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Why Waterhemp in Two Illinois Fields Just Made Weed Control a Lot Harder

Multiple herbicide-resistant waterhemp that now resists Group 15 preemergence residual herbicides complicates a total herbicide-dependent strategy.

Weed control became a lot harder this month, due to the confirmation of Group 15 herbicide resistance in two Illinois waterhemp populations. While this resistance was confirmed in two field populations in the central Illinois counties of McLean and Champaign, it’s possible waterhemp biotypes that resist Group 15 herbicides lurk elsewhere, says Aaron Hager, University of Illinois (U of I) Extension weeds specialist.  

“It highlights the fact that we have to think differently about soil-applied herbicides,” says Hager. “Weeds don’t just resist foliar herbicides but also soil-applied ones, as well.”

“Preemergence residuals still work well, but this means we have some things to think about,” says Gordon Vail, technical product lead for Syngenta, which collaborated with U of I scientists in confirming the Group 15 resistance. “This is one thing people don’t like to hear, but we need to do something else besides (relying on) herbicides.”

The Story Behind Group 15 Resistance 

Group 15 (long-chain fatty acid inhibitors) herbicides include:

  • S-metolachlor (Dual Magnum)
  • Acetochlor (Harness, Warrant, Degree, Surpass NXT, Topnotch)
  • Pyroxasulfone (Zidua)  
  • Dimethenamid-P (Outlook) 

They’re popular herbicides, used by many corn and soybean farmers on a preemergence basis to snuff weeds before canopy closure or application of a postemergence herbicide. Before this discovery, Group 15 herbicides encountered few resistance problems worldwide, and those that had occurred surfaced just in grasses. These waterhemp populations are the first broadleaf species in the world discovered to resist Group 15 herbicides. 

The Group 15 waterhemp resistance is rooted in 2009, when U of I researchers found a waterhemp population that resisted HPPD inhibitor herbicides (Group 27) in the McLean County field. Group 27 herbicides include ones like Laudis and Callisto.

This seed corn field, along with a seed corn field in Iowa the same year, were the first confirmed cases of waterhemp that resisted HPPD-inhibitor herbicides. The McLean County waterhemp population also resisted ALS inhibitors (Group 2 herbicides like Pursuit and Classic) and Photosystem II inhibitors (Group 5 herbicides like atrazine.)

There was more, though. 

“When our graduate student was doing a field trial, it was evident that level of control that we would normally associate from S-metolachlor (Dual Magnum) was much lower than expected,” says Hager. 

It wasn’t the case with all Group 15 herbicides, though, as application of another Group 15 preemergence residual herbicide (acetochlor) held weeds back for several weeks. 

Flash forward to the middle of this decade, when a waterhemp population from neighboring Champaign County was found to not only resist the HPPD inhibitor site of action, but four other sites of action that included:

  • ALS inhibitors (Group 2 herbicides like Pursuit and Classic)
  • Photosystem II inhibitors (Group 5 herbicides like atrazine)
  • PPO inhibitors (Group 14 herbicides like Flexstar and Cobra)
  • Synthetic auxins (Group 4 herbicides like 2,4-D and dicamba)

U of I scientists continued to monitor the population and also applied the Group 15 herbicides S-metolachlor and acetochlor to the waterhemp population.

“Thirty days later, I went to look at it and saw the same thing we saw previously in McLean County,” says Hager. While acetochlor performed well, the S-metolachlor application failed to hold the waterhemp at bay. Subsequent greenhouse dose-response experiments to calculate resistance ratios showed resistance ratios were higher for S-metolachlor than for the Group 15 herbicides acetochlor, dimethenamid, and pyroxasulfone. However, Hager says they were able to calculate resistance ratios and, thus, resistance for all of those Group 15 active ingredients.

Waterhemp resistance and weaker control in preemergence residual herbicides has been more common in PPO inhibitors (Group 14), says Bill Johnson, Purdue University Extension weed specialist. 

“Typically, if you have resistance to postemergence PPO-inhibitor herbicides, the length of residual that you get with preemergence PPO herbicides is also less,” he says. “The whole population of waterhemp might not blow through it, but rather than getting five or six weeks of residual protection, you might only get three.” 

Change in Thinking 

For years, applying a preemergence residual herbicide has been a time-tested agronomic recommendation. Syngenta data points to the use of preemergence residual herbicides on about 75% of U.S. soybean acres. (This amount can vary by geography.)

However, continual applications – particularly using the same herbicide site of action – forms predictability, which resistant weed biotypes pierce. 

“Resistance is nothing more than an evolutionary outcome,” says Hager. “This (preemergence resistance) will happen. We just don’t know how fast it will develop or when it will strike. But it’s already here.”

As with postemergence herbicides, including several herbicide sites of action in a preemergence mix can work. For example, Vail says mixing a Group 15 herbicide with a Group 5 herbicide like metribuzin (TriCor, and an active ingredient in Authority MTZ) or Prefix (a premix of the Group 15 S-metolachlor with fomasafen (Reflex, a Group 14 herbicide) are good options. 

Problems, though, can still occur when a mix contains just one effective herbicide site of action. The enormous pressure placed by weeds on that site of action can eventually render it ineffective, says Vail. 

Further complicating the situation is the specter of metabolic resistance. This is spurred by the metabolic detoxification of a herbicide by a weed. By itself, metabolic detoxification isn’t bad. It’s the reason why selective herbicides spare crops while killing weeds. Atrazine, for example, can be used on corn because corn is able to metabolize – or rapidly break down – atrazine before it injuries corn, says Hager. 

Unfortunately, several weed populations resist herbicides though nontarget site mechanisms, including metabolism.

Theoretically, a weed species could resist a herbicide to which is has never been exposed. 

It’s also challenging to identify resistance in preemergence residual herbicides. Failure could be due to lack of rainfall needed for activation. 

“It is so much easier to recognize resistance in a foliar herbicide,” says Hager. Not as much of a research base exists for detecting resistance in preemergence herbicides, he says. 

What to Do

It’s been 30 years since the last corn and soybean herbicide site of action – the HPPD inhibitor (Group 27) –  was discovered. Since then, all “new” herbicides contain mixes of existing sites of action. 

Several companies have active herbicide discovery programs searching for a new herbicide site of action. Huge hurdles exist, ranging from efficacy to today’s intense regulatory environment. 

“Not many companies will advance material short of a blockbuster,” says Hager. “Those don’t come along often.”

Vail says Syngenta is still searching for a new site of action. Still, metabolic resistance is changing the way chemical companies conduct research. 

“It might not be a new site of action, but maybe a different chemical type in an existing site of action that a plant cannot metabolize,” says Vail. 

A new herbicide is no guarantee problems will not result in the future. Using a new herbicide over and over again is a model for development of resistance. 

“We did that several years with Roundup (in glyphosate-tolerant soybeans), and we broke that hammer,” says Hager. “We will break other ones over time. We have to rethink that strategy.”

Waterhemp requires multiple tactics due to its long emergence pattern, says Purdue’s Johnson.

“The reality is this is a $50-per-acre weed in soybeans,” says Johnson. “What I hope doesn’t happen is that we get back into the Roundup mentality, now that we have Xtend (dicamba-tolerant) and Enlist (2,4-D) soybeans, where if it doesn’t work, we spray it again and again. That will kill those technologies.” 

Full Rates

Applying full rates of preemergence residual herbicides can help the situation, says Vail.

“A lot of times, farmers will apply one half, two thirds of the labeled rate of preemergence herbicides,” he says. Up front, it saves money. Still, any money saved on the front end can cost farmers at harvesttime in lost yields, he says. A clean field at harvest is no guarantee that satisfactory weed control occurred if a preemergence residual herbicide did not fully control early-season weeds. 

“You may have 230- to 240-bushel (corn) yields, but you may have had 240-, 260-bushel yields if you controlled weeds the first three to four weeks of a crop’s life,” he says. 

Cultural Techniques

“At some point in time, we need to resign ourselves to the fact that we will have to open up something else other than what’s in a jug,” says Hager. 

“Cultural practices need to come into play,” adds Vail. “It might be (narrow) row spacings, it might mean planting other crops, it might mean (adjusting) planting times,” he says. 

In some cases, it might mean a long-forgotten piece of iron – the cultivator – to remove stubborn herbicide-resistant weeds.

“You have to go back and do everything you can to prevent weeds from going to seed,” adds Vail. Southern farmers who have weeds remaining after harvest could either till or spray them with Gramoxone (a nonselective, nonresidual herbicide), he says. 

Think Differently 

“We should not be surprised by this,” Vail says. “We have had HPPD resistance since 2009 and atrazine resistance since 1970.”

The good news is that even with resistance, these products still are widely used in weed-management programs.

“It’s important to put this in context,” Vail adds. “It (Group 15 resistance) was found in two counties, not a large area. In the vast majority of cases, Group 15 herbicides like S-metolachlor still give excellent control.”

However, there may be other Group 15 resistance cases in other fields, he adds. 

“Now is not the time to panic, but to start thinking about what to do differently,” Vail says. 

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