Today’s weeds know how to fight back against long-used herbicides and adapt in ways that spell trouble in production agriculture, says Kevin Bradley, University of Missouri Extension weed scientist.

But 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.

Rise of resistance

Metabolic resistance is becoming a problem in weed control. This type of resistance was reported in waterhemp in 2003.

The process allows plants to convert the herbicide’s active ingredient into inactive metabolites that don’t kill the plant. The concern around metabolic resistance is the possibility that it can confer resistance to other herbicides within the same chemical groups, and perhaps even to herbicides in other groups.

It is 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, Bradley says.

This herbicide metabolism started garnering more attention at universities across the Midwest in 2019 as it showed up in Illinois, Arkansas and Kansas. And it doesn’t appear to be stopping.

“Unfortunately, the trend with resistant pigweeds like waterhemp and Palmer amaranth right now is metabolic resistance,” Bradley says. “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.”

New control measures

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.

He says that it is going to take more than herbicides alone to solve this problem with resistant weeds. The current solution to these resistant weeds is mixing and rotating herbicide sites of action.

“But remember,” Bradley adds, “herbicides alone aren’t a silver bullet to solve the resistance problem.”

For more information, visit weedscience.missouri.edu.

University of Missouri Extension and Iowa State Extension contributed to this article.