Farm Progress

Glyphosate resistant weeds a reality for cotton growers

Roy Roberson 2

February 10, 2006

6 Min Read

Glyphosate resistance is a reality that should scare some cotton growers into changing the way they do business, says North Carolina State Weed Scientist and Researcher Alan York.

Speaking at the Beltwide Cotton Conference in San Antonio, Texas, York further stresses the potentially dire situation by explaining that in the past herbicide resistance was dealt with simply by switching to a herbicide with a different mode of action. The economic reality of the cotton market, he contends, has left no new herbicides with different modes of action in the pipeline.

Mode of action, York explains, is the biochemical process by which a chemical kills a plant. For cotton growers in the Southeast, resistance to ALS-inhibiting herbicides is of critical concern because of the widespread use of these materials on cotton and peanuts. In the Southeast, ALS-inhibitors have been widely used and successful in controlling cocklebur and pigweed.

“Having no new modes of action in the pipeline means that the next time we have widespread resistance problems, we may not have the magical silver bullet to fix it,” the North Carolina scientists explains.

Herbicide resistance is not a new problem. York points out that there are already 107 biotypes of weeds within 63 weed species that have shown resistance in the U.S.

York says, “we began seeing resistance to MSMA by cocklebur in the Cotton Belt several years ago. Now, we are seeing goosegrass, johnsongrass and Palmer amaranth weeds with resistance to dinitroanalin (DNA) products, such as Prowl and Treflan. And, we have johnsongrass with resistance to Poast and Fussilade,” he explains.

Glyphosate resistance is by far the biggest concern for cotton growers in the Southeast. Among the 13 states reporting resistance to glyphosate, six are cotton producing states. With the high percentage of Roundup Ready cotton grown in the Southeast, resistance to glyphosate has the potential of being a catastrophic problem.

Horseweed resistance to glyphosate has been widely reported in the Cotton Belt. This particular weed has very small seed that makes it very adaptable to spread over a wide area in a short period of time.

York showed the audience a map of a cotton-producing area in Tennessee where glyphosate resistant horseweed spread over a large area in three years (2001-2003). “Horseweed is a two-edged sword, York explains — on the one hand, it is easy to control, but on the other you have to know you have the problem before you plant, because your options are very limited once cotton plants emerge.

In the summer of 2005, University of Georgia researchers found glyphosate resistant Palmer amaranth, commonly called pigweed. In the Southeast, York contends, this discovery took herbicide resistance to a much higher level of concern.

The North Carolina scientist describes Palmer amaranth as a “big, ugly pigweed’, noting that it can grow an inch a day in ideal conditions and in a short period of time can become the dominant plant in a field.

In tests at the University of Georgia, a 4X rate of glyphosate, applied three times had little negative affect on this species of pigweed. “If you grow cotton in the Southeast, and you have Palmer amaranth in your fields, looking at side-by-side comparisons of resistant and non-resistant pigweed should scare you to death,” York stresses.

This particular pigweed has separate male and female flowers and tests indicate resistant males can cross pollinate with non-resistant females to produce resistant seeds, meaning Palmer amaranth has the potential to spread quickly, York explains.

“We know we have glyphosate resistant Palmer amaranth in Georgia, and I strongly suspect we have it in North and South Carolina, York says.

Capron, Va., cotton grower Cliff Fox, who farms in Southeast Virginia says this pigweed is a problem on his farm, but so far they have not seen any indication of resistance to glyphosate.

“We have always been too complacent about herbicide resistance, and we became even more complacent with the onset of Roundup Ready cotton. We were told weeds weren’t likely to develop resistance to glyphosate, but deep down some of us didn’t believe it. Now, glyphosate resistance is a reality, bringing our fears to the forefront,” York says.

For resistance to start, there has to be a plant somewhere in the population that has resistance to a particular herbicide. In a typical plant population, it is expected less than one plant per million would be resistant, York says. By putting pressure on non-resistant plants with a target herbicide, resistant plants are left to produce seed. If continued pressure is exerted on the non-resistant population, the resistant population continues to increase, the North Carolina scientist explains.

Growers have no way to know when a resistant plant occurs in a field. However, farmers can reduce the chances of that resistant weed causing problems by rotating herbicides with different modes of action, along with other cultural and management practices.

“The key to a good herbicide resistance program is to more wisely use the materials we have,” York urges. He stresses that growers should know the mode of action of all herbicides used on cotton and on rotation crops and use this knowledge in planning weed control programs. He says, “for example, farmers using ALS materials on cotton should not use these herbicides on peanuts grown in rotation with cotton.”

“Ideally, growers should want multiple modes of action, using sequential pre- and post-applications and tank-mixing,” York says.

In Georgia and the Carolinas, the herbicide resistance program recommended is geared toward Palmer amaranth, because this pigweed is such a potential problem. Growers are recommended to use a residual herbicide, such as Cotoran, MSMA or Treflan, followed by Dual mixed with glyphosate at post-emergence, followed by a lay-by treatment, such as Caparol, Direx, Suspend or Valor plus glyphosate. Using at least two of these treatments will provide at least two different modes of action, York points out.

Why Staple is not a part of this recommendation is a good question, according to York. The answer is clear, Staple provides excellent control of Palmer amaranth, which provides selective pressure on this pigweed, which could lead to more rapid resistance problems, if resistant plants are in the growers field.

It is much easier to deal with isolated incidences of herbicide resistance than on a wide scale, as was the case with horseweed resistance in Tennessee. For this reason, York urges growers to look for resistance problems. Such things as failure by a known herbicide to control weeds it has controlled in the past, control and lack of control of the same weed by the same herbicide in the same field, and getting good control by a herbicide on other weeds, but not the target weed are good indicators of resistance problems.

York concludes that herbicide resistance is not the end of the world for cotton growers, but that it does have the potential to become an extensive problem.

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