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Herbicide resistance haunts growers

With mounting concern, weed scientists throughout the region — and the world — are cataloguing a growing list of weeds that have developed resistance to glyphosate, which comprises the cornerstone of crop planting systems throughout the Southeast.

The bugbear of weed resistance — the one that keeps weed scientists awake at night — is a type of pigweed known as Palmer amaranth. One biotype of this pigweed has shown resistance to glyphosate, while others have developed resistance to ALS herbicides, a class of chemicals comprising several key herbicide products.

The good news for Alabama growers is that the weed is still confined to southern Georgia. The bad news is that if it ever makes it into Alabama, the results could be catastrophic. And there is every indication it inevitably will get into the state.

Mike Patterson, an Alabama Cooperative Extension System weed scientist and Auburn University professor of agronomy and soils, describes Palmer amaranth as the mother of all monkey wrenches. If these resistant biotypes gain a firm toehold in Alabama, they potentially could undermine the crop system that cotton and soybean growers have depended on for years to control weeds and save on operating costs — one that combined use of glyphosate-resistant plants with limited-tillage systems to reap huge savings in chemical and equipment costs.

“For a lot of Alabama growers, their whole system is based on reduced-tillage and the use of postemergence herbicides, such as Roundup and the ALS herbicides,” Patterson says. “If weed resistance forces them to abandon these products and return to some of the older soil-based herbicides, they may to have to go back to some of the older tillage systems, too.”

That could deal a major blow to many Alabama growers who are still reeling from last year's drought.

“I think a lot of our growers, especially because of last year's drought, are standing on the edge economically,” Patterson says, “and if they can't produce the crop economically anymore due to this increased weed resistance problem, they could be put out of business.”

At least for now, there is not much Palmer amaranth to speak of in Alabama, and, as far as Patterson is aware, there are none of the biotypes causing farmers in neighboring Georgia such fits. Fortunately for Alabama growers, these non-resistant pigweed biotypes are easily controlled by Roundup.

Moreover, Patterson believes that the threat from these resistant biotypes will not necessitate a wholesale abandonment of Roundup Ready technology, which is, after all, effective on a broad range of weeds — roughly one hundred different species in all.

“I don't believe we're going to stop using Roundup, but we're going to have to supplement it with soil-applied products to try and slow down pigweed until our crop reaches an adequate height.”

It's a strategy that has been employed with cotton in the past.

“If you have a big cotton plant and a small weed, you can post-direct your spraying underneath it with a non-selective herbicide and stay ahead of the weed,” he says.

Even so, Patterson says the threat of pigweed should not be discounted or minimized.

“If the pigweed is a resistant biotype, and it comes up with the plant, our options are limited. It is a nightmare situation, and if it turns up, we'll be virtually powerless to deal with it.”

Weed resistance has become a pressing problem not only in the Southeast but also worldwide, Patterson says. Scientists already have documented 313 resistant biotypes of weeds and 183 different species worldwide (110 broadleaf species and 73 grasses).

Alabama is not immune. Researchers already have identified three such species in the state, including marestail, a winter annual predominant in the Tennessee Valley and resistant to glyphosate; common cocklebur, which has developed resistance to the MSMA and DSMA classes of herbicides; and goosegrass, found in Cherokee county and resistant to Prowl and Trifluralin, which are used to control small-seeded broadleaf species such as pigweed.

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