Hard times for herbicides

Seven years ago, Dave Button had about had it with head-high Palmer amaranth (pictured at right, to the left of waterhemp) that infested one of his soybean fields. 

“I tried everything I could do to kill it,” says the Great Bend, Kansas, farmer. 

He even resorted to old weed wiper technology by dousing canvas in glyphosate (Roundup’s active ingredient) to physically contact the weeds. He also increased rates above what he normally applied. 

“They didn’t even wilt,” Button says. “At that point, it was like, ‘Houston, we have a problem.’ ”

Flash forward to 2021. Button applied a preemergence residual herbicide, which initially suppressed the field’s Palmer amaranth. Then he followed up with postemergence applications of dicamba (Group 4). 

“It knocked it down, but it still kept coming back,” says Button. He then applied glufosinate (Group 10) and finally fomesafen (Group 14).  

Nothing worked. 

Finally, he mechanically removed Palmer amaranth through a Row Shaver he invented that cuts the weed at its stem. 

“It’s a booger of a field, to say the least,” he says. “I think it [Palmer amaranth] resists everything. It’s gotten to the point to where I clean any piece of equipment that’s been in that field so I don’t spread it to other fields.” 

Low Risk Doesn’t Mean No Risk 

Resistance isn’t new. For example, velvetleaf first resisted atrazine — commercialized in 1958 — in a Maryland cornfield in 1984.

However, widely adopted herbicide-tolerant crops have had the unintended consequence of hastening the development of herbicide-resistant weeds. University of Delaware weed scientists confirmed glyphosate-resistant marestail in a Delaware soybean field in 2000, just four years after Monsanto (now part of Bayer) launched glyphosate-tolerant (Roundup Ready) soybeans in 1996. 

A 2017 research paper by Andrew Kniss, a University of Wyoming Extension weed specialist, found that glyphosate applications on U.S. glyphosate-tolerant cotton and soybeans increased by displacing other herbicides. Use didn’t shift in corn, though, as glyphosate complemented other herbicides rather than replacing them. 

Kniss echoed Monsanto’s initial belief that glyphosate was at low risk for developing resistance compared with other herbicide sites of action. 

Still, low-risk isn’t no risk, as selection pressure for resistant weed biotypes increased due to the adoption rate at which U.S. farmers applied glyphosate on glyphosate-tolerant crops, says Kniss. By 2006, U.S. farmers planted glyphosate-resistant seeds on nine out of every 10 soybean acres, according to USDA data. By 2018, glyphosate-resistant weeds surfaced in most soybean-producing states, according to USDA. 

“When you spray hundreds of millions of acres every year for decades, the result is predictable,” says Kniss. 

Keeping Pace

As more weeds resisted glyphosate, the industry developed additional soybean weed management systems that included tolerance to: 

• Glufosinate (Group 10, Liberty)
• Dicamba (Group 4, XtendiMax, Engenia, Tavium)
• 2,4-D choline (Group 4, Enlist) 
• HPPD inhibitors (Group 27, Alite 27 for use only in labeled counties) 

Companies also stacked soybean varieties with multiple herbicide tolerances that included:

• Enlist E3 (tolerance to 2,4-D choline, glyphosate, and glufosinate)
• Xtend (tolerance to dicamba and glyphosate)
• XtendFlex (tolerance to dicamba, glyphosate, and glufosinate)
• GT27 (tolerance to isoxaflutole and glyphosate) and LibertyLink GT27 (tolerance to glyphosate, glufosinate, and isoxaflutole)

Two pugnacious pigweeds, though, followed suit. Weed scientists have confirmed Palmer amaranth resistance to herbicides from nine sites of action. Meanwhile, waterhemp now resists herbicides from seven sites of action. 

Grassy weeds are resisting herbicides, too. Weed scientists first confirmed glyphosate-resistant johnsongrass in 2007 in Arkansas. 

“We have two other major [glyphosate-resistant] grasses that are creeping up in the South,” says Austin Scott, operations leader and herbicide specialist for Beck’s. 

“Italian ryegrass is becoming a major issue, especially in corn,” he says. Jungle rice is another grass we’re having difficulty controlling.” 

Scott adds that if glyphosate is ineffective against these grasses, herbicides such as clethodim (Group 1) are an option.

Still, an ominous sign pointing toward future resistance looms. Jungle rice that resists Group 1 herbicides has been confirmed in countries such as Venezuela. 

Pesticide Treadmill 

"The biggest problem facing American agriculture right now in big-time row crop agriculture is weed management,” says Charles Benbrook, president of Benbrook Consulting Services, who has provided consulting services for the organic industry and chemical companies. Development of more herbicide-tolerant systems leads to a “pesticide treadmill” where more applications and more herbicides are needed to successfully control weeds, he says. 

“At some point, these systems will break down due to weed resistance,” he says.

Weed resistance has hit glyphosate particularly hard: 89% of Iowa waterhemp samples screened by the University of Illinois Plant Clinic resisted glyphosate in 2016. However, it’s likely a high percentage of samples were submitted from fields with known glyphosate-resistant waterhemp, says Bob Hartzler, retired Iowa State University Extension weed specialist. This bias yielded a higher frequency of resistance than if waterhemp plants were collected from randomly selected fields, he says. 

Newer herbicide-tolerant systems also fare better when it comes to resistance. Glufosinate resistance is so far limited to a Palmer amaranth biotype in Arkansas and Italian ryegrass in the Pacific Northwest. Waterhemp and Palmer amaranth that resist synthetic auxins, such as 2,4-D and dicamba, have occurred in just a handful of cases. 

Still, it’s important to take steps to forestall resistance with these herbicides. “This includes applying effective multiple herbicide sites of action,” says Jeff Herrmann, Bayer crop management engagement manager. If a field contains glyphosate-resistant waterhemp, this means a tank mix must include other effective herbicides sites of action to manage it, he adds. 

Postemergence herbicide applications also must be properly timed. “Target applications when weeds are 2 to 3 inches high,” says Aaron Hager, a University of Illinois Extension weed specialist. 

Timing also applies to corn. “Weeds at a 6-inch height can create a 6% yield loss,” says Kurt Maertens, BASF technical services representative. “When weeds hit the 12-inch mark, yield loss can be as high as 22%.”

More Similarities

Differences exist among various postemergence herbicides used in herbicide-tolerant soybean systems. 

“Liberty [applied to glufosinate-tolerant soybeans] is a fickle herbicide,” says Scott. “It likes sunlight. It likes humidity. It likes water [at least 15 gallons per acre are recommended]. It likes AMS [ammonium sulfate as a surfactant]. If you don’t give it everything it needs, it’s not going to do the job for you.”

Dicamba and 2,4-D choline control also may differ on certain weeds. “Dicamba is stronger on smartweed, where 2,4-D is weak,” says Hager. 

With waterhemp, Palmer amaranth, and giant ragweed, though, little difference exists, says Hager. 

“I’ve always thought that dicamba is just a hair more efficacious on the pigweed species than what 2,4-D is, but not enough to impact a chemical decision,” he says. “They are more similar than dissimilar.”  

Think Long Term 

Still, look beyond differences when managing weeds, advises Hager. 

“If [efficacy] differences are the main driving factor as to which [herbicide-tolerant] technology you will use, you’re partially missing the boat on resistance issues,” he says. “All of these will never be immune to resistance to Amaranthus [pigweed]species. Search instead for technologies that allow you more flexibility of use, stewardship, and effective tank mix partners that give effective control, especially on driver weeds.

“It’s not a question of if they will become resistant,” Hager adds. “It’s a question of when.”

Think Tank Mixing

Tank mixing effective herbicide sites of action is an excellent way to forestall resistance. In 2015, USDA-ARS and University of Illinois weed scientists released a study showing that a field in which 2.5 herbicide sites of action per application were used was 83 times less likely to select for glyphosate-resistant waterhemp within four to six years than a field in which only 1.5 herbicide sites of action per application were used. 

Limits exist, though. The researchers stressed this works only if each tank-mix component effectively controls the target weed species. 

Different chemistries and adjuvants also don’t always play well together. For example, Fusilade (Group 1) antagonism that exists with dicamba (Group 4) often complicates volunteer corn control, says Dean Grossnickle, a Syngenta agronomic services representative.  

“You may have to increase your rate [of Fusilade] or add another surfactant when mixing it with dicamba,” he says. He reports the Syngenta trials he conducted showed that increased volunteer control resulted when crop oil was added to the tank mix.

The Resistance 

Palmer amaranth - Weed scientists have confirmed Palmer amaranth resistance to:

• ALS inhibitors (Group 2, Pursuit, Scepter) 
• Photosystem II inhibitors (Group 5, atrazine) 
• PPO inhibitors (Group 14, Flexstar) 
• HPPD inhibitors (Group 27, Balance Flexx, Callisto) 
• Glyphosate (Group 9)
• Synthetic auxins (Group 4, 2,4-D and dicamba) 
• Microtubule inhibitors (Group 3, trifluralin) 
• Very long-chain fatty acid synthesis inhibitors (Group 15, Dual Magnum) 
• Glufosinate (Group 10, Liberty) 

Waterhemp - Weed scientists have confirmed waterhemp resistance to:

• ALS inhibitors (Group 2, Pursuit, Scepter) 
• PPO inhibitors (Group 14, Flexstar) 
• HPPD inhibitors (Group 27, Balance Flexx, Callisto) 
• Glyphosate (Group 9)
• Synthetic auxins (Group 4, 2,4-D and dicamba) 
• Very long-chain fatty acid synthesis inhibitors (Group 15, Dual Magnum) 
• Photosystem II inhibitors (Group 5, atrazine) 

Metabolic Resistance 

Historically, target-site resistance was the only resistance game in town. There’s a new kid on the resistance block, though, that could render current weed management recommendations useless.

“Metabolic resistance is occurring even in fields where herbicides have rarely or never been used,” says Meaghan Anderson, Iowa State University (ISU) Extension field agronomist. “This is a huge concern because there is now potential for resistance to develop in weed populations to herbicide sites of action that have not even been discovered.”

Target-site resistance occurs when a mutation changes the protein that is the herbicide’s target site. This ensures weed survival, as the herbicide no longer inhibits its target in the weed, says Bob Hartzler, retired ISU Extension weed specialist. 

Metabolic resistance surfaces in a “here and there, maybe once in a while” manner. It’s normally keyed by P450 and glutathione S-transferases weed enzymes that can metabolize herbicides, just as crops can metabolize herbicides, says Stephen Powles, a weed scientist at the University of Western Australia.

With target-site resistance, weeds resist just those herbicides that inhibit the target site. Not so with metabolic resistance. Weed enzymes that metabolize one herbicide group may be able to metabolize other groups, says Hartzler.

That’s what happened in a waterhemp population from an Illinois continuous seed cornfield that thrived even after applications of Group 27 herbicides such as Callisto. Initially, researchers suspected target-site mutation keyed the resistance. Instead, they found that enzymes in the waterhemp genes also triggered increased metabolism to Group 2 herbicides (like Pursuit and Classic) and Group 5 herbicides (such as atrazine). In 2019, University of Illinois scientists confirmed the field’s waterhemp also metabolically resisted Group 15 herbicides (such as Dual Magnum).

“Our recommendation has been that multiple effective products with different sites of action together is a better way to slow the evolution of herbicide resistance than just rotating to single herbicides between years,” says Aaron Hager, University of Illinois Extension weed specialist. “However, remember that these recommendations are all based on target-site resistance. We may learn something with metabolic resistance five years from now that this is the worst thing you can do. That’s what’s so worrisome about metabolic resistance.” 

Psst...

Want to whip pigweeds such as waterhemp?  

Stop seeds. 

“You can win the battle against these out-of-control waterhemp populations by targeting the most vulnerable stage in the life cycle, and that’s seed,” says Aaron Hager, University of Illinois Extension weed specialist. “Waterhemp seeds lose most of their viability after seven years. If you prevent seed production for three to four years, you will see population numbers plummet.”

About the author: Gil Gullickson is executive editor for crops technology for Successful Farming and our website, Agriculture.com. You can reach him at Gil.Gullickson@meredith.com. Follow him on Twitter at @GilGullickson.