The marvels of metabolism occur every time a crop plant breaks down a selective herbicide that kills adjacent weeds.

These days, though, it’s almost like the weeds are watching. That’s because biotypes of two pigweed species – waterhemp and Palmer amaranth – are mimicking the same type of metabolism field crops use to render a herbicide harmless.

Welcome to the world of metabolic resistance.

Farmers haven’t heard much about it, since target-site resistance is the source of so many of the herbicide-resistant weeds they face. Yet, metabolic resistance looms large in the weed science community. A 2018 University of Nebraska resistance survey conducted in collaboration with Kansas State University found 50% of Nebraska waterhemp plants that resisted Group 5 herbicides (atrazine) were metabolically resistant.

“The scary part is this resistance could chew up a herbicide site of action that’s not now even in the marketplace,” says Aaron Hager, University of Illinois (U of I) Extension weed specialist.

For farmers like Chad Leman, dealing with metabolic resistance is just the latest weed-management challenge. He first started noticing lambsquarters that resisted Group 2 (ALS inhibitors like Pursuit) on his family’s Eureka, Illinois, farm about 10 years ago. That’s when he and his family started forming an integrated weed-management plan featuring scouting, tillage, preemergence, and postemergence residual herbicides, and roguing weed escapes.

“You constantly have to adjust,” says Leman. “Boxer Mike Tyson once said everybody has a plan until they get punched in the face.”

Resistance Types

Repeated use of the same herbicide can spur both types of resistance, says Bob Hartzler, Iowa State University Extension weed specialist. They differ, though, in the way resistance develops.

Target-site resistance occurs when a weed mutation changes the protein that is a herbicide’s target site. This ensures weed survival, as the herbicide no longer
attaches to its target on the weed, says Hartzler.

Metabolic resistance surfaces in a ‘kind of 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.

Weed scientists know much more about target-site resistance than metabolic resistance, says Hager.

“Target-site resistance was easier to identify and study prior to advances in molecular weed science techniques,” he says. “I often say that we don’t even know what all we don’t know about metabolic resistance.”

Good news exists. Metabolic resistance pathways are complicated. “It takes longer for metabolic resistance to spread compared to target-site resistance,” says Gordon Vail, Syngenta technical product lead.

A weed that metabolically resists one herbicide in a herbicide site of action group won’t necessarily resist other herbicides within that group, says Hartzler. Waterhemp that metabolically resists atrazine, for example, may not be able to metabolize another Group 5 herbicide, metribuzin (Sencor, Lexone).

If Australia’s experience is any indicator, though, the U.S. should worry. One Australian weed, rigid ryegrass, has metabolically resisted herbicides that have never been applied to a field. “We have ryegrass biotypes resistant to herbicides not yet discovered,” says Powles.

“As we develop new herbicide sites of action, we might spend millions and millions of dollars on a product that weeds already resist,” adds Arlene Cotie, senior product development manager with Bayer CropScience.

Resistance Differences

“The good thing about target-site resistance is that weeds resist just a single herbicide group,” says Hartzler.

Not so with metabolic resistance. “With this resistance type, weed enzymes that metabolize one herbicide group may be able to metabolize other groups,” says Hartzler.

In 2009, U of I scientists analyzed a waterhemp population from a continuous seed corn field that thrived even after applications of Group 27 herbicides like Callisto. Initially, researchers suspected target-site mutation keyed the resistance. Instead, they found that enzymes in the waterhemp genes triggered increased metabolism to mesotrione (Callisto’s active ingredient), Group 2 herbicides (like Pursuit and Classic), and Group 5 herbicides (like atrazine).

What to do?

“Widespread metabolic resistance – plus target-site resistance – helped us realize in Australia that we needed to be more diverse than simply reaching for the next jug of weed control,” says Powles. “U.S. farmers, who, for decades, could control weeds easily by simply changing the jug, are now grappling with this reality.”

“We have to change our focus from protecting crop yields to minimizing the weed seed bank,” adds Hartzler.

Ways to do it include the following.

Tilling where feasible. “Tillage will help manage weeds before planting and in the crop where topography and soil types allow,” says Dane Bowers, a Syngenta technical product lead. “They will be controlled before they form seed.”

Expanding the crop rotation. “A corn-soybeans-wheat rotation, or including alfalfa in a rotation, adds diversity to a weed-control program,” says Bowers.

A 2002-2006 ISU trial that found a four-year rotation of corn-soybeans-triticale/alfalfa-alfalfa sliced weed seed production compared with a two-year corn-soybean rotation. As a result, herbicide use declined by 82%, says Hartzler.

Using herbicides wisely. “We tell farmers to start clean (with a burndown) and use overlapping residual herbicides with two effective sites of action,” says Cotie. Overlapping residual herbicides can nix weed escapes that could set and drop seed near harvest, she adds.

That’s easier said than done in a spring with prolific precipitation like 2019. If everything is falling apart, though, farmers should aim to at least apply a burndown herbicide before planting, says Curt Hoffbeck, a Pioneer field agronomist. This can help control weeds before they exceed label height and, subsequently, form seed, he says.

Narrowing rows. “Narrow row spacing can help a crop be more competitive and prevent weed emergence and weed seed formation by closing the canopy earlier,” says Dan Waldstein, BASF technical marketing manager.

Impact On Herbicide Research

Finding a new herbicide site of action is the Holy Grail in the corn and soybean space. Group 27 herbicides (Callisto, Balance Flexx) were the last ones to be discovered in the late 1980s. FMC, BASF, and Bayer CropScience executives say they are developing new herbicide sites of action that may debut in five to 10 years.

All could be for naught, though, if metabolic resistance spreads.

“That is the scariest thing for all of us (in the industry),” says Arlene Cotie, senior product development manager with Bayer CropScience. Once established, multigene metabolic resistance can quickly overwhelm existing chemistries, she notes.

Metabolic resistance is changing the way chemical companies conduct research, says Gordon Vail, Syngenta technical product lead. “It might not be a new site of action, but maybe a different chemical type in an existing site of action that a plant can’t metabolize,” he says.

Seed Destruction

Australian farmers aren’t only harvesting small grains with their combines. They’re also harvesting weed seeds that otherwise would germinate and spur future herbicide-resistant weed infestations. Several such tools, like the Harrington Seed Destructor (HSD), do so by collecting and pulverizing weed seeds at harvest using internal weed seed mills.

“Sometime down the road, I see tools like this helping farmers in Iowa (and other states) in managing weeds,” says Bob Hartzler, Iowa State University Extension weed specialist. “This reduces the likelihood of new resistant weeds getting started.”

Kill rates for seed destructors rank 99.9% and above for waterhemp, lambsquarters, giant ragweed, and cocklebur, says Hartzler.

Challenges exist that include the following:

Weed seed status. To work, seed has to be on the weed at harvest. “It wouldn’t work for weeds like foxtail that are already on the ground at harvest,” says Hartzler.

Green stems. University of Arkansas researchers who’ve tested the HSD have found that green soybean stems can interfere with chaff flow to the mill. Satisfactory performance occurred in corn, though.

Mill wear. A survey of 32 Australian farmers who tested the HSD in 2017 found that, although the mill pulverized the seed, wear occurred sooner than expected.

Harvest efficiency. “There needs to be minimal disruption of harvest efficiency,” says Hartzler.

Cost. HSD prices may range from $85,000 to $117,000.

Combine capacity reduction of 12% to 20%. “Australian farmers (in the 2017 survey) revamped their combine engines but had concerns over doing this because it could violate their warranties,” Hartzler says.

Fuel consumption. It can take an extra $1.20 per acre in fuel to run the unit.

Hartzler says solutions to many of these challenges may occur. Unit price and other costs also must be weighed against future herbicide costs and resistance problems that a high weed seed bank can key, he says.

Group 15 Resistance

Group 15 residual herbicides like acetochlor (Harness), S-metolachlor (Dual Magnum), dimethenamid (Outlook), and pyroxasulfone (Zidua) form the backbone of many weed-management plans. By suppressing early-season weeds, they can reduce weed-control pressure on later-applied postemergence herbicides.

That may be changing, though. This year, University of Arkansas weed scientists confirmed a Palmer amaranth population that resists S-metolachlor.

Ditto for University of Illinois (U of I) weed scientists, who confirmed two waterhemp populations that resist all Group 15 herbicides they tested. This marks the first time globally that broadleaves have resisted Group 15 herbicides. Previously, resistance just had surfaced with weed grasses.

“It’s important to put this in context,” says Gordon Vail, technical product lead for Syngenta, which collaborated with U of I scientists in confirming the Group 15 resistance in the Illinois waterhemp. “In the vast majority of cases, Group 15 herbicides like S-metolachlor still give excellent control. But this means we have to think about things. This is one thing people don’t like to hear, but we need to do something else besides (solely relying on) herbicides.”

The specter of metabolic resistance is also present in these cases, as the Arkansas Palmer amaranth biotype was metabolically resistant.

U of I weed scientists are currently analyzing laboratory data for metabolic resistance. So far, it appears these waterhemp populations are metabolizing S-metolachlor about as quickly as corn can, says Aaron Hager, U of I Extension weed specialist.

Target-Site Resistance

1. Impacts just one herbicide site of action.

2. Normally spurred by a single amino acid substitution in a weed’s long amino acid string. Due to this change, the herbicide no longer binds to that site (in the weed). However, resistance in Group 14 herbicides (PPO inhibitors like Flexstar) can occur by deletion of a codon that consists of three amino acids.

3. Mostly spread by pollen but can also be spread by seed.

Metabolism-Based Resistance

1. Single mechanism can trigger resistance to multiple herbicide sites of action.

2. Resists herbicide sites of action not yet discovered.

3. Features enhanced activity of glutathione S-transferases and P450 enzymes. Metabolic-resistant waterhemp has naturally occurring levels of these enzymes that enable it to metabolize herbicides. Glutathione S-transferases can metabolize some herbicides, but there are few genes that do so and are, thus, less important than P450 genes.

4. Can be spread by either pollen or seed.

No Resistance to Iron

Groans among farmers often accompany any talk of cultivation for managing weeds. That’s understandable, for past generations endured aching necks and cultivator blight from off-target passes.

Today, though, cultivation is easier, and no weed yet resists cold steel. Chad Leman, who farms with family near Eureka, Illinois, says it’s one tool they use on soybeans to supplement a herbicide program that features overlapping residual preemergence and postemergence herbicide.

weed menace farmer chad leman

“Cultivation is easier now with guidance,” he says. “The (Cat) Whiskers on the cultivator enable it to (automatically) shift from side to side. With autosteer, we rarely see crooked rows anymore.”

Cultivation also helps incorporate residual preemergence herbicides the Lemans apply. They normally split soybean acres between two-thirds GMO varieties and one-third non-GMO cultivars. Both receive a preplant residual mix of Group 14 herbicide (PPO inhibitors like Flexstar), Group 15 herbicide (long-chain fatty acid inhibitors like Zidua), Group 2 herbicide (ALS inhibitors like Pursuit), and Group 5 herbicide (Photosystem II inhibitors like Sencor).

GMO soybeans receive a mix of Group 9 herbicide (glyphosate), Group 4 herbicide (dicamba), and Group 15 herbicide, while non-GMO soybeans receive a mix of Group 1 (ACCase inhibitors like Select), Group 14, and Group 15 herbicide.

On corn, they apply a preplant residual of Group 27 herbicide (Balance Flexx) and Group 5 herbicide (atrazine). Postemergence residual herbicides include those from Group 9 and another from Group 27.

He adds flexibility is key, though, particularly in a challenging spring like 2019.

Diverse weed-control programs also have their challenges. Chemistry switches mandate sprayer cleanouts to prevent contamination. To ease cleanout labor, they apply all liquid formulations.

“We budget between $20 and $25 per acre for herbicides on GMO crops,” Leman says. “For non-GMO beans, we budget between $40 and $45 per acre.”

Leman also enlists the help of his daughters and their friends to walk soybean fields for late-season escapes. “I tell them that walking beans is a character-building experience,” he jests.

6 and 13

Missouri farmers have every right to be discouraged by a 2018 discovery of a waterhemp biotype that resists six herbicide sites of action.

Still, Australian farmers have it worse. A grass weed in Australia, rigid ryegrass, has resisted 13 herbicide sites of action from 1982 to 2013. Many of the resistance cases are metabolically based. Several cases of multiple herbicide sites of action resistance exist, including a five-way resistant rigid ryegrass biotype that was confirmed in pasture seed in 2010.