Cotton Links


How To Control Pigweed?
Use Residual Herbicides


It is no secret that glyphosate-resistant Palmer amaranth (pigweed) has become a tremendous agronomic challenge for Southeast and Mid-South cotton producers.

First confirmed in Georgia in 2005, it has now spread all across the Cotton Belt, making its presence felt from the Carolinas to the Delta.

“Glyphosate-resistant Palmer amaranth had an enormous impact on our yields this season,” says Ken Smith, Extension weed scientist at the University of Arkansas.

“When pigweeds go through the spindles on those cotton pickers, they make a lot of noise, and they are also costing our growers lots of money. In some cases, we had entire fields that were bush-hogged because they could not be harvested.”

In three years, glyphosate-resistant Palmer amaranth has built a well-deserved reputation as a production nightmare. Plants emerge throughout the season and can grow as much as two inches a day. Once they get about knee high, viable control options are all but non-existent.

With such a bleak picture, many producers may wonder what kind of success they can actually have against such a formidable pest.

Meeting The Challenge

The good news is that producers are learning how to overcome this problem, but it requires vigilance and a proactive approach toward weed management programs.

“We are seeing some real success managing this problem,” says Stanley Culpepper, associate professor and Extension agronomist at the University of Georgia.

“Many of our growers who put forth a good effort, and were blessed with timely rains to activate residual herbicides, had a pretty good year keeping resistant pigweed under control.

“The awareness is there, but there is a difference between being aware and having the willingness to do something,” says Culpepper.

“The people who have fought it since the beginning, adopted new practices and made the necessary changes in their weed management programs did well. Many of those growers who had the mindset that they weren’t going to have to face this problem and didn’t take precautions had a very difficult year.”

Producers Making Changes

Implementing residual herbicides into their weed management program is one of the key changes many producers have made that has led to success. These herbicides have become invaluable tools, in part, because they prevent pigweeds from emerging.

“Syngenta was involved in a collaborative effort with the University of Arkansas and University of Warwick in England to create a model that simulates how the use of residual herbicides in cotton can affect the evolution of glyphosate resistance in Palmer amaranth,” says Chuck Foresman, manager of weed resistance strategies for Syngenta.

“The model clearly showed that a residual herbicide is the way to slow down glyphosate resistance.”

Value Of Residuals

Reflex, a pre-emergence residual herbicide product, was used to control glyphosate-resistant Palmer amaranth in cotton, and the results were good.

“It has had a tremendous impact on managing pigweed,” says Culpepper. “A single herbicide isn’t going to do the trick; this problem requires a system approach. But Reflex has been a critical component in our system.”

Smith agrees.

“Our producers will be prepared to go in with a lot more residual herbicides next season,” he says.

Gibbs & Soell, which represents Syngenta, contributed information for this article.

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