What is in this article?:
- Total removal is goal for resistant weed population
- Few herbicides
- Herbicide resistant weeds are prolific seed producers.
- Abusing the chemistry may create significant problems within three years.
- Rotating mode of action is a critical management tool.
Tom Eubank, Mississippi State University; Shannon Morsello, Syngenta; and Jason Boyd, Mississippi State University, pose following a panel discussion on herbicide resistant weed control at the annual Ag Technology conference in Commerce, Texas.,
The problem is worse because of limited availability of new herbicides. “The last new cotton herbicide was released more than 10 years ago,” he said. “We are in a dire situation for finding new chemistry to control weeds.” ALS herbicides “are very prone to resistance.”
He also noted that at least 24 weed species have been identified as resistant to glyphosate. “Within four or five years, we could see 30 or more weed species resistant to glyphosate. Palmer amaranth is the one we deal with the most in the Delta,” he said. “It’s the perfect weed, a desert annual plant that will grow in water in Mississippi. It has been seen in Michigan, so we know it’s adaptable.”
A farmer’s weed control program has a significant effect on the probability of developing herbicide-resistant weeds. “For example, using two or more herbicide modes of action equals low possibility for resistance. Using just one mode of action means a high probability.”
Using multiple means of weed control—chemical, cultural and rotation—results in a low probability of resistance. Doing the same thing and planting the same crop year after year makes the probability of resistance high.
The key is never letting resistant weeds get established. “Start clean at planting,” he said. “Overlay a residual herbicide. That’s your first line of defense. Focus on the soil seedbank. That’s Palmer amaranth’s biggest asset.”
A combination of preplant, pre-emergence and post-emergence herbicide treatments is crucial. “Don’t let the weed come out of the ground.”
Scouting early to identify escapes is also important. “Identify weed escapes while they are small. Control depends on size of the weed. Apply the appropriate product and then identify performance issues as early as possible.”
Producers should follow labeled use rates and to assure adequate coverage. Spray volume and crop interception of the herbicide will increase success rate.
“Rotate the mode of action. The Weed Science Society of America now identifies the mode of action for each herbicide,” he said.
Bond recommends farmers at last consider control methods other than chemical. “Cultural practices (such as cultivation) may be difficult for larger farms but they can alter row width and other practices.”
Preventing seed production is key, he said. “The goal is to remove all non-controlled weeds. An economic threshold does not apply for herbicide-resistant weeds. Remove every one. Rapid viable seed production is typical.”
He also recommends steps to prevent seed movement within fields and across farms. That includes cleaning equipment such as harvesters, tillage tools, and sprayers. Farmers should try not to move weed seed in gin trash or in the field with irrigation, especially furrow irrigation.
Post-harvest management is also critical. Bond said any plants that escaped all control efforts should be removed from the field. Burning is a good option. “Post-harvest management that encourages seed production means everything you did during the summer to prevent resistance will go out the window.”
Palmer amaranth originated in the Sonoran desert of Mexico but has adjusted well to varied climates across the United States. It has also adjusted to the over-reliance on one herbicide or one mode of action.
“Try different options and not the same thing all the time,” he said.