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Meet Metabolic Resistance

It’s why weeds could possibly resist a herbicide to which they have never been exposed.

Metabolic detoxification is a term you may have happily left behind in a college chemistry class. It’s the reason, though, why selective herbicides spare crops while killing weeds.

“The reason we can use atrazine in corn is because corn is able to metabolize – or rapidly break down – atrazine before it causes injury to corn,” says Aaron Hager, University of Illinois (U of I) Extension weed specialist.

Unfortunately, weeds are starting to mimic this.

Most weeds resist herbicides due to alterations in their gene target site. A few now also resist herbicides through nontarget site mechanisms, including metabolism.

“Metabolic resistance is the ability of a weed plant to break down or metabolize a herbicide that is damaging to the weed,” says Hager. “They’re now mimicking the selectivity metabolism that occurs in crops. This is resistance. Theoretically, a weed species could resist a herbicide to which it has never before been exposed.”

He adds this could neuter a new herbicide site of action before it is even applied to a weed population.

Bulletproof Waterhemp

Back in 2009, U of I scientists analyzed a waterhemp population that infested a central Illinois continuous seed cornfield. It thrived even after applications of HPPD-inhibitor herbicides like Callisto. 

Initially, researchers suspected resistance was due to target-site mutation. Instead, they found certain enzymes in waterhemp genes triggered increased metabolism to mesotrione (Callisto’s active ingredient) and also atrazine. Since then, several other Midwestern locations have reported similar occurrences. U of I researchers have confirmed single-gene atrazine metabolic resistance. Multiple genes are likely keying the metabolic HPPD-resistant populations, though. However, researchers have not yet found those genes.  

The fact that multiple genes are involved in such metabolic resistance cases makes it tougher to tackle, says Gordon Vail, Syngenta technical product lead. “The resistance we find in Palmer amaranth and waterhemp degrades the chemistry with three, four, or five more genes."

This differs from the target-site mutation that occurred with ALS site of action herbicides (Pursuit, Scepter). “With ALS herbicides, we’re dealing with just one gene,” Vail says.

The genes involved in the resistance spurred by metabolism may number in the hundreds, says Dean Riechers, a U of I herbicide physiologist. The U of I researchers are now looking for new ways to overcome the HPPD-inhibitor resistance mechanism by blocking weed enzymes using a different chemical inhibitor. If the scientists can find inhibitors of waterhemp’s herbicide metabolism pathways, it may be possible to get around the resistance without injuring the crop, he says.

Resistance to target-site mutations is easier to predict, as it’s often spurred by repeated applications of the same herbicide site of action. Not so with metabolic resistance.

“It will become less likely to accurately predict that this will happen with a specific herbicide or herbicide group,” says Hager.

There’s some good news in that the pathways that lead to metabolic resistance are complicated. “It takes a lot longer for metabolic resistance to develop and spread compared with target-site resistance,” says Vail.  

What to Do

In the meantime, applying a tankmix of herbicides with effective multiple sites of action each time an application is made can forestall resistance, says Hager.  

So is tillage. That’s anathema from an erosion standpoint, as reduced tillage and no-till are the belle of the soil health conservation ball these days. Unfortunately, there are lots of unwelcome crashers. “Palmer amaranth is well adapted to reduced tillage,” says Mike Weber, Bayer Crop Science senior technical representative.   

Jayme Dick-Burkey, who farms with his wife’s family near Dorchester, Nebraska, notes they planned to continue no-till across their entire farm. However, a herbicide-resistant weed review has prompted them to complement herbicides by planting cover crops followed by tillage on some fields.

“We saw those fields had an advantage over fields with a herbicide application alone,” Dick-Burkey says.

Erosion is a concern. Still, he says, some no-till fields with cover crops killed prior to planting the 2016 crop had worse erosion than those where cover crops had been incorporated into the soil.

“The cover crops incorporated into the soil seem to hold back weed pressure,” he says. “Reincorporation into the soil looked as though it gave soil a more even and sturdy structure.”

This finding and subsequent research has prompted Dick-Burkey and his family to invest in proper tillage tools for such fields. Seed bank management is another tool for managing herbicide-resistant weeds.

“Last year was not the best year for waterhemp control (in Iowa),” says Mike Owen, an Iowa State University Extension weed specialist. “There were lots of plants out in the middle of fields prior to harvest. If we don’t take care of those scattered plants via hand-rouging, we will be in real trouble. Combining those patches at harvest is an incredibly effective way of spreading waterhemp seed across the field.”

In the meantime, keep metabolic resistance in mind the next time you see weeds that just won’t die. “It doesn’t follow the rules,” says Hager.

What’s Coming Down the Pike

From the 1950s through the mid-1980s, agricultural chemical companies steadily discovered new corn and soybean herbicides with new sites of action (SOA). Then discoveries stopped.

HPPD inhibitors (Callisto, Impact, and Laudis) were the last new herbicide SOAs to be commercialized.

So is a new herbicide SOA coming down the pike? Probably not for a long time, says Gordon Vail, Syngenta technical product lead.

“It is really difficult to do,” he says “We have discovered all the easy ones.” He adds that some research compounds that have excellent weed control could damage the crop. Meanwhile, a herbicide with a new SOA that is gentle on the crop could only result in 40% to 50% weed control.

“Everyone thinks we need a new site of action,” says Vail. “We won’t find one today. Maybe we will find one in the future. But developing a new site of action to a weed that can readily metabolize anything you spray may not be a proper strategy."

“A better alternative may be to search for new active-ingredient molecules in existing sites of action,” he says.

Differentiating factors like pH and acidity from existing active ingredient molecules may make it impossible for weeds like waterhemp to degrade it, Vail adds.

Syngenta has one such compound that may debut next decade, he says. Although it’s an HPPD inhibitor like Callisto, it has a different binding affinity to the weed target sites.

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