When it comes to Palmer pigweed, you can pay me now or pay me later

Vicky Boyd
Vicky Boyd, Editor

An old joke has it that cockroaches and whatever other object that seems to have more than nine lives will be the only things to survive a nuclear bomb. More recently, many growers have laughed that cockroaches and Palmer amaranth will be the only things to make it out alive.

All joking aside, Palmer amaranth — also called Palmer pigweed — is a formidable foe that seems to quickly evolve to resist just about any control method thrown at it. As with most pests, prevention is a lot easier and more economical than dealing with the invader once it becomes established.

Along with prevention goes a zero-tolerance policy. Earlier this decade, University of Arkansas Cooperative Extension conducted grower education on the importance of zero tolerance for glyphosate-resistant Palmer amaranth, says Dr. Bob Scott, who at the time was Extension weed specialist. He is now director of the university’s Rice Research and Extension in Stuttgart.

Even a single resistant pigweed plant that went through a combine could forever infest a field, Scott says, citing research by colleague Dr. Jason Norsworthy. Instead, Extension specialists urge growers to pull the few pigweeds in their field to prevent a much larger problem from developing.

And he says he believes the zero-tolerance message got through to a lot of growers.

When Scott first became director of the rice research facility in 2018, a neighboring grower called him out about pigweed along the station’s south border.

“Around here, they don’t let those things to go seed,” the grower told Scott, who promptly grabbed a hoe and removed the 14 offending pigweed plants.

The problem of herbicide-resistant Palmer pigweed has continued to expand, both in acreage as well as the number of chemicals to which the weed has grown resistant.

Earlier this year, Kansas State University researchers confirmed a Palmer amaranth biotype resistant to Group 4 herbicides, including dicamba and 2,4-D.

University of Tennessee Extension weed specialist Larry Steckel is following up on similar grower reports that the latest generation of dicamba products is not controlling pigweed like they did when the Xtend system was first introduced in 2013.

In addition, Norsworthy and fellow University of Arkansas weed scientist Tom Barber have confirmed Palmer pigweed populations in Northeast Arkansas with multiple resistance to S-metolachlor, a Group 15 herbicide, and glyphosate.

These latest findings join the growing list of herbicides to which Palmer amaranth has become resistant. They are Group 2 (ALS inhibitors), Group 3 (dinitroaniline such as Treflan), Group 5 (atrazine), Group 9 (glyphosate), Group 14 (PPOs) and Group 27 (HPPD inhibitors).

Although several Palmer amaranth populations resistant to two different modes of action have been confirmed around the country, researchers in Kansas and Arkansas have found populations resistant to five different modes of action.

On top of that, North Carolina researchers have identified a pigweed population resistant to glufosinate, a Group 10.

All of these discoveries lend even more credence to weed Extension specialists’ long-standing mantra, “Start clean, stay clean.”

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