too


JACKSON, Tenn. — Palmer amaranth, commonly known as pigweed, is currently considered the most troublesome and costly weed in U.S. soybean production. It has developed resistance to the widely used glyphosate and more recently, to a group of herbicides known as protoporphyrinogen oxidase (PPO) inhibitors.

Writing on that topic, Drake Copeland, a graduate student at the University of Tennessee Institute of Agriculture, won first place for a poster by a Ph.D. candidate at the 2018 Weed Science Society of America (WSSA) meeting. And although Palmer is not as widespread in Illinois, researchers there are warning farmers against apathy concerning this fast-spreading menace.

Copeland’s poster was titled “Influence of Residual Application Rate on PPO-Resistant and Susceptible Palmer Amaranth in Tennessee.” The initial findings of his study suggest Palmer, an invasive species from the Sonoran Desert area, could be exhibiting greater tolerance and resistance to even more herbicide modes of action.

He is studying under the direction of Larry Steckel Ph.D., professor and extension weed specialist with UT’s Plant Sciences Department. His studies show that pigweed resistance is continuing to evolve.

“About 83 percent of the agricultural fields in West Tennessee have developed this resistance,” Steckel said. “The main issue is you don’t realize you have it until it is already there. With the PPO resistance, they confirmed it in 2015, but it was probably there three or four years before that.

“The issue is not being able to know when you have resistance because it starts out at a small level and you don’t realize it until it is a huge problem,” said Copeland. “Within that time it spreads and takes over. For soybean and cotton growers it is a huge problem; it is in all of West Tennessee.”

One solution for farmers might be integrating herbicides and cultural practices, Steckel explained.

“I’ve said many times that the long-term solution to resistant Palmer amaranth cannot just be poured out of a jug,” he added. “We have proven that where we can integrate the herbicide with the cover crop, that has been the most effective method in controlling these resistant weeds, particularly Palmer amaranth.”

Palmer needs three things to germinate: heat, water and light, Steckel said. Cover crops can act as mulch, keeping out the light, thus preventing and slowing germination of the noxious weed.

“If you keep it in the dark with cover crops or narrow row widths, we can cut the ones that come up by 50 percent, and the ones that do come up are delayed. Half of the struggle is over.”

If farmers terminate their cover during or even after planting, it will delay the weed’s emergence, Steckel said. Probably 30-40 percent of Tennessee’s agricultural acres now have a cover crop.

Copeland’s poster was co-authored by Steckel and Matthew Wiggins of FMC Corp. The WSSA is a nonprofit society founded in 1956 to encourage and promote the development of knowledge concerning weeds and their impact on the environment.

Meanwhile, up north at the University of Illinois, Aaron Hager, Ph.D., associate professor in the Department of Crop Sciences, and his colleagues have been advising farmers not to underestimate this aggressive weed.

“According to research done in Arkansas, only three years after the introduction of Palmer amaranth in a cotton field, you cannot farm that cotton field if you did not do anything to preclude that seed production,” he said. “We have a zero-tolerance threshold for Palmer amaranth. We’ve never had that before for any other weed species before.”

Hager agreed with Steckel that herbicides are essential for the management of Palmer, but are not enough on their own. He suggested a cultivator, even a hoe, or chopping the weeds out by hand.

“Just to think you can spray your way to a clean field at the end of the year doesn’t always play out because of how much seed the female plants can make,” Hager explained.

The most devastating thing about Palmer is apathy, he said. People don’t think a few scattered plants here and there are important – but 50 female plants are capable of scattering hundreds of thousands of seeds.

“Then, at the end of the year you run this marvelous weed seed-spreading machine called a combine through those females, and you will spread that seed from one side of your field to the next,” he pointed out.