Research focuses on residual herbicide resistance

URBANA, Ill. — Waterhemp populations continue to evolve in herbicide resistance and weed scientists are investigating its move to residual herbicides.

To evaluate how widespread Group 15 herbicide resistance is, a project funded by the Illinois Soybean Association checkoff program is collecting and analyzing waterhemp populations from soybean fields across Illinois with a focus on metabolic resistance.

Results from this project will be used to provide farmers with recommendations on how best to incorporate these herbicides into integrated weed management programs.

Resistant populations will be used in subsequent research to identify the gene or genes conferring the metabolism-based resistance.

This research is led by the University of Illinois Department of Crop Sciences, spearheaded by Aaron Hager, professor and U of I Extension weed science specialist, along with professor Patrick Tranel and graduate student Travis Wilke.

Resistance in Illinois waterhemp has been documented to herbicides from seven site-of-action classes — ALS inhibitors, triazines, PPO inhibitors, glyphosate, HPPD inhibitors, auxinic herbicides, such as 2,4-D and dicamba, and very-long-chain fatty acids inhibitors, Group 15.

Soil-residual herbicides are components of an integrated weed management program that provide several benefits that include reducing the intensity of selection for resistance to foliar-applied herbicides. The discovery of resistance to Group 15 herbicides challenges the herbicide-only weed management approach.

Group 15 herbicides with preemergence activity were discovered in the 1950s. The herbicides starve the plants of VLCFA that are essential for the formation of cuticle waxes and cellular membranes.

Finding Solutions

“This project is really going to help us determine how long-term our solutions will be and determine what the future of waterhemp control really looks like,” said Wilke in an ISA Field Advisor video.

The research included documenting the extent of resistance to Group 15 herbicides in waterhemp. Researchers collected 126 populations of waterhemp this past fall from 84 counties.

“We are going to put them through a greenhouse trial where we will be able to determine how much metabolic resistance they have to chloroacetamide herbicides, part of the Group 15 site-of-action,” Wilke said.

“Most of it has been target site resistance, meaning that to a reasonable extent, no matter how much herbicide you applied, it was going to be resistant to it. Now, we have metabolic resistance.

“We’re looking at whenever we make a field application of those herbicides, metabolically the plant cannot overcome that original rate that we spray. It cannot detoxify that high of an original rate whenever you make that first pass through the field.

“Those residual herbicides are meant to offer several weeks of residual activity and keep waterhemp in particular from emerging. What we’ve seen now is those residuals are lasting much shorter than what they were originally intended to.”

Weed scientist are recommending that farmers do everything possible to prevent a single waterhemp from going to seed.

“We’ve essentially kind of taken on a mindset that we should not even allow those few plants to go to seed. So, practices that have become super outdated like walking a soybean field, something that no one’s done for decades,” Wilke said.

“If we’re going to truly prevent resistance, that’s one way to make sure that we minimize the selection pressure as much as possible on our herbicides because to date we haven’t found any weeds that are resistant to being pulled out of ground and manually removed.

“As long as we’re using chemical on it we’re always selecting for some sort of resistance. It’s a problem that you definitely have to stay ahead of. You have to be as proactive as possible to a problem that you are normally reacting to in the field.”