Glyphosate-resistant Palmer amaranth pigweed knows no boundaries, just ask farmers in Alabama.

GLYPHOSATE RESISTANT pigweed was found on a farm in east-central Alabama in 2009.

For a while now, growers in the state have wearily watched their neighbors across the river in Georgia, where resistant pigweed has established a stronghold and is expected to be present in every crop-producing county by the end of this year.

(For an update on the situation in Georgia click here).

Historically speaking, glyphosate-resistant pigweed hasn’t been a problem in Alabama cotton fields, but as of now, it is. Most recently, in 2009, it was documented on a farm in east-central Alabama’s Barbour County, where approximately 2,000 acres were infested, says Mike Patterson, Auburn University Extension weed scientist.

Reports of additional fields containing escaped pigweed in Roundup Ready cotton indicates, he says, that this problem will spread across south Alabama fields in the next few years. A field of soybeans infested with Palmer amaranth was discovered in the Tennessee Valley region of north Alabama in 2009, where the weed was not controlled with applications of glyphosate. A further investigation of this infestation will be conducted this year.

In response to this threat, a large, replicated weed control demonstration was conducted by Auburn University researchers in one of the cotton fields on the Barbour County farm in 2009. This was done, says Patterson, to demonstrate to local farmers and Extension employees which herbicide systems could be used to combat glyphosate-resistant Palmer amaranth in an area already infested with the weed pest.

A four-acre site located on the farm was selected to demonstrate different weed management systems for Palmer amaranth control in cotton. Phytogen 375 WRF seed was planted on May 14 in a reduced-tillage, stale seedbed culture. Row spacing was 38 inches and seed were planted at the rate of two per foot of row.

Twelve herbicide systems were used to demonstrate Palmer amaranth control. Pre-emergence herbicides were applied immediately after planting and good activating rainfall occurred within two days. Early postemergence treatments were applied on June 11 and layby treatments were applied on July 14.

Visual crop injury and Palmer pigweed control was evaluated on June 11, July 23, and July 22. Ratings were made on a scale of zero to 100 where zero is no injury or control and 100 is cotton death or complete control. No yields were obtained due to excessive rainfall throughout the fall and winter.

Timely activating rainfall within two days following planting and pre-emergence herbicide application resulted in optimum conditions for pre-emergence activity, reports Patterson. Minimal crop injury was observed 28 days following application (12 percent or less) and control of Palmer amaranth ranged from 68 to 77 percent (Prowl H2O alone) to 90 to 93 percent (Prowl plus Reflex).

Some early postemergence treatments resulted in significant crop injury, with Ignite 280 plus Dual Magnum injuring cotton 15 to 23 percent at 12 days after treatment. Palmer amaranth control for all systems 12 days following early postemergence applications ranged from 85 to 95 percent.

Crop injury was less than 11 percent for all layby treatments eight days following application.

Palmer amaranth control ranged from 90 to 95 percent for all 12 herbicide systems at this time, with most plots having only three to four weeds per plot (four rows by 400 feet).

These plants were hand weeded from the entire demonstration following the evaluation of layby treatments.

Glyphosate-resistant Palmer amaranth had already emerged in many cotton fields in Georgia by late April, according to Stanley Culpepper, University of Georgia Extension weed scientist. “It is critical that growers control these emerged plants before planting their cotton crop. If the Palmer amaranth population is resistant to Roundup, then one of the more effective mixtures would be an application of paraquat (Gramoxone, others) plus diuron (Direx, others) plus crop oil,” he says.

Mixtures of diuron with paraquat are often far more effective than paraquat applied alone, says Culpepper. “Other effective options do exist including a burndown application of Ignite. Ignite can be an effective treatment depending on the rate of Ignite applied and size of Palmer amaranth at application timing. Currently, we are evaluating more closely the rate of Ignite needed for Palmer amaranth as it increases in size,” he says.

With the data generated to date, Culpepper estimates that 22 ounces per acre of Ignite would control a 2-inch Palmer, 29 to 32 ounces per acre of Ignite would control a 3-inch Palmer, and 40 ounces per acre of Ignite would control 6-inch Palmer amaranth. These recommendations likely will be adjusted once more data is generated, he says.

Combinations of Ignite plus diuron are being studied, but the effectiveness of this mixture is currently not completely understood, notes Culpepper.

“It is critical that paraquat or Ignite applications be made in at least 15 gallons of water per acre using flat-fan — some type of drift guard, hopefully — spray nozzles. Read and follow all labels regarding plant back restrictions and rates of herbicides to be used on your soil type,” he says.

Culpepper warns growers not to become over-dependent on Ignite herbicide. “Numerous factors contributed to the development of glyphosate-resistant Palmer amaranth, but certainly one factor was our over-dependence on glyphosate with growers often making three or more applications per season. Glyphosate-resistant Palmer amaranth was confirmed in 2004 but was most likely first present in 2001 or 2002. Therefore, it only took four to five years once Roundup Ready cotton was commercialized until glyphosate-resistant Palmer amaranth was present in Georgia.”

Numerous growers have now started down the over-dependence path with Ignite, he says. “Palmer amaranth resistance to Ignite is inevitable if we decide to abuse this herbicide, and its loss will likely occur much more rapidly than noted with glyphosate resistance. All growers must adopt a programs approach when using Ignite, and those growers making three or four applications of Ignite per season with few to no other herbicide in the management program must change their approach or they likely will ruin the effectiveness of the Ignite system for us all.”

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