Managing herbicide resistant weeds
Effective weed control strategies can help farmers facing invasive weeds that are resistant to conventional application of glyphosate. Vipan Kumar, Ph.D., Extension weed management specialist for Cornell’s College of Agriculture & Life Sciences, presented on the topic at the annual Corn Congress.
“Palmer amaranth is the number one challenging species with glyphosate resistance in these counties,” Kumar said, indicating fields growing soybeans, alfalfa and corn in 2024 in Ontario, Yates, Wayne, Genesee, Orange and Steuben counties. “I am always looking for an operator who has these herbicide resistant weeds.”
His latest information indicates that glyphosate- and atrazine-resistant Palmer amaranth is becoming widespread in affected counties and doesn’t appear to be slowing down.
His research indicated that with application of HPPD inhibitors Callisto and Impact, “we saw pretty high level of resistance.”
The results of trials of post-emergence herbicide for multiple herbicide resistant Palmer amaranth control have led Kumar to believe that in New York fields, increased resistance is developing “and we suspect resistance to group two,” he said.
Waterhemp (aka roughfruit amaranth), a new pigweed species, is also spreading quickly in New York, making its way across 25 counties so far. Nearly all of its appearances are in soybeans, but in four of the reported sightings, the field was planted to corn.
In greenhouse trials, his research team also tested herbicides at different rates.
“More than half the population showed survival with multiple herbicide applications – 70% to 100% survival with glyphosate,” he said. “Only a few populations showed gene mechanisms. Is it fast metabolism or poor absorption? We are still investigating.”
His team is also researching glyphosate dose response.
“Synchrony resistance is widely spread in the Midwest,” Kumar said. “Callisto is also showing resistance. We’re also looking at pre-emergence application.”
Out of his team’s 23 tested populations, nine exhibited four-way resistance of glyphosate, mesotrione, chlorimuron/thifensulfuron and atrazine.
“Whole plant dose responses confirm the first report of multiple herbicide resistant waterhemp populations,” Kumar said. Applications of dicamba, lactofen and glufosinate “provided effective control of multiple herbicide resistant populations.”
In field trials in Walworth, Wayne County and Albany for the past two seasons, Kumar’s team applied 10 pre-emergence and 15 post-emergence herbicides. The control areas were infested with waterhemp.
“Heavy rainfall affected pre-emergence application,” Kumar noted.
Acuron™ proved the most effective with Dual II Magnum the least effective.
Italian ryegrass is another problematic weed on Kumar’s radar. It’s shown resistance to glyphosate. In trials of post-emergence herbicide for glyphosate resistant Italian ryegrass, “most worked well, especially nicosulfuron,” Kumar said.
Researchers looked at clethodim, glufosinate, paraquat, pinoxaden and quizalofop.
For use as burndown herbicides for Italian ryegrass control, Kumar’s team looked at Liberty and Gramoxone, the “two most effective burndowns with ryegrass fully headed,” Kumar said. He plans to repeat trials in 2026.
Kumar also presented results from testing herbicide-resistant horseweed in a greenhouse, which is also showing multiple herbicide resistance.
The highest response was with Liberty Ultra, Sharpen and Gramoxone, which he said are “pretty good at controlling glyphosate-resistant horseweed.”
One of the ways farmers can better control resistant weeds is integrating cover crops and herbicides in corn. Kumar said on-farm research in 2025 attempted early May termination and planting green about two weeks after.
“If you delay termination, you have better luck in suppressing biomass,” Kumar said.
The team applied a variety of herbicides at each termination pre-burndown, including Roundup PM; Roundup and Dual I Magnum; Roundup and Bicep Lite II Magnum ; Roundup, Dual II Magnum and Callisto. Those tried pre-burndown to mid-post included Roundup and Dual II Magnum, followed by Clarity; Roundup and Bicep Lite II Magnum, followed by Clarity; and Roundup and Dual II Magnum plus Callisto followed by Clarity.
Researchers planted Roundup ready corn. The cover crop mix included Austrian winter peas, crimson clover, purple top turnip, radish, sunflower and volunteer wheat. Early termination was April 30, 2025 and planting green took place May 20, 2025.
All applications proved helpful with the exception of Roundup alone (the highest weed biomass by far) – Roundup and Dual II Magnum; Roundup and Dual II Magnum followed by Clarity; and Roundup, Dual Magnum II and Callisto.
Three months after planting, the group with the highest weed biomass in the corn was the early termination Roundup, followed by early termination Dual II Magnum and Clarity; and then early termination Roundup and Dual II Magnum.
As for planting green, the treatments suppressed weed biomass overall better than the early termination plots.
One of Kumar’s graduate student’s projects is researching electrocution for control of Palmer amaranth. They’re “trying to see if zapping them in the heading stage would reduce the production,” he said.
It didn’t seem effective; however, the project has shifted to see if seeds are viable after electrocution.
Kumar is also studying the use of a seed impact mill in a combine. The researchers were able to grow Palmer amaranth from ground seed in the chaff; however, it reduced the number of plants.
