If anybody is qualified to discuss how to properly deal with glyphosate resistant Palmer amaranth, it’s Georgia Extension Specialist Dr. Stanley Culpepper. The resistant pigweed epidemic began in Georgia, due in part to a homogenous reliance on the same technologies in weed control. After three years of suffering from the destructive pigweed, Culpepper and Georgia are still fighting an uphill battle.
“Back in 2005, we probably had 3,000 to 5,000 acres of agronomic land infested with glyphosate resistant Palmer amaranth. By the end of 2007, we’re looking at a minimum of 300,000 acres,” Culpepper says. Though Georgia’s growers have taken a hard financial hit because of the resistance epidemic, Culpepper credits their determination in fighting against the weeds. In order to keep other areas of the cotton belt from facing a similar struggle, he issues a stern warning.
“We’re in a boat that’s already set sail. We’ve got a disaster; we’re having to deal with it. You don’t necessarily have to get in that boat with us, but it’s up to you. If you’re going to ignore the situation, pretend the situation doesn’t exist, and continue to just spray Roundup, you will join us.”
Understandably, Culpepper advocates that growers study their chemistries in order to diversify and better manage resistance. In order to rotate chemistries effectively, growers must understand how they differ from each other. He does recommend specific herbicides.
“If you’re a cotton producer in our state, you need a residual herbicide at planting, early post and at layby. Residual, residual, residual is what you need, and we’ve got to pray for timely rain on the dryland production,” he says. “The three most effective residual herbicides for Georgia cotton producers include Reflex, Valor and Staple (where ALS-resistance is not present).”
Culpepper says that in Georgia, layby directed application mixes of diuron and MSMA, or diuron combined with MSMA and Valor were common in 2008. Hooded applications of Gramoxone plus diuron were also common. While late-season action is sometimes futile, Culpepper’s advice on managing escapes is simple: yank them out before seed production and seed rain occur.
Hand pulling is often the only option to remove escapes after layby, according to Culpepper, because once glyphosate becomes powerless against the plants, pigweeds larger than approximately 6 inches are too tough for other chemical options to handle.
Dr. Alan York, of North Carolina State University, also recommends residual herbicides as the primary weapon against resistance, though he says that preplant and preemergence is the time span when they are most effective. “If you miss it preemergence, you have missed it for the season,” York says. As for late-season escapes, both experts agree that the basic action of hand pulling pigweed is an effective way to handle the problem.
“If you think you don’t have a resistance problem, but you see one plant out there after applying Roundup, you’re a fool if you don’t go pull it,” York says.
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By 2007, glyphosate-resistant Palmer amaranth had infested around 300,000 acres in Georgia.