By Clint Thompson
Georgia’s cotton farmers continue to be consumed with weed management issues, specifically with Palmer amaranth. University of Georgia Cooperative Extension weed scientist Stanley Culpepper said during the University of Georgia Cotton/Peanut Research Field Day that grower’s No. 1 priority is resistance management in Palmer amaranth, also known as “pigweed.”
“Palmer amaranth, last year, based on 1,737 growers filling out a survey, noted it was the most problematic pest in all of Georgia cotton, by far and away. What we’re seeing unfortunately is the trend for this plant to become resistant to just about every class of herbicide chemistry that we have,” Culpepper said. “In Georgia, obviously, we have resistance to roundup. We also have resistance to the ALS chemistry, for example, STAPLE®. We also have resistance to Atrazine. We also have resistances to your yellow herbicides; for example, your Treflan or your Prowl.”
This year there was confirmed resistance to the PPO herbicides: Cobra, Reflex, Blazer. A 10-x rate against one-inch pigweed in the greenhouse and they still could not control the weed, he said.
It is essential that growers make sound management programs, which means don’t rely too much on one class of chemistry post emergence. Do not overdo your Liberty, Dicamba or 2-4,D.
“Everybody thinks resistance is complicated, it’s really not. It’s simple math. The more plants you try to kill with the same class of chemistry, the more likely you’re going to get resistance, the more quickly you’re going to get resistance,” Culpepper said.
Culpepper encourages producers to practice a systems approach to avoid further resistance issues. These tactics include pre-emergence herbicides, conservation tillage, including a roll rye as a cover crop; and lay-by with a hooded sprayer. In one of Culpepper’s research projects, the use of pre-emergence herbicides reduced weeds that needed to be killed post-emergence by 95%.
“Sustainable programs are the key to our program,” Culpepper said. “It’s important that we have a systems approach. If we have a systems approach, we have very effective programs and systems that are sustainable long-term.”
If not managed correctly, Palmer amaranth can quickly overtake a field. It can reach heights of 7 to 10 feet, rob cotton plants of needed water and nutrients and snag cotton equipment moving through a field. Palmer amaranth can also reproduce at alarmingly high rates. A female plant can produce approximately 400,000 seeds in dryland production. In irrigated fields, seed production can exceed 750,000 per female plant.