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Develop Sound Weed Management Program

Cotton Farmers Urged To Get Back To The Basics In Controlling Palmer Amaranth

• By Amanda Huber,
Southeast Editor •

palmer pigweed

Resistant pigweed will continue to be a major priority in cotton production.

Cotton farmers have made monumental advancements in weed control and pesticide stewardship across Georgia during the past five years, according to University of Georgia Extension weed specialist Stanley Culpepper. Additionally, pesticide drift complaints have dropped 76% during that time.

“Although clear progress has been made managing pigweed, the pest remains resilient. In 2019, Palmer amaranth was confirmed to be resistant to herbicides including Reflex, Cobra and Ultra Blazer (PPO herbicides).”

Pigweed-Free Planting Zone

Culpepper says producers should review the basic principles of a sound weed management program. Step one is to not plant into fields with emerged pigweed.

“Planting into fields with emerged Palmer amaranth remains one of the greatest mistakes growers make. Valor, Direx, Gramoxone, 2,4-D and/or dicamba are among the most effective tools to prepare for planting into a Palmer-free field.”

Producers should apply burndown at least 14 days prior to planting. Then, scout fields a day or two prior to planting, and if Palmer has come up, control those before planting.

Preemergence Applications

The next step is to apply preemergence herbicides, which are the most effective tool to prevent resistance to postemergence herbicides.

Culpepper says, “Research conducted across Georgia during 2018/19 showed that an effective preemergence herbicide mix reduced the number of pigweed needing to be controlled by the first postemergence or topical spray by 99.8%.”

The second most effective approach is a rolled rye cover crop, reducing Palmer emergence by 75%.

“Growers should always mix two active ingredients effective on pigweed, applying them preemergence at rates that will not harm cotton.” He also says, removing the at-plant herbicide application is not a good decision and will lead to rapid resistance to topically applied herbicides.

Timeliness Is Critical

Even when at-plant herbicides perform well, pigweed has often emerged by 14 days after planting. The next step is to apply topical herbicides sequentially in a timely manner to control Palmer that is 3 inches or less.

“If the at-plant herbicide does not perform, the first topical application may need to occur around 10 days after planting. Otherwise, the first application is likely needed at 14 to 18 days.

“Liberty, 2,4-D or dicamba mixtures can all be effective, if timely, but these products will not consistently control Palmer larger than 3 inches,” Culpepper says. “Do not make more than two applications of these active ingredients during a cotton crop and follow all label restrictions.”

Final Directed Application

The final step of an effective weed management program is a directed or hooded herbicide application, which Culpepper says is needed in most fields.

“This application will improve weed control, reduce cotton injury and decrease selection pressure that creates resistance to topically applied herbicides. Layby applications with products like diuron, Valor, Cotoran, Envoke or other non-auxin/Liberty-type products are encouraged.”

Drought conditions, which are usually experienced during at least a portion of nearly every season, will affect weed management.

“Most weeds, especially Palmer amaranth, become more tolerant to topically applied herbicides under dry conditions.

Although it is not always possible to eliminate drought periods, herbicide applications made in the morning, when plants are less stressed, may be more effective,” Culpepper says.

Remember to follow label directions for drift.