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• Palmer amaranth, also known as pigweed, is the most commonly discussed or just plain cussed weed across the Sunbelt, but a host of others, nearly 100 separate species, show resistance to one or more classes of herbicides.
Shane Osborne, agronomist at the Oklahoma State University Research and Extension Center in Altus, said during the recent No-till Oklahoma Conference in Norman that folks harbor a lot of misconceptions about herbicide weed resistance but the problem is getting worse.
“Not all weed control failures occur because of resistant weeds,” Osborne said, “and there is a difference between resistance and tolerance. Resistance is an acquired ability of a plant or population to survive an herbicide application. This trait passes from one generation to another.
“Tolerance is an inherent ability of a species to survive and reproduce following an herbicide application, but there is no change over time.”
Some weeds, for instance, are more susceptible to some herbicides than they are to others.
Horseweed has shown some herbicide resistance in Oklahoma for some time, he said. “And we heard questions about potential Palmer amaranth resistance in adjacent regions last year.”
He recommends farmers order herbicide early to make certain they get what they need for the target weeds they’ve identified in their fields. “Make each application count and don’t sacrifice accuracy for speed.”
Ken Metcalf, an independent crop consultant in the Oklahoma Panhandle, also discussed resistant weed issues at the No-till Oklahoma Conference. Northern Oklahoma farmers may have a few different weed problems than farmers in the southern part of the state, he said. Knowing the specific weed population is the first step in developing a control strategy.
The resistant weed populations may differ somewhat across the Sunbelt and herbicide selection will vary according to the weed species, the crop being grown and the management system in place. But the basic tenets of herbicide resistant weed management apply whether the problem occurs in Wilson, N.C., Jackson, Tenn., or Apache, Okla.
The most common denominators include:
• Change to a different mode of action;
• Don’t rely on a single mode of action all season;
• Add residual herbicides to the weed control program, including pre-plant incorporated and pre-emerge herbicides;
• Use full label rates;
• Target small weeds;
• Start clean in the spring, using a burndown and then residuals to take pressure off post emergence products.
Herbicide resistance is also beginning to make inroads in the western states. Weed resistance to an herbicide recently showed its ugly head for the first time in Arizona. Greenhouse assays last fall conducted by University of Arizona weed scientist Bill McCloskey confirmed Palmer amaranth resistance to glyphosate in cotton.
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The resistance was found in an 80-acre field rotated with wheat in the Buckeye area. When McCloskey first visited the field, a severe Palmer amaranth infestation had outlasted several applications of glyphosate.
“I am not surprised that glyphosate resistance was found in Arizona,” McCloskey said. “It was only a matter of when.”
Glyphosate is the predominant weed-management strategy used by Arizona cotton growers year after year.
McCloskey expected to find herbicide resistance first in tree crops since glyphosate can be applied up to eight times annually for weed control.