Superweed in New York State Found Resistant to Widely Used Herbicide

By Caitlin Hayes

It can grow an inch or two a day, produce nearly a million seeds and emerge at almost any point in the growing season. Waterhemp is among the most destructive weeds U.S. growers contend with, and new research has confirmed that populations in New York state are resistant to one of the most common herbicides.

In the study, published Jan. 13 in Weed Technology, Cornell researchers found that waterhemp samples from Seneca County soybean fields were five to 12 times more resistant to glyphosate than controls. They also found classes of herbicides that are effective in controlling the weed, as well as evidence of resistance to additional herbicides.

"Having the confirmation of this resistance in New York gives us the opportunity to spread the message that we should use these chemistries more judiciously and diversify our weed control practices," said first author Vipan Kumar, associate professor of weed science in the School of Integrative Plant Science, in the College of Agriculture and Life Sciences (CALS).

"Along with Palmer amaranth, waterhemp is one of the worst species in the country right now, and we already have reports from midwestern and southern states of resistance to many different herbicides. We need to educate folks and get ready in terms of management."

Widely documented in other states, the research confirms glyphosate-resistant waterhemp for the first time in New York state, with potentially significant consequences for growers. Kumar said 75% to 80% of soybean farmers in the state use glyphosate as their primary method of weed control.

Waterhemp was discovered in New York state in 2014 and has spread quickly to 23 counties. Dubbed a "superweed" for its ability to proliferate and evolve resistance, the weed threatens soybean and corn crops especially, with 43% reduction in soybean and up to 74% reduction in corn yields reported in other states.

With glyphosate resistance confirmed, Kumar and his team are now spreading the message to growers at annual meetings, through publications and newsletters, and via individual consultations. "We're trying to let everyone know that we need to look beyond glyphosate," he said.

The researchers found evidence of resistance to three other herbicide chemistries—mesotrione, atrazine, and chlorimuron plus thifensulfuron. Additional herbicides—dicamba, glufosinate, lactofen and 2, 4-Dichlorophenoxyacetic acid, applied alone or in combination with others—were effective in killing the weed.

While swapping herbicides can help growers manage the weed in the short term, Kumar said waterhemp's ability to evolve multiple resistance calls for a more integrated weed management strategy over the long term.

"In my education and outreach, I'm suggesting to producers that they integrate multiple, different methods of weed control, so that we're not only relying on herbicides," Kumar said. "That will extend the viability of the herbicides we do have."

Planting cover crops and introducing more competitive hybrids or varieties can give the advantage back to crops, Kumar said, and vigilance about cleaning equipment between fields can limit the spread of the seeds.

The team is investigating strategies new to the U.S., such as pulverizing waterhemp seeds in the soil post-harvest. They're also tracking the resistance of samples from around the state.

"New York is unique in the sense that we have fewer herbicides label registered than some other states," Kumar said. "To sustain the options we have, we need to work outside the box."