Arizona cotton growers should know by Thanksgiving whether two suspected cases of glyphosate herbicide resistance to Palmer amaranth (pigweed) test positive or negative.

Greenhouse tests under way by University of Arizona weed specialist Bill McCloskey, Tucson, will prove whether pigweed is resistant to glyphosate, and if so whether the resistance level is moderate or high.

“It is resistant pending greenhouse confirm studies,” McCloskey said.

McCloskey believes test results will come back positive.

If resistance is proven, glyphosate would be the first weed-resistant herbicide in Arizona. Glyphosate is the active ingredient in Roundup herbicide and many generic products.

“If the test results are positive, this will be the largest challenge facing Arizona cotton growers,” McCloskey said.

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McCloskey discussed the issue during a UA field crops workshop in Marana, Ariz., in late September.

Both suspect locations in Arizona are in the western Greater Phoenix area in Maricopa County.

The first suspect find was reported in Buckeye in early July in two fields located about a mile apart. The first field was a no-till field in a double crop rotation with Roundup Ready Flex cotton and wheat.

Half of the field contained very high numbers of Palmer amaranth, Amaranthus palmeri. The grower had sprayed Roundup exclusively over the last several years.

After two, 44-ounce-per-acre Roundup applications early this season, the herbicide provided less pigweed control. After the suspect find was reported, a custom applicator hired by Monsanto applied another 44-ounce-per-acre Roundup application. The result was minimum pigweed control.

Monsanto’s Arizona sales representative Paul Sawyer sprayed a 5 percent Roundup solution on the pigweed which also provided little weed control.

About half of the field was disked down. A hand crew chopped down and removed the remaining pigweed.

A nearby field in Buckeye, about 20 acres in size, included about one pigweed per acre. McCloskey believes glyphosate resistance was present in the field for about one year.

The second suspect area, located in Glendale, included a less-infested pigweed field than the Buckeye locations. Sawyer applied a 5 percent Roundup solution which killed most of the pigweed.

McCloskey and Arizona Cotton Growers Association executive vice-president Rick Lavis are working closely to disseminate key information to growers.

“This is an issue at the top of our priority list in the Arizona cotton industry,” Lavis said. “It’s not a crisis at this point. We are taking every step to prevent it from becoming a serious problem.”

One of those steps is the ACGA’s creation of a resistance-management committee which includes industry leaders. A series of letters mailed to the association’s grower members emphasize how to best prepare for and manage glyphosate resistance.

“This is a profitability issue,” Lavis said. “It requires more costly chemical applications to control it which boils down to increased costs.”

Best options for growers

How much could glyphosate resistance to pigweed increase cotton production costs in Arizona?

McCloskey shared PowerPoint data on increased costs for the same problem in Georgia. Georgia’s initial case of glyphosate resistance to pigweed was confirmed in 2004.

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Prior to the resistance, typical weed control costs in Georgia cotton cost growers about $25 per acre annually. The cost to manage moderate-to-severe pigweed resistance increased the cost by about 50 percent or more.

If glyphosate resistance is confirmed in Arizona, McCloskey says growers will likely reduce glyphosate use and rely more heavily on other herbicides in their arsenal for pigweed control.

Roundup Ready Flex has worked well at weed control in the past, McCloskey says. However, some cotton growers across the state have been “lulled into a false sense of security.”

“We’ve been on cruise control with RR Flex. It’s worked great for a long time,” McCloskey said. “Growers are accustomed to the convenience and low cost.”

He says adding $20 or more per acre in additional herbicide costs is a great concern.

McCloskey said, “A cotton grower cannot sustain $100 per acre in chemical- and hand-weeding costs and remain economically viable.”

What is the best glyphosate resistance control option for growers?

“The No. 1 preseason pigweed control option I recommend is a dinitroaniline (DNA) herbicide application including Prowl, Treflan, or similar 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.”

Despite the likely herbicide-resistance case, McCloskey remains somewhat upbeat. He says Arizona’s unique farming system could help mitigate the effects of glyphosate-resistant pigweed and slow its spread by seed.

“Arizona agriculture lies in isolated valleys surrounded by barren, arid desert landscape; instead of riparian areas found around many fields in the Mid-South and Southeast,” McCloskey explained. “This is a huge advantage for Arizona growers.”

Another positive is many growers rotate crops due to year-round growing conditions. Rotation provides improved herbicide effectiveness through the use of multiple technologies.

McCloskey says conventional alfalfa is a good rotation choice with cotton in a herbicide-resistant environment. Alfalfa is cut and mowed which reduces the pigweed population. Water-run herbicides can help suppress pigweed emergence. Herbicides for winter annual weed control can also provide pigweed control in the spring.

In the U.S., about 372 resistant weed biotypes exist including 200 species on more than 570,000 fields. Globally, glyphosate resistance is found in 21 weed species, 11 grasses, and 10 broadleaves.

McCloskey says herbicide resistance should be suspected when other causes of herbicide failure have been ruled out. Growers should suspect a herbicide if the same product(s) with the same mode of action are used year after year, or if a normally-controlled weed species is not controlled.

Other herbicide-resistance indicators include healthy weeds in a field intermixed with killed weeds of the same species, or when a single-species weed patch of uncontrolled plants spreads.

“A dead ringer for herbicide resistance is when a grower finds a mix of dead and living pigweed in the same field after a herbicide application.”

To reduce resistance spread by pigweed seed, McCloskey suggests cleaning custom-harvest equipment between fields. Increased mechanical cultivation is another method, especially in crop rotation.

If the greenhouse tests confirm glyphosate resistance, McCloskey will file a documented case report on the weedscience.org website.

He plans to discuss the Arizona resistance case during the 2013 Beltwide Cotton Conferences in San Antonio, Texas in January.

McCloskey concluded, “Cotton growers should adopt a zero tolerance as the best, long-term and most cost-effective way to prevent glyphosate-resistant pigweed from infesting the land.”

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