Grant no mercy to suspected herbicide-resistant weeds. If it’s alive after harvest or remains green in-season following prescribed herbicide applications, do whatever is necessary to remove the offensive pest from the field.
It’s a numbers game, says Jason Bond, Mississippi State University weed scientist out of the Delta Research and Extension Center in Stoneville. If an average of 1.5 female Palmer amaranth plants escape control in a field (in a non-competitive environment), they can produce up to 2 million seed in the first year. In year two, if a producer accomplishes 99.9 percent weed control, as many as 1,000 plants may survive. If just 400 of those are female, they may produce as many as 160 million seed.
“We can’t count the number of seed that could survive in year three,” he said.
Bond, who has had ample experience with glyphosate-resistant weeds, especially Palmer amaranth or pigweed, in Mississippi cotton and soybeans fields, told a group of Texas farmers that if they let the weeds get a head start they’ll soon see more weed in their fields than crop.
“Herbicides do not create resistant weeds,” Bond said at the annual Ag Technology Conference in Commerce, Texas. “We create resistant weeds through selection pressure.”
Using the same herbicide or the same mode of action over and over allows weeds that are resistant to that product to survive while the weeds susceptible to the material die. In several years, a field is infested with weeds that can no longer be controlled by a particular herbicide.
“The resistance is already present in the weed population,” Bond said. “Abusing the chemistry may create significant problems within three years.”
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.