Why some waterhemp do not die

Metabolic resistance poses another challenge to waterhemp management in this article written by Lauren Quinn, a media specialist at the College of Agricultural, Consumer and Environmental Sciences at the University of Illinois.

Maize naturally tolerates some herbicides, by detoxifying chemicals before they cause damage. This allows farmers to spray a class of herbicides called HPPD inhibitors that destroy weeds such as waterhemp and amaranth and leave the corn unaffected. In more and more areas, however, this method fails. Waterhemp will not die.

Metabolic response to topramezone

Scientists have studied the reaction of waterhemp to two common herbicides that inhibit HPPD, mesotrione (trade name Callisto) and tembotrione (Laudis). They discovered that the weed uses the same cellular mechanism as corn to detoxify chemicals.

However, no one had studied the metabolic response of waterhemp to a third HPPD inhibitory herbicide, topramezone (Impact or Armezon). Topramezone belongs to a chemical subclass different from that of mesotrione and tembotrione, but is also widely used in corn.

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

Unfortunately, this discovery is not good news for corn farmers. Metabolic resistance is a concept that has been proven in recent years. "Our initial theory was that hemp water would mimic corn as it does for the other two HPPD inhibitors. But no, she has found a different solution, "says Dean Riechers, a scientist and herb specialist from the University of Istria, who also participated in the Frontiers in Plant Science study, also supported by Syngenta. "We do not know how or why, but there is a mechanism different from that of corn. Bottom line – you can not use any of the three HPPD inhibitors to control this population. "

Resistance to HPPD inhibitors

The waterhemp population mentioned by Riechers comes from a field located in McLean County, in central Illinois. Over the last decade, the continuous seed corn field was treated with all three HPPD inhibitors, and the waterhemp showed resistance to all. Riechers and his co-authors planted the seeds of this population in a greenhouse and sprayed them with the three herbicides to assess the degree of damage. Compared with two chemical-sensitive populations, McLean County's lime water treatment plants looked great.

Researchers also grew water hemp plants from a Nebraska field treated only with mesotrione and tembotrione. Although they have never been exposed to topramezone, the plants appeared to be resistant. They did not look as good as the people of McLean County, but they were much better than those of the susceptible populations, says Riechers.

"The greenhouse experiment showed that the Nebraska population had resistance to an herbicide that they had never been exposed to," he says. "Did the other two herbicides select topramezone resistance? My colleagues from Syngenta and I believe that. Our long term goal is to determine if each herbicide has its own resistance gene or if there are genes that one or the other could select. "

The research team found that plants in McLean County used a different route than corn used to detoxify pramezone.

Hemp Management Dilemma

Riechers says the discovery is scientifically interesting, but that it could be a tough pill to swallow for the corn industry.

"It's scary because these hemp populations find a way to metabolize these compounds, making chemical control of weeds even more difficult," he says. "At the present time, you can spray any of these three HPPD inhibitors on corn, not kill corn, but potentially kill weeds. But if the weeds use a different mechanism to detoxify the chemical, you will need to develop another type of herbicide that does not use the same metabolic pathways. This could be effective on weeds, but who knows if corn would tolerate it?

Chemical companies could use the information in discovery research to develop new products, but farmers may not have the opportunity to wait. In the meantime, Riechers reports the work of his colleagues at the University of Israel on tank mixing of several herbicide sites to limit resistance. The management of the weed seed bank via the Harrington seed killer is a non-chemical method to limit resistance.

"We're looking more and more to find out what these hemp-water populations can do for detoxification and it's discouraging. Our research highlights how important it is to take further action to limit the spread of these resistant plants or to prevent them from occurring in the first place, "he said.

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