Herbicide resistance has gone from a potential threat to a reality in Georgia cotton fields, with Extension specialists confirming documented cases in several counties.
“We have done extensive work and have confirmed glyphosate-resistant Palmer amaranth in the central Georgia counties of Dooly, Macon and Taylor,” says Steve M. Brown, University of Georgia Extension cotton specialist.
Confirming resistance in a weed species is a rigorous process, says Brown. “We've looked at five specific sites in Macon County, and we've gone 35 miles from there in every direction collecting pigweed. We've planted the seed in greenhouses and sprayed about 80 different collections. Glyphosate was not effective on about 56 percent of those. There's about a 70-mile spread from Macon County where we've seen resistance, so it's not extremely isolated — there could be others who have it,” he says.
This year, University of Georgia Extension specialists and researchers will be looking at control programs for glyphosate-resistant Palmer amaranth, says Brown, including residuals and the Liberty Link system with Ignite herbicide.
In looking at pigweed resistant to the ALS herbicides, Georgia researchers have collected samples from about 70 locations within the state where problems have been detected, he says.
The large majority of those samples could not be killed with the ALS herbicide Cadre, he adds.
Other ALS herbicides include Strongarm, Pursuit and Classic. Populations of ALS-resistant pigweed have been identified in Burke, Colquitt, Cook and Mitchell counties.
A Section 18 Crisis Exemption for the herbicide Reflex could help Georgia cotton producers battle the ever-increasing threat of resistance. The exemption, granted recently, applies only to Georgia and could help growers cope with looming resistance problems. The Georgia Department of Agriculture and the EPA approved a Section 18 Crisis Exemption for the use of Reflex as a pre-emergence treatment in cotton for controlling pigweed.
While the label allows for use rates of 1 to 1.5 pints per acre, University of Georgia Extension specialists suggest that the state's growers not exceed 1 pint per acre. Also, Georgia growers should not add Dual Magnum as a pre-emergence tank mixture with Reflex because of the potential for injury on coarse-textured soils, they say. The exemption is effective until July 1, 2006.
Application instructions call for Reflex to be applied as a pre-emergence treatment using broadcast or banded application methods in a minimum of 10 gallons spray solution per acre. Application should be made only to coarse-textured soils (sandy loam, loamy sand, sandy clay loam). Adequate rainfall or irrigation within seven days of application is required for the herbicide to be activated.
In addition to providing residual control of glyphosate-resistant Palmer amaranth, the exemption states that Reflex also provides control of ALS-resistant pigweed.
Cotton plants are tolerant to pre-emergence applications of Reflex when applied at recommended rates, according to the exemption.
Some crinkling or spotting of cotton foliage or stunting may occur, especially if heavy rainfall occurs during or soon after cotton emergence, but cotton plants normally outgrow these effects and develop normally.
To broaden the weed control spectrum, Reflex may be tank-mixed with other pre-emergence herbicides such as Caparol, Cotoran, Direx, Karmex, Staple or Zorial. For control of emerged weeds, Reflex may be tank-mixed with a burndown herbicide such as Gramoxone Max, Gramoxone Inteon or glyphosate products such as Touchdown and Roundup that are labeled in cotton.
In reduced-tillage plantings, Reflex can be applied up to 14 days prior to planting or at planting with a burndown herbicide. Growers are asked to refer to the tank-mix partner label for use directions, restrictions and limitations.
The most restrictive labeling applies to the use of Reflex, including that it should not be applied as a pre-emergence treatment to medium or fine-textured soils because crop injury likely will occur, and Reflex should not be applied later than 70 days before harvest.
As for rotational crop plant-back intervals, soybeans, cotton, dry beans and snap beans may be replanted immediately. For wheat, a plant-back interval of four months should be observed.
For barley, rye, corn, peanuts, peas and rice, observe a plant-back interval of a minimum of 12 months after treatment. To avoid crop injury, do not plant alfalfa, sunflowers, sugar beets, sorghum or any other crop within 18 months.
The exemption recommends that growers avoid drift to all other crops and non-target areas.
Resistance should not be confused with other herbicide performance failures, say Extension specialists.
Among the factors that can reduce weed control with glyphosate are issues linked to environmental effects, sprayer calibration, application procedures, spray coverage, herbicide rate and weed size.