Not managing resistant pigweed hurts everyone

If you’re not managing glyphosate-resistant pigweed in your cotton fields, you’re not only causing problems for yourself, but you’re causing them for many other growers as well.

“This weed pest is spreading through pollen movement,” says Stanley Culpepper, University of Georgia Extension weed scientist. “If your neighbor is doing everything right and you’re not, you’re hurting yourself and him. Everyone needs to properly manage this pest.”

During the recent Sunbelt Expo Field Day, Culpepper said that 95 percent of the calls he was receiving at the time came from growers who had resistant Palmer amaranth pigweed that was 20 to 24 inches tall.

“If you call me or your county Extension agent, and you have a 2-inch Palmer amaranth, we might can help you,” he says. “But if you call when the Palmer amaranth is 20 to 24 inches high, get your shovel and start digging it up because there’s no way you can manage it with a pesticide.”

Over the past two years, the only relevant weed control issue in cotton has been glyphosate-resistant Palmer amaranth pigweed, says Culpepper. “In 2005, we confirmed resistance in three counties. In 2006, we found it in seven more counties, and in 2007, we found it in 10 more counties. This is a very serious pest, and it’s spreading rapidly. The only way to economically manage this pest is to start with a residual herbicide at planting or before planting.

“If you think you can run Roundup Ready programs and just put everything down at planting, then this pest will put you out of cotton production, without a doubt,” he says.

And nothing will change, he adds, at least for the next five to seven years. “Residual-based programs must be used throughout the season. We don’t even want to see a pigweed — that’s the only we can truly manage this pest economically,” he says.

Unfortunately, he adds, there are also growers using sound residual programs who are facing a late-season pigweed challenge.

“Most of these failures are in dryland fields where residual herbicides were not activated in a timely manner or in conservation-tillage fields where the residual herbicides did not reach the soil. Regardless, each of these growers faces some very difficult management challenges for the rest of 2008. Management options include herbicides, cultivation, and/or hand weeding,” says Culpepper.

Herbicide options are very limited, he says, and likely will control only pigweeds that are 4 inches or less in the row and 6 to 8 inches in the row middle when using hooded applications.

The most effective herbicide treatment for directed and hooded applications include the following, says Culpepper:

(1.) Diuron (Direx, others) or Layby Pro plus MSMA:

• Diuron rate: 1.6 to 2.4 pints per acre. See label for use on your soils and cotton must be 12 inches tall.

• Layby Pro rate: 2 pints per acre. Cotton must be at least 16 inches tall.

• MSMA rate: 2.5 pints of a 6.6-pound active ingredient per gallon or 2.67 pints of a 6-pound active ingredient per gallon.

• Add crop oil when applying Diuron or Layby Pro plus MSMA alone.

• The addition of Valor to this mixture will improve control. If adding Valor to this mixture one MUST USE SURFACTANT and NOT CROP OIL. Never apply Valor with a crop oil at layby.

• The addition of Aim or ET to Diuron or Layby Pro plus MSMA likely will improve control.

• Valor, Aim and ET should be applied only to cotton that is at least 16 inches tall having 3 inches of a “barky” stem. Spray should not contact higher than 2 inches up on the cotton.

• Apply in approximately 15 gallon of water per acre.

(2). Paraquat (38 fluid ounces of Gramoxone Inteon) plus Diuron (1.6 to 2.4 pints) plus crop oil.


• See label for use of Diuron on your soil.

• Spray or even spray drift cannot contact any part of the cotton or very serious injury will occur.

• Of course, this application will not control weeds in the cotton row.

• Apply in 15 to 20 gallon of water per acre.

Cultivation can be used to control small pigweeds in the row middle, says Culpepper. “Although most growers remember the days of cultivation, it is critical to remove the root from the soil while trying to avoid cultivating within 72 hours of a rainfall as the plant would re-root itself. It is also worth mentioning that the cultivation process will stimulate a new flush of weeds that will have to be managed in the next couple of weeks. Also, of course, cultivation does not provide control in the row where yield loss is the greatest,” he says.

Hand weeding Palmer amaranth can be very successful but is more complicated than once thought, he says. Growers who hire crews with machetes or hoes likely will spend a lot of money with minimal results.

“If a Palmer amaranth plant is ‘cut’ off above ground, it will re-grow and likely be very green and aggressively growing at the time of defoliation and harvest, not to mention seed production still will be enormous.

Palmer plant roots must be removed from the soil, he says. “However, the roots cannot be in contact with the soil as moisture on ground or rainfall that occurs within 72 hours will facilitate plant re-growth. Thus, the only effective way to actually hand weed Palmer amaranth is to pull the plant, including the roots, from the soil and either set the roots on top of the cotton or carry it from the field.”

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