Palmer amaranth (Amarnthus palmeri S. Wats.) is a member of the Amaranthaceae or pigweed family. It is also known as carelessweed or palmer pigweed. It is an erect, branched summer annual that can grow to over seven feet tall and emerge throughout the growing season. One Palmer amaranth plant per 30 foot of row can reduce cotton yields by 6 to 12%, making it the most competitive of pigweeds found in cotton. Plants have either male flowers that shed pollen or female flowers that can produce up to 600,000 seed per plant.
In Virginia, Palmer amaranth has only been documented in 13 counties, but will likely enlarge its territory. Its spread can be rapid because of custom harvesting, failing to clean vehicles and equipment after exiting infested fields, and failing to hand remove escapes. More troublesome is that extensive use of herbicides has led to resistance to glyphosate and ALS-inhibiting herbicides. In Virginia, Palmer amaranth populations resistant to ALS-inhibiting herbicides have been confirmed and glyphosate-resistant Palmer amaranth is suspected. Because of the weed's rapid spread and its tendency to develop resistance to herbicides, experiments were conducted to evaluate the efficacy of herbicides and herbicide programs in a Suffolk field where ALS-inhibiting herbicide resistance has been confirmed. This publication summarizes those experiments.
Site Description and Experimental Methods
Experiments were established in Suffolk, VA in 2008 and 2009. In 2008, a cover crop of rye and vetch was established in the fall of 2007. Glyphosate was sprayed in April to kill the cover crop. Rye was killed, but vetch was not adequately controlled (Fig. 1a). Additional glyphosate applications eventually killed the vetch, but more rapid control of vetch was obtained with Gramoxone Inteon applied with the preemergence herbicide. No cover crop was present in 2009, but a wheat-soybean residue provided good soil cover (Fig. 1b). In April, the field was sprayed with glyphosate, 2,4-D, and Valor herbicides to kill existing vegetation. Cotton was strip-till planted in 36-inch rows on May 21, 2008 and May 3, 2009. In both years, part of the field was planted to Roundup-Ready Flex® cotton (Deltapine DP143B2RF in 2008; PhytoGen PHY315RF in 2009) and part was planted to Liberty-Link® cotton (FiberMax FM1735LLB2 in 2008; FiberMax FM958LL in 2009). Roundup-Ready Flex® cotton has been genetically engineered to tolerate glyphosate herbicide and Liberty-Link® cotton has been genetically engineered to tolerate glufosinate herbicide. Herbicide treatments were applied with a CO2 unicycle sprayer traveling at 3 miles per hour and using 80015 flat fan spray tips at 15 gallons per acre and 32 pounds per square inch of operating pressure. Application timings and dates are shown on Figures 2 through 5. Applications were generally made before Palmer amaranth plants reached 3 to 4 inches in height. Several weed species were present, but only Palmer amaranth control ratings are presented.
Control in Roundup-Ready Flex® Cotton
In 2008, greater than 90% control of Palmer amaranth was obtained with an early glyphosate application (Fig.2). Preemergence herbicides provided 65 to 80% control. Control in all plots improved to greater than 95% after glyphosate was applied to 5- to 6-leaf cotton.
In 2009, weed pressure was not as great. But, as in 2008, glyphosate alone provided excellent control of Palmer amaranth (Fig. 3). In addition, Reflex or Prowl + Reflex controlled more than 95% of the weeds present. Control with Prowl was similar to 2008. Control with Cotoran was better than in 2008, but not as good as Reflex. As in 2008, control was greater than 95% after glyphosate was applied to 4- to 6-leaf cotton.
Control in Liberty-Link® Cotton
In 2008, two of four replications did not contain a cover crop. This provided for evaluation of the cover crop effect on control of Palmer amaranth. When preemergence herbicides were used, the cover crop improved control by 10 to 30%, depending on the herbicide used (Fig. 4). However, if no preemergence herbicide was used, control declined with glufosinate (Ignite 280). This was likely due to interception of the herbicide by the vetch cover, not allowing good coverage of the weeds. After the Ignite application to 2- to 4-leaf cotton, differences between plots with and without residue cover disappeared. All herbicide treatments provided over 90% control.
In 2009, control was generally better than in 2008. It is likely that the wheat and soybean residue remaining from the previous year plus residual control from the pre-plant application of Valor minimized the weed pressure. Also note from the photograph on page 1 that more weeds emerged within the tilled strip than between the strips. By incorporating residue and herbicide with the strip-tillage operation, the residue effect is largely negated and the herbicide is diluted. In addition, it brings buried seed to the germination zone. An early application of glufosinate and a preemergence application of Reflex gave greater control than in 2008 (Fig. 5). On the other hand, control with preemergence applications of Prowl and Cotoran was less than in 2008. After glufosinate was applied at the 2- to 4-leaf cotton stage, control increased to greater than 90% for all treatments.
Palmer amaranth was adequately controlled with all glyphosate treatments at this location. While this may imply that a glyphosate alone weed control program is a viable option, the use of residual herbicides will reduce the likelihood of glyphosate-resistant Palmer amaranth of being established. In a soybean experiment conducted in Greensville County in 2009, three applications of glyphosate did not fully control this weed. Glufosinate weed control programs also adequately controlled Palmer amaranth at this location. Liberty-Link® cotton offers a good alternative to Roundup-Ready Flex® systems, especially if growing continuous cotton or rotating with another Roundup-Ready® crop such as soybean. Rotating between these two systems will allow one to better integrate other herbicide mechanism of actions into their weed management, therefore forestalling potential resistant weeds. However, one must keep in mind that glufosinate must be applied to weeds 3 inches tall or less. In addition, it works best when integrated with preemergence residual herbicides. Finally, note that all treatments in these experiments included a layby residual herbicide application. Cotton does not form as good of a canopy as other crops, therefore does not naturally shade out late emerging weeds. It is recommended that a layby herbicide continue to be included in cotton weed control programs to prevent lateseason weed emergence and prevent Palmer amaranth seed production.
The authors would like to acknowledge the following for their financial support of this research: The Virginia Agricultural Council, Cotton Incorporated, the Virginia Cotton Board, and Bayer CropScience. We also would like to thank Ms. Gail White and Mr. David Horton for their technical expertise and assistance in establishing these experiments, applying herbicide treatments, and summarizing the data. We especially wish to thank Mr. Tommy Roundtree and David Bosselman for use of land for this study.
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Issued in furtherance of Cooperative Extension work, Virginia Polytechnic Institute and State University, Virginia State University, and the U.S. Department of Agriculture cooperating. Edwin J. Jones, Director, Virginia Cooperative Extension, Virginia Tech, Blacksburg; M. Ray McKinnie, Administrator, 1890 Extension Program, Virginia State University, Petersburg.
December 22, 2009