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Why Some Waterhemp Doesn’t Die

A University of Illinois study explains why metabolic resistance is a concern for managing waterhemp.

Metabolic resistance poses another challenge to waterhemp management in this article authored by Lauren Quinn, a media specialist in the University of Illinois college of agricultural, consumer, and environmental sciences. 

Corn naturally tolerates certain herbicides, detoxifying the chemicals before they can cause harm. It’s what allows farmers to spray fields with the class of herbicides known as HPPD-inhibitors, which kill weeds such as waterhemp and Palmer amaranth and leave corn unscathed. In more and more fields, though, this method is failing. Waterhemp won’t die. 

Metabolic Response to Topramezone

Scientists have studied waterhemp’s response to two common HPPD-inhibiting herbicides, mesotrione (trade name Callisto) and tembotrione (Laudis). They found the weed uses the same cellular mechanism as corn to detoxify the chemicals. 

However, no one had studied waterhemp’s metabolic response to a third HPPD-inhibiting herbicide, topramezone (Impact or Armezon). Topramezone is in a different chemical subclass than mesotrione and tembotrione, but is just as widely used in corn.

A new study from the University of Illinois (U of I) identifies the detoxification pathway in two Midwest waterhemp populations that plays a role in rapidly metabolizing topramezone.

Unfortunately, the finding is not good news for corn growers. Metabolic resistance is a concept that has perked up in recent years.  “Our initial theory was that waterhemp would mimic corn as it does for the other two HPPD-inhibitors. But no, it found a different way,” says Dean Riechers, a U of I weed scientist and coauthor on the Frontiers in Plant Science study that was also supported by Syngenta. “We don’t know how or why, but it has a different mechanism from what corn naturally has. Bottom line – you can’t use any of the three HPPD-inhibitors to control this population.”

HPPD-Inhibitor Resistance 

The waterhemp population Riechers refers to is from a field in McLean County in central Illinois. During the past decade, the field of continuous seed corn has been treated with all three HPPD-inhibitors, and waterhemp was showing resistance to them all. Riechers and his coauthors planted seeds from that population in a greenhouse and sprayed the plants with all three herbicides to assess the degree of damage. Compared with two populations sensitive to the chemicals, the McLean County waterhemp plants looked great.

The researchers also grew waterhemp plants from a Nebraska field that only had been treated with mesotrione and tembotrione. Despite never having been exposed to topramezone, the plants appeared to be resistant. They didn’t look as good as the McLean County population, but they looked much better than the sensitive populations, says Riechers.  

“The greenhouse experiment showed the Nebraska population did have resistance to an herbicide it had never been exposed to,” he says. “Did the other two herbicides select for topramezone resistance? My colleagues at Syngenta and I believe so. Our long-term goal is to find out if each herbicide has its own resistance gene or if there are genes that one or the other could select for.”

Using an excised leaf assay they developed to identify herbicide-detoxifying enzymes, the research team discovered the McLean County plants were using a different pathway than corn to detoxify topramezone.

Waterhemp Management Dilemma

Riechers says the finding is scientifically interesting, but it might be a tough pill to swallow for the corn industry.

“It’s scary because these waterhemp populations find a way to metabolize these compounds, so it makes chemical weed control that much more difficult,” he says. “Right now, you could spray any of these three HPPD-inhibitors on corn, not kill the corn, but potentially kill the weeds. But if the weeds are using a different mechanism to detoxify the chemical, you’d have to develop a different kind of herbicide that doesn’t use these same metabolic pathways. It might be effective on the weeds, but who knows if the corn would tolerate it.”

Chemical companies could use the information in discovery research to develop new products, but farmers may not have the option to wait. In the meantime, Riechers points to U of I colleagues’ work on tank-mixing multiple herbicide sites of action to limit resistance. Managing the weed seed bank through the Harrington Seed Destructor is a nonchemical method to limit resistance.

“We’re finding out more and more about what these waterhemp populations can do for detoxification, and it’s disheartening. Our research just underscores how important it is to take alternative steps to limit the spread of these resistant plants or prevent it from happening in the first place,” he says.


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