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Research Highlights
Researchers are Seeking How to Improve Control of Herbicide-Resistant Waterhemp

In this article, you’ll find details on:

  • Illinois researchers are determining the extent of waterhemp’s resistance to Group 15 herbicides.
  • Waterhemp has become the No. 1 driver of herbicide usage decisions for farmers.
  • Soybean farmers need to consider their herbicide plan for the season to reduce problematic waterhemp populations.

Map of Illinois showing sample sites
Waterhemp sampling distribution in 2023. Random collections were made from 127 fields spanning 84 Illinois counties. Source: Aaron Hager

By Carol Brown

In some ways, farming looks a lot like it did 30 years ago. But in other ways, major changes and developments have greatly affected crop production. In Illinois, researchers are working to get better control of waterhemp, which was basically unheard of in the state 30 years ago.

“Waterhemp has become a prevalent and problematic weed. It is the number one driver species across the majority of acres in Illinois, which means it is the weed that’s driving farmer decisions on herbicide usage,” says Aaron Hager, professor of extension weed science at the University of Illinois. “Thirty years ago, the only place we knew to find waterhemp was primarily across the southern third of Illinois. Today, it is in all 102 counties in the state.”

Hager is leading a project supported by checkoff funding from the Illinois Soybean Association that will determine the prevalence of waterhemp resistance to Group 15 herbicides. The weed’s propensity to evolve herbicide resistance is one explanation to why it has spread across the state so quickly. The project will discern just how resistant it is and their research results could impact herbicide application recommendations or even change herbicide formulation in the future. 

The Group 15 herbicides — including Warrant, Zidua, Dual and others — control weeds through their soil residual action. They are applied pre- or post-emergence and are activated in the soil with moisture, infecting the weed seedlings before they emerge. These products are commonly used in both corn and soybean fields, but the Group 15 products are the only choices the soybean farmer has when managing weeds with a layered residual plan. 

“We know these soil-applied herbicides are not going to last the entire season,” Hager explains. “At some point, there will be an emergence of waterhemp that will need to be controlled with a post-applied herbicide. The Group 15 herbicides can be added to a post-application to gain several more weeks of residual control. But they will only work with a timely rain to activate the product. And if the waterhemp population is resistant, the length of control won’t be a long as it would be if the waterhemp was sensitive to the herbicide.”

Determining Waterhemp Resistance

Before the team could establish how resistant waterhemp is, they had to collect samples to test. Hager’s field research team and graduate student Travis Wilke traversed the state in the fall of 2023, pulling waterhemp plants from farm fields in more than 80 counties. Farmers also provided plant samples to the team. 

“In total, we collected more than 120 waterhemp samples to work with,” says Hager. “Travis has prepped the seeds from the samples to grow them in the greenhouse. The young waterhemp plants are treated with a ‘discriminating dose’ of S-metolachlor, one of the active ingredients in the Group 15 herbicides, then he gives them sufficient moisture to activate the herbicide.”

Hager describes a discriminating dose as high enough to control the sensitive populations, but the resistant plants will germinate and emerge through it. So far, Wilke has screened 50 accessions, categorizing them into three groups: as resistant, less resistant, or more resistant, based on comparisons with a population exhibiting known resistance.

Once Wilke has processed all the collected waterhemp samples, they will select around eight populations, or accessions, within those resistance categories and screen their resistance against six to eight herbicide rates to determine the range of resistance. 

After these measurements are made, they will deliver the results to Patrick Tranel, a weed science professor at the University of Illinois, to conduct the next phase of the project. Tranel will be searching for the waterhemp genes responsible for herbicide resistance using an array of populations, both resistant and sensitive. His work could inform chemical companies to formulate a herbicide appropriately around the waterhemp resistance genetics.

Recommendations in the Short-Term

Hager offers several tips to help farmers reduce waterhemp pressure in their fields now. 

Waterhemp doesn’t play by the same rules as other weeds, he says, as it first emerges in late April or early May. It is a long-germinating and emerging species, making it difficult to control with any single tactic. In ideal situations, it can continue to emerge through August and September, so it is essential to have a season-long herbicide plan. 

“Waterhemp resistance exists in all of the Group 15 herbicides. We have seen a range of magnitude within the resistance, and the highest-resistance levels are with S-metolachlor,” he says. “The resistant plants are those that first emerge through any of the Group 15 herbicides.”

Lastly, Hager says there is no substitute for field scouting, it is crucial to continue to scout for weeds all season long.

Other Resources

Progress Made in Waterhemp Resistance Research for Group 15 Herbicides – Illinois Field Advisor article

The Basics of Herbicide Resistance – GROW article

How Herbicide Resistant Weeds Spread – GROW article

Weed Control Research – SRIN page

Published: Dec 16, 2024

The materials on SRIN were funded with checkoff dollars from United Soybean Board and the North Central Soybean Research Program. To find checkoff funded research related to this research highlight or to see other checkoff research projects, please visit the National Soybean Checkoff Research Database.