Glyphosate resistance has become a hot topic in weed management. Vipan Kumar, Ph.D., associate professor and Extension weed management specialist in Cornell’s Soil & Crop Sciences Section, addressed the issue in his presentation, “Integrated Weed Management Strategies in New York Soybean Production” at the recent Soybean & Small Grains Congress hosted by CCE’s Northwest New York Dairy, Livestock & Field Crops Program.
“Palmer amaranth is a very genetically diverse species,” Kumar said.
It typically appears in early to mid-May – planting time for many growers. Palmer’s aggressive growth and competition with crops make it a troublesome weed that can cause up to 70% reduced harvest in soybean fields heavily infested with it. Palmer is also cross-pollinated and a prolific seed producer, growing more than 600,000 seeds per plant.
Kumar said Palmer has become resistant to 10 different herbicides in the U.S., including glyphosate. And glyphosate-resistant Palmer has been spreading.
In Kumar’s studies of controlling it, Roundup PowerMax 3 “barely did anything,” he said. The best results among the control methods tested were achieved with Liberty 280 applied at 32 oz./acre; Sharpen at 2 oz.; Cobra and Clarity 12.5 oz. and 16 oz., respectively; Cobra and Liberty 280 at 12.5 oz. and 32 oz.; Sharpen and Clarity at 2 oz. and 16 oz.; and Sharpen and Liberty at 2 oz. and 32 oz.
Tall waterhemp (also known as roughfruit amaranth) is a new weed species appearing at farms in New York. An annual pigweed species, Kumar reported glyphosate resistance among Empire State populations of the weed.
“Tall waterhemp … is similar in biology to Palmer amaranth,” Kumar said. “It has been in New York since 2014, when the first population was identified. There are 22 different counties where it has been identified. We tested some of the population for resistance to glyphosate.”
Researchers observed herbicide resistance in seven sites of action in a few selected populations of tall waterhemp. Kumar added that potential multiple resistance to ALS, HPPD and PS II inhibitors may occur. In testing interventions on tall waterhemp that has shown resistance, higher application of Clarity, Enlist, Liberty, Roundup, Synchrony, Atrazine, Callisto and Cobra still resulted in surviving plants.
“We are not just talking about glyphosate resistance but multiple herbicide resistance,” Kumar warned. “We are investigating other mechanisms.”
Vipan Kumar, Ph.D., an Extension weed management specialist in Cornell’s Soil & Crop Sciences Section, studies the effects of herbicides on resistant weeds. Photo by Deborah J. Sergeant
Some common cover crops also show the possibility of developing resistance to glyphosate, such as annual ryegrass.
“What can growers do to control annual ryegrass?” Kumar said. “The U.S. is number one in herbicide resistance. Why? We have so much selection pressure. Since 1975, we’ve been using glyphosate.”
Repeatedly using the same herbicide breeds resistance into the target plant.
“Another reason could be because of chemical-based weed management,” Kumar said.
In 2023 while studying pre- and post-herbicides for glyphosate-resistant waterhemp in Enlist soybeans, Kumar found that “we did not see high control with those pre- programs,” he said.
But in 2024, “pre- programs did a pretty good job in controlling glyphosate-resistant waterhemp.”
He added that although covers are grown for many reasons, “if you’re dealing with herbicide-resistant weeds, you’ll have another reason,” since covers can help in suppressing weeds.
“Cover crops with a pre- program can do wonders for you,” Kumar said.
He also said a post-application program can take weed suppression even further.
Other weed control methods like a tractor-mounted inter-row mower and weed electrocution of the weed seed bed can also help.
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