It is official: Arizona has its first documented case of herbicide resistance to a weed — Palmer amaranth — and the state’s cotton growers should implement production changes this season.

Bill McCloskey, University of Arizona Extension weed specialist, confirmed the first case of resistance of Palmer amaranth (pigweed), Amaranthus Palmeri, to the herbicide glyphosate (Roundup).

McCloskey made the conclusive finding after a series of UA greenhouse tests with pigweed seed conducted last fall in Tucson.

“I am not surprised that glyphosate resistance was found in Arizona,” McCloskey told a crowd of cotton growers gathered for a UA Extension agronomic workshop in Yuma, Ariz., in January.

“It was only a matter of when,” McCloskey said. “Glyphosate is the predominant weed-management strategy used by Arizona cotton growers year after year.”

Yet McCloskey is surprised that the first Arizona case of resistance was found in cotton. He expected resistance first in tree crops since glyphosate can be applied up to eight times annually for weed control.

The resistance alarm first sounded last summer when a Buckeye cotton grower and several pest control advisers (PCAs) contacted McCloskey. The concern was an 80-acre cotton field with a severe pigweed infestation after several applications of glyphosate.

McCloskey visited the field — plus another suspect field farmed by the same grower about a mile down the road.

McCloskey’s greenhouse tests from December confirmed glyphosate resistance in the first field. In fact, the resistance levels were among the worst found in the U.S.

“It seems to be endemic in the Buckeye area in a wheat-cotton double crop scenario without tillage and pre-emergence herbicides,” McCloskey said. “Roundup was the only herbicide applied during the warm times of the cotton season.”

McCloskey received a call several weeks later from a PCA in Glendale with a problem related to pigweed control with glyphosate. UA greenhouse results were negative for glyphosate resistance.

Buckeye and Glendale are located in Maricopa County in central Arizona, just west of Phoenix.

As the 2013 Arizona cotton planting nears, McCloskey says the spread of the resistance gene depends on production practices utilized by growers. He urges growers to implement production changes to minimize the spread of resistance.

First, start with a clean field. Utilize mechanical tillage at preplant, in-crop cultivation, and post harvest. Hand rogue before seed set. Use multiple herbicides with different modes of action. “The top preseason pigweed control option I recommend is a dinitroaniline (DNA) herbicide application of Prowl, Treflan, or generics,” McCloskey said. “The best way to apply a DNA herbicide is on the flat with a boom on a disk or field cultivator to incorporate the product.”