Herbicide resistance is also beginning to make inroads in the western states. Weed resistance to an herbicide recently showed its ugly head for the first time in Arizona. Greenhouse assays last fall conducted by University of Arizona weed scientist Bill McCloskey confirmed Palmer amaranth resistance to glyphosate in cotton.

The resistance was found in an 80-acre field rotated with wheat in the Buckeye area. When McCloskey first visited the field, a severe Palmer amaranth infestation had outlasted several applications of glyphosate.

“I am not surprised that glyphosate resistance was found in Arizona,” McCloskey said. “It was only a matter of when.”

Glyphosate is the predominant weed-management strategy used by Arizona cotton growers year after year.

McCloskey expected to find herbicide resistance first in tree crops since glyphosate can be applied up to eight times annually for weed control.