Project tracking Group 15 herbicide resistance

BLOOMINGTON, Ill. — A statewide survey is underway by the University of Illinois to determine the extent of waterhemp resistance to Group 15 herbicides.

To evaluate how widespread Group 15 herbicide resistance is, an Illinois Soybean Association checkoff program is collecting and analyzing waterhemp populations from soybean fields across the state.

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.

“As long as we’re using chemicals on it, we’re always selecting for some sort of resistance.”

—  Travis Wilke, master’s student, University of Illinois

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

“We collected a total of 126 populations of waterhemp from 84 counties throughout the state last fall. We are putting 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.

“We are documenting the extent of Group 15 herbicide resistance in waterhemp throughout Illinois. Most of it has been target site resistance, meaning that to a reasonable extent, no matter how much of herbicide you applied, it was going to be resistant to it. Now we’re looking at metabolic resistance.”

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,” Wilke said.

“People like those at the University of Illinois are now recommending that we do everything we can to prevent a single waterhemp plant from going to seed in that field. Essentially, we’ve taken on a mindset that we should not allow even those few plants to go to seed.

“As long as we’re using chemicals 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.”

Multiple Resistance

Waterhemp has developed resistance to herbicides from multiple site-of-action groups across Illinois and other states.

Confirmed resistance in Illinois waterhemp has been documented to herbicides from seven site-of-action classes — ALS inhibitors, triazines, PPO inhibitors, glyphosate, HPPD inhibitors, auxin herbicides (2,4-D and dicamba), and VLCFA, or very long chain fatty acids, inhibitors (Group 15).

“It’s been five years ago since we first made the confirmation about resistance to Group 15 herbicides. These products are somewhat unique in that they have really only pre-emergence activity. They really don’t have any foliar activity, and this is why these are going to be sprayed and come in contact with the soil,” Hager said.

The Group 15 inhibitors target very long chain fatty acids and essentially the plants starve for those acids that are very important for various plant components, for example, for cell membranes, as well as waxes on the leaf surface.

“And generally, a lot of times when we use these products, a sensitive plant will either fail to emerge from the soil or, if it does emerge, it will remain in a very, very small state, arrested growth state, and probably not even visible unless you get down and look very, very closely,” Hager said.

Good, Bad News

In the last five to seven years, there has been a lot of momentum toward using layered residuals, especially in soybeans.

“We would make an application of a residual product close to soybean planting time and then we’d come back for that first post-application to control any emerged weeds,” Hager said. “The common practice now is to add one of these Group 15 products in there with the idea of trying to extend this residual control for several additional weeks into the season.

“If there’s good news around this, of course, it’s that really, relatively speaking, resistance is rare to the Group 15. There’s approximately somewhere around 13 species worldwide that have evolved resistance to Group 15 products and only three dicot species.

“The bad news is two of those three dicot species are amaranths. One is waterhemp and the other is Palmer amaranth.”

Mechanism Change

Enhanced metabolism is a mechanism of resistance the plants use to survive Group 15 herbicides.

“The plants, the seedlings are taking up the herbicide, it’s getting into the plant, but they’ve evolved mechanisms now that allow them to rapidly break down that herbicide before it can actually cause a lethal effect to the plant,” Hager said.

“The unnerving part is that these waterhemp populations now are using the same technique that many crop plants use in order to survive being treated with the herbicide. The reason that we don’t control corn with atrazine is simply because the corn can rapidly metabolize the atrazine.

“Now we’re seeing more and more of these populations that are able to break down some of these Group 15 herbicides literally as rapidly as corn can. And rapid metabolism is the mechanism by which corn is resistant or tolerant to Group 15 herbicides.

“What we want to emphasize is this is continuing to happen and any waterhemp plants that have survived the growing season and any females that now have seed produced, all those females have gone through one additional season of selection. Those seeds then enter into the soil seed bank and be part of the problem for future years.”

Shorter Residual Control

One of the most significant implications of this work is resistance manifests in the field, not necessarily the greenhouse or in the laboratory, but in a farmer’s field where all these products are used, and the length of residual control lessens.

“Hypothetically, we would use a soil residual herbicide on a sensitive population that gave us, let’s say four weeks, perhaps five good weeks of residual control. If we would use that same active ingredient on a confirmed resistant population, maybe that length of residual control is now only three weeks, maybe it’s been two weeks, maybe those survivors in the field have a different mechanism. Some of these may be only a week,” Hager said.

“That’s really the concern about this. If our residual control is less and less, that means that we may have to think about moving up the timing literally on the calendar of our post-application because our length of residual control is simply not going to be as it once was on sensitive populations.

“We talked about this idea of overlapping residuals. The overlapping residuals that we use in soybeans are all Group 15 products. We need to be aware that we may not be able to extend that length of residual control following the in-season application of our Group 15s.”

Best Recommendation

Producers are encouraged to use multiple effective herbicides.

“In all honesty, we think that’s the best recommendation, but we have to admit that we really don’t have a lot of data to support that anymore,” Hager said.

“We really need to think about the idea that we’re going to continue to treat virtually every acre of corn and soybeans, not only in Illinois, but perhaps many areas of the Midwest with herbicides.

“But because of this continued evolution of resistance, we really have to have a serious conversation about what else we can do to try to ensure that by the end of the growing season that these female plants have no seed because amidst all this uncertainty that we have around which herbicides may work yet, which ones may not work, or do these females in the field right now contain the next generation of resistance mechanism.

“The only thing that we really know is that if there is no seed, there’s no change in the frequency of any of these resistance mechanisms.”

Looking Ahead

“The biology of waterhemp is really the key, and the key really is that we’re not necessarily going to wait and open up a new jug of herbicide that the industry brings us at some point, because that’s probably not going to be very sustainable. But what we do know is that the weakness of waterhemp, the weakness of Palmer amaranth for that matter, is that the seeds do not remain viable in the seed bank indefinitely,” Hager said.

“And so these other practices, whatever they are, maybe it is picking up a hoe or a weed hook and walking a bean field. Maybe it is the idea of harvest weed seed control in the back end of the combine. Maybe, in some cases, it’s the electrocution of the weeds after they get a height differential over the soybeans. Any of these things that can reduce that amount of seed, that’s the win that we need to look for.

“We also want to caution folks that not every time we see a failure of a field treated with a herbicide is not always caused by resistance.”