Navigating certainty of herbicide resistance

EAST PEORIA, Ill. — History tells us that weeds developing herbicide resistance is inevitable and figuring ways to navigate this is essential in crop production.

Kathryn Seebruck, University of Illinois Extension commercial agriculture educator for agronomy and crop sciences serving Jo Daviess, Stephenson and Winnebago counties, spoke of the weed challenges during the recent Farmland Owners Conference at Illinois Central College.

The first documented case of herbicide resistance was in 1957 with 2,4-D, just 16 years after it was first rolled out to the public.

The triazine herbicide was discovered in 1955, and the first case of resistance was documented in 1970.

Glyphosate was brought to market in 1974 and in 1998 there was the first documented case of resistance.

In 1982, the ALS-inhibiting herbicide was rolled out. Resistance was documented just a few years later.

“Are we noticing a pattern? We have a herbicide that’s discovered. It does its job very well, and some weed population develops a resistance to it,” said Seebruck at the conference, hosted by U of I Extension’s Fulton-Mason-Peoria-Tazewell Unit.

Today’s Resistance

There are currently 523 unique documented cases of herbicide resistance across the globe, unique cases being a species that’s resistant to a certain site of action.

There are 131 of those unique cases in the United States. Illinois has 29 species of weeds documented with resistance.

“These are documented, so these have been recorded, studied and characterized. So, it doesn’t include all of the undocumented cases,” Seebruck noted.

“Of those 29 cases, they include upwards of two-, three-, four-, all the way up to six-way multiple resistance. This means that there are populations in Illinois, or at least a population in Illinois, that exhibits resistance to six different herbicide sites of action.

“This means that whatever unfortunate grower has that population, that’s six different types of herbicide that they don’t have an option of using because they’re not going to control that population at all.”

She referred to an example a troublesome population of waterhemp in Illinois that has been a focus of U of I researchers.

“They found it had resistance at first in 2019 to five different sites of action. They studied it further and a year later they published another paper saying they found another site of action it’s resistant to. So, now this single population in Illinois is resistant to six different sites of action,” she said.

“What was interesting about that is one of the herbicides that they characterized as being resistant to was 2,4-D which is an auxin inhibiting herbicide. That population had never been exposed to 2,4-D before. So, that was kind of an anomaly because usually it’s such that the population gets exposed to a certain type of herbicide and then it becomes resistant to that herbicide.

“Since if it was resistant to 2,4-D they wondered if it also is resistant to dicamba which is becoming a more and more popular herbicide. They did find the population was resistant to dicamba. This was the first case of dicamba resistance in waterhemp in Illinois.

“Resistance evolves very easily and also very quickly.”

Quick Evolution

Under the conventional belief, evolution happens over thousands and thousands of years, but the evolution of resistance in weeds is extremely fast. Resistance to ALS-inhibiting herbicides only took eight years.

Weeds acquire resistance to herbicides in an evolutionary process. This is because the weeds are wild plants, so their natural genetics, their phenotype, is that they’re naturally sensitive to herbicides and they die when they’re exposed to them.

“Resistance evolves very easily, very quickly.”

—  Kathryn Seebruck, agronomy and crop sciences educator, University of Illinois Extension

When a weed acquires resistance, basically that population has evolved from being sensitive to being resistant.

“The reason for this is because applying herbicides is a huge selection pressure. We’re selecting for the strongest individual in a population, and when that herbicide comes and it’s supposed to be doing its job really well, but there’s one plant in the population that has somehow developed resistance, it’s now the strongest individual in that population. It’s going to survive and if it’s not caught, it’s going to produce seed and pass on those resistance traits to its progeny,” Seebruck said.

“The resistance individual survives. It stays in that field and if the grower doesn’t catch it, it’s going to produce seed and the following season that population is going to have more and more resistant individuals.”

Nontarget Wrench

Adding to the challenge is the two primary types of resistance — target site and nontarget site.

The target site resistance is a result of a genetic mutation that causes a significant change in the protein structure of an enzyme so that a herbicide can no longer find it and then the plant is no longer effected by that herbicide. Nontarget-site resistance involves metabolic processes not related to the target site.

These processes prevent the herbicide from reaching the target site at toxic concentrations. Nontarget-site resistance mechanisms include reduced translocation, isolation of the herbicide in vacuoles and metabolism-based resistance.

On the other end of the spectrum, nontarget-site resistance involves more change in the plant than just at the target site alone.

In this type of resistance, weeds develop the ability to rapidly metabolize, or break down, the herbicide before it can cause significant bio-toxic effects to the weed.

“A nontarget-site resistance is very different because it has nothing to do with the target site or a change in the target site. Most commonly plants that have nontarget-site resistance can instead metabolize or detoxify the herbicide molecule. They basically break that molecule down so it doesn’t get a chance to reach that target site,” Seebruck said.

“Nontarget resistance has kind of thrown a wrench into our understanding of resistance evolution. First and foremost, it’s definitely increasing in frequency. Over the past 15 years or more we’ve been focused on target-site resistance because that was most common. We got really good at characterizing all of the different target-type resistance mechanisms and mutations until we started to see this rise in nontarget-site resistance.

“What’s even more interesting is nontarget-type resistance can confer some really unpredictable cross resistance.”

Herbicide Rotation

One recommendation to combat resistance is rotating with herbicide X one year and herbicide Y the next year and another is using herbicide tank mixes.

“With herbicide tank mixtures, you’re spraying herbicide X and Y in the same application to hopefully have the same results. We started studying this and we started to see patterns,” Seebruck said.

“The herbicide rotation researchers found that that resulted in an increase of target-site resistance. Then a study in England saw herbicide tank mixtures resulted in an increase in nontarget-site resistance.”

No Simple Solution

There is no silver bullet.

“There’s not going to be some novel product or practice that comes out that’s going to single-handedly wipe out this issue and result in never having this issue again,” Seebruck said.

“Also, we certainly cannot rely on chemical control. This issue with those tank mix and rotation recommendations is we were using herbicides to try and solve a problem that was caused by herbicides. So, we have to do more than just rely on chemical control because when we rely on a herbicide, resistance is evolved to that herbicide.”

Since 1980 there have been large gaps between new herbicide discoveries that would help address weed resistance.

“In addition, some researchers who are in contact with industry have said that the industry partners they have were in the process of creating new herbicides, but they stopped because they were already seeing based on research that it was very likely that populations were just going to evolve resistance to that. That’s how significant it is,” Seebruck said.

“Just a few months ago, FMC Corporation announced the trade name for a brand new herbicide, Dodhylex, that has a novel mode of action. It was the first novel mode of action in the industry in over three decades.”

Diversified Control

“I’m not trying to tell everyone to stop using herbicides. That’s never going to happen, certainly not any time soon. If you use chemical control, don’t only use and rely on chemical control,” the agriculture educator said.

“How can you do that? There are a number of options. It depends on your operation what works best for you. No solution is going to be the same for everybody.”

One option is using cover crops to suppress weeds early in the season. Farmers in Australia uses a special machine to crush the weed seeds behind the harvester.

“Diversifying the crop rotations can help break up different lifecycles of certain weed species. Doing all of these things in combination with chemical control are going to really help with weed control,” Seebruck said.

“But at the end of the day I want to stress that the ultimate goal is that we’re ensuring no weed seed production. Because if you do have a resistant individual in your population and it survives, it’s just going to end up with you having more resistant individuals down the line in subsequent seasons.

“If we don’t let them reproduce, we’re not allowing them to pass on those resistance traits in their progeny in the subsequent season.”