Battling weeds in corn crops can be a never-ending fight as new weeds encroach in the Northeast and an increasing number of weed species become resistant to the most commonly used herbicides.

Vipan Kumar, Ph.D., weed scientist and professor for Cornell University in the Soil & Crop Sciences Section, presented “Emerging Weed Control Challenges and Path Forward for Corn Production in New York” at the recent 2024 Corn Congress, hosted by the Cornell Cooperative Extension Northwest NY Dairy, Livestock & Field Crops Program.

Kumar has worked for Cornell for the past year after working in a similar role in the Midwest. A year’s worth of research and farm visits offered him a good picture of the current weed issues in the Northeast.

He listed as problematic annuals in corn common lambsquarters, horseweed, common ragweed, redroot pigweed, giant foxtail, Palmer amaranth and green foxtail – especially the last four. As for challenging perennials, field bindweed, hemp dogbane, horsenettle, Canada thistle, yellow nutsedge and quack grass top his list.

Kumar said that Palmer amaranth is both a recent newcomer and a weed to take seriously. “It can reduce corn yield by up to 70% if widespread,” he warned. A single female plant can produce millions of seeds.

Another weed species new to the region is tall waterhemp, aka common waterhemp. It has a similar biology as Palmer amaranth. Both weeds have spread to several counties, mostly in central and western New York.

Kochia (tumbleweed) represents yet another newcomer to the Northeast, as well as Johnson grass. The latter is an invasive and herbicide-resistant weed. With potential changes in atrazine use (the label is under review with the EPA), “there could be changes in use and rate. We need to be proactive,” Kumar said.

As weeds become resistant, it becomes even more important to use herbicides carefully. In Kumar’s research with resistant waterhemp populations, he evaluated Roundup at 64 oz./acre, Cobra at 20 oz./acre and Callisto at 6 oz./acre. The glyphosate plot appeared unaffected after treatment. The Callisto plot showed better success, with nearly all the waterhemp exhibiting severe damage. In the Cobra plot, “one population survived and was able to produce seeds,” Kumar said.

He also assessed glyphosate-resistant annual ryegrass cover crop from Western New York. Those plants survived 64 oz. of Roundup – a testament to the nature of these resistant plants.

This summer, Kumar plans to collect a larger variety of weed seeds to look at their response to herbicides.

One field study last summer illustrated that pre-mixes showed “overall pretty good control” for fields with low to moderate populations of giant foxtail, common lambsquarter and common ragweed in corn. These mixes included:

  • Resicore XL + Aatrex
  • Resicore XL + Aatrex + Roundup
  • Resicore XL fb Kyro + Aatrex + Roundup
  • Surestart II fb Kyro + Aatrex + Roundup
  • Keystone NXT fb Kyro + Aatrex + Roundup
  • Resicore XL fb Kyro + Aatrex + Accent

Even 35 days post-application, “all the treated areas looked pretty good,” Kumar said.

He saw similar results when comparing herbicide programs with and without atrazine against lambsquarter.

In addition to using the appropriate herbicides as directed, Kumar said that farmers would benefit from using precision ag tools.

“Taking crop and weed biomass measurements to add data to an AI database can help detect issues,” he said. “We have started looking at other means for weed control, working with Getting Rid of Weeds.”

Getting Rid of Weeds (or GROW) is a scientist-led network that coordinates research to help farmers with herbicide resistance issues. Kumar is one of the Mid-Atlantic representatives of GROW, which seeks to help growers use numerous means for weed control, like cultural, mechanical and chemical methods.

Another method Kumar mentioned is the Redekop Seed Control Unit, which uses two impact mills to pulverize seeds during harvest.

“In Kansas, it eliminated more than 90% of Palmer seeds in sorghum,” Kumar said. “I’m pretty sure we can do it in corn as well.”

He also promotes using cover crops as a system approach for controlling weeds.

by Deborah Jeanne Sergeant