Lack of cold weather means more nematodes, possibility of seedling disease.

Is Crop Production Moving Toward ‘Stacked Resistance?’ 
Growers urged to stop replacing one herbicide with another.

Finding ways to halt the “resistance treadmill” was a key message from weed scientists at the recent Pigposium III, an event hosted by the University of Arkansas System Division of Agriculture focused on herbicide-resistant Palmer amaranth.

About 300 producers, Extension agents, consultants and members of industry, heard weed scientists from four states describe research and strategies for managing weeds that have developed resistance to the most-used herbicides.

Jason Norsworthy, weed scientist with the U of A Division of Agriculture, opened with a situation report, noting that it’s been 12 years since the existence of glyphosate-resistant Palmer amaranth, better known as pigweed, was first confirmed.

A year later, in 2006, glyphotsate-resistant pigweed was confirmed in Arkansas and Tennessee.
In just over a decade, “31 states now have glyphosate-resistant Palmer amaranth,” he says. “The world’s greatest herbicide is no longer effective against pigweed.”

Get Off The Treadmill

In defining what he called the resistance treadmill, “we’re taking one herbicide and replacing it with another, and taking another herbicide and replacing it with another, and taking another herbicide and replacing it with another,” Bob Scott, weed scientist for the University of Arkansas System Division of Agriculture, says. “Today, the goal is to stop doing this.”

“If we follow our resistance pattern … we’re just going to add dicamba to the growing list of resistance that we have,” he says. “We’ve proved this in a laboratory study.”

Scott says ALS resistance perpetuated glyphosate resistance, glyphosate resistance perpetuated PPO resistance and asked “what will PPO resistance perpetuate?”

Running Out Of Herbicide Options

This pattern will eventually lead to “multiple stacked resistance,” Scott says. “I’m a weed scientist and that scares me. Stacked resistance is a game changer.”

Scott’s comments were echoed by fellow Division of Agriculture weed scientist Tom Barber: “if you continue to rely just on PPOs, you’ll shift the population in that field to PPO resistance.”
Norsworthy, in talking about the future of herbicide use says it’s just a matter of time before we have a six-way stack in herbicide resistance.

He quoted Eric Maupin, a Tennessee farmer and board member of the American Soybean Association, saying “going into 2016, I have one post-emergence option for pigweed, and if I were to lose that option, I wouldn’t be farming soybean.”

Effective Experiences

Larry Steckel, a Tennessee Extension weed scientist, talked about cover crops being highly effective and could likely reduce the number of herbicide applications by one.

Michael Popp, a U of A ag economist, walked the audience through Palmer amaranth manager, or PAM, a PC-based program that enables growers to develop a strategy in managing pigweed. The software is available at

Three producers shared their experience with various weed management methods, including Harry Stephens, of Helena, Ark., who talked about his successful use of narrow windrow burning.

Dane Coomer, of Piggott, Ark., told the audience about his experience with zero tolerance in cotton. “It’s a mindset and goal. If you see one on your farm, you pull it.” Coomer uses chopping crews to eliminate remaining weeds by hand.

Producer Adam Chappell talked about the successful use of cover crops, telling the audience he’d been using the same beds since 2010. He listed all of his weed management tactics, noting that only prayer is the free one.

The primary message to producers: Do something different this year to combat Palmer amaranth and to preserve herbicides.

Article by Mary Hightower, University of Arkansas Division of Agriculture. For more information about weed management, contact your Extension office or visit or

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