Linda Geist

COLUMBIA, Mo. – Weeds today aren’t like “what Mom used to make,” says University of Missouri Extension weed scientist Kevin Bradley.

Today’s weeds know how to fight back against long-used herbicides and adapt in ways that spell trouble in production agriculture, says Bradley. Officially, Missouri already has 11 different herbicide-resistant weed species. Waterhemp and Palmer amaranth, both weeds in the pigweed family, rule the roost when it comes to resistant weeds in Missouri, he says.

Resistant weeds are fast outpacing the development of new herbicides. It’s not just resistance running amok; it’s the type of resistance that concerns Bradley. “Some of the mechanisms responsible for resistance in these weeds like waterhemp and Palmer amaranth aren’t like anything we’ve seen before,” he says.

Herbicides focus on specific enzymes that bind to target sites. Historically, weeds have adapted through mutations in their internal enzymes that result in changes to the those target sites. This has been one of the most common resistance mechanisms identified in weeds for the past several decades.

More recently, weeds have been increasingly developing metabolic-based resistance, or non-target-site resistance, which lets plants convert the herbicide’s active ingredient into inactive metabolites that don’t kill the plant. Worse yet, metabolic resistance can confer resistance to other herbicides within the same chemical groups and perhaps even to herbicides in other groups. It’s possible that metabolic resistance can confer resistance to new herbicides that have never been sprayed in that field. This makes weed control even more unpredictable and concerning, says Bradley.

“Unfortunately, the trend with resistant pigweeds like waterhemp and Palmer amaranth right now is metabolic resistance,” says Bradley. “When weed scientists have investigated the 2,4-D, dicamba or group 15 resistant pigweed populations that have been found in some states in recent years, they have found metabolic resistance mechanisms in these weeds more often than not,” he says.

 “Keep mixing and rotating herbicide sites of action, but remember, herbicides alone aren’t a silver bullet to solve the resistance problem.”

Meanwhile, Bradley is studying other ways to control weeds and prevent weed seeds from returning to the soil. Methods include weed electrocution and a seed destructor that crushes seeds during harvest. Bradley says that it is going to take more than herbicides alone to solve this problem with resistant weeds.

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