As no-tillers, researchers and chemical companies scramble to control increasingly tough herbicide-resistant weeds, University of Missouri extension weed specialist Kevin Bradley says the current science of weed control will likely soon need to be broadened.

He compares fighting resistance in weeds like waterhemp and pigweed is much like a treadmill. These weeds, already resistant to 2,4-D and dicamba, are now beginning to show signs of resistance to Group 15 herbicides.

“For years we’ve been on this treadmill,” Bradley says. “We keep using a herbicide and breaking it and trying to go to another herbicide and breaking it, and so on, over and over. That’s why the industry has adopted integrated weed control (IWC) programs with their ‘one surviving weed is too many’ zero-tolerance policies. It’s also the reason so many multi-pass applications are made with various tank mixes of herbicides from different activity groups.”

Resistance Types

“Most of the time throughout our history of using herbicides we have had weeds developing resistance,” Bradley says. “That type of resistance is what we call target-site resistance, named for herbicide modes of action (MOA), which target specific sites — or systems — within a plant’s physiology.”

Plants have substances or substrates in them that, when triggered, develop into a product, usually an amino acid that the plant needs to thrive. In these instances, a specific enzyme or group of enzymes is needed to trigger the process that converts those substrates in the plant into useful proteins.

These two diagrams illustrate how target-site (top)…