Why Won’t Herbicides Alone Stop Waterhemp?

Waterhemp continues to defy control across the Midwest. Once resistant to herbicides from only a few groups, populations in Illinois now shrug off six different sites of action, and more may be coming.

"It's only gotten worse," said Aaron Hager, a University of Illinois Extension weed scientist. "We're now at six, and that number will increase soon."

Even more troubling, researchers are documenting metabolic resistance, meaning weeds are evolving the ability to break down herbicides before they can work.

"These waterhemp populations are now as efficient, sometimes even more efficient, at metabolizing products like S-metolachlor as a corn plant," Hager said. "That's a scary thought."

What does metabolic resistance mean? Traditional "target-site" resistance occurs when a herbicide can no longer bind to its target protein. Metabolic resistance is far less predictable.

"When you're dealing predominantly with metabolic resistance, you have no predictability at all," Hager explained. "A herbicide that worked last year might fail this year, even if you've never sprayed it on that field before."

Few Herbicides Still Work Reliably

In soybeans, glufosinate (Liberty) remains the last widely effective postemergence option, but its future isn't guaranteed.

"Technically, it's the only post- product we haven't documented resistance to," Hager said. "But I'm sure it exists somewhere. We just haven't walked that field yet."

Residual herbicides still play a key role, but even they are losing longevity. "What once gave six weeks of control may now only give three," Hager said. "In another decade, we may be lucky to get one or two weeks of residual activity."

New Chemistry Will Not Save Us

Several new actives, including diflufenican, which Bayer plans to launch in time for the 2026 season in U.S. row crops, offer some promise. Hager cautioned against viewing them as silver bullets.

"Every time a brand-new jug hits the market, everyone uses it," he said. "That creates enormous selection pressure from year 1. If we do what we've always done, the effective lifespan of that chemistry will be dramatically reduced."

"We've opened different jugs for 30 years and still have waterhemp," Hager said. "Chemistry alone is not the solution."

Stacked-trait soybeans, such as Enlist E3 and XtendFlex, have given farmers more flexibility, but Hager said they also reinforce a "just spray it" mentality.

"The more herbicide options you have, the more likely you are to have a spray-only approach," he said. "If you don't prevent seed production, it doesn't matter how many traits are in the seed."

The Future of Dicamba

Even if dicamba products return with new label restrictions, Hager doubted off-target issues can be eliminated.

"We've had volatility and off-target issues with dicamba since the 1960s," he said. "You can reduce them, but you can't take them to zero. It's a matter of scale. Spraying a few research plots isn't the same as spraying half a million acres a day."

Hager explained that dicamba's chemistry makes it inherently prone to vapor drift under the right environmental conditions. Even low-volatility formulations can volatilize when temperatures rise or humidity drops, moving off-target hours or even days after application. While drift-reduction agents and nozzle technology help minimize particle drift, they cannot completely prevent vapor movement once dicamba is in the atmosphere.

"When you look at the amount being applied across the landscape, even a very small percentage of volatility becomes a big problem," Hager said. "That's why we continue to see injury on sensitive vegetation, even when applicators are following the label."

Illinois lawmakers have introduced bills to ban dicamba statewide because of damage to trees and ornamentals. "Public outcry is real," Hager said. "If such a ban happens, it wouldn't just affect soybeans. It would impact corn, pastures, and even lawn care."

In 2024 and 2025, seasons without post-applied dicamba in soybeans, Hager noticed more clean soybean fields. "People are figuring out how to make the Enlist system work," he said.

Managing the Soil Seed Bank

"The single biggest mistake farmers make is allowing seed production," Hager said. "That's always been the biggest mistake, and it always will be."

He pointed to growers who stay ahead of waterhemp by pairing effective soil-applied programs with timely posts, and by physically removing survivors before harvest.

"If you can prevent seed production for three or four consecutive years, populations will plummet," he said. "That's how you get back in front of it."

But vigilance must remain. "Seeds move," he added. "Deer, birds, equipment, they'll bring it right back if you let up."

An Integrated Strategy

Successful programs now depend on multiple, complementary tactics:

Residual herbicides, layered and timed precisely.
 

Mechanical control such as cultivation or hand-pulling where possible.
 

Cover crops that provide dense biomass. "It's like growing your own mulch," Hager said. "The more cover, the more suppression."
 

Harvest weed-seed control tools such as seed impact mills. "They're not a magic fix, but they can help reduce the seed load," he said.
 

Emerging technologies, including robotics and automation, which show promise but are still years away from widespread use.
 

"Anything that helps reduce seed set is beneficial," Hager said. "The key is doing something in addition to spraying."

The Road Ahead

Waterhemp is not going anywhere, but farmers who adapt and stop letting plants go to seed can win back control.

"Weeds never increase yield. They only decrease it," Hager said. "The question is how much yield loss you're willing to tolerate before you change your approach. This weed plays by its own rules, and if you want to beat it, you'll have to play by them too."