Control, Assessment and Glyphosate Resistance of Palmer Amaranth (Amaranthus palmeri S. Wats) in Virginia

Glyphosate resistant crops were rapidly adopted by farmers since their introduction in 1996 and currently, greater than 90% of cotton and soybean crops are glyphosate resistant. Glyphosate has been an effective mean for controlling Palmer amaranth, however overreliance on glyphosate based systems resulted in weeds that can no longer be controlled with glyphosate. Palmer amaranth resistance to glyphosate has been confirmed in ten US states including Virginia's bordering neighbor North Carolina. The objectives of this study were to i) determine the spread of Palmer amaranth and evaluate awareness among farmers and agribusinesses of herbicide resistant weeds in Virginia; ii) determine the efficacy of commonly used cotton and soybean herbicides programs for Palmer amaranth control; and iii) conduct greenhouse experiments to quantify the level of glyphosate resistance in a Greensville County, Virginia population. Using a communication network of Virginia county extension agents and crop advisers, Palmer amaranth was found in 15 Virginia counties. A survey was conducted to evaluate awareness of herbicide resistance and management of weeds in Virginia. Ninety percent of producers had fields planted to Roundup Ready® crops for each of the last 3 years. One hundred percent of the responders claimed awareness of the potential for weeds to develop resistance to glyphosate, but when asked about how serious they consider weed resistance to herbicides, the responders average rating was of 7.9 (on a scale of 1 to 10 where 1 is "not at all serious" and 10 is "very serious" ). Eighteen percent of the responder population claimed no awareness of glyphosate resistant weeds documented in Virginia. Herbicide efficacy experiments were established in soybean and cotton fields infested with Palmer amaranth. In soybean, experiments were established in a field where Palmer amaranth was not adequately controlled with glyphosate in the previous year. Glyphosate applied at 0.87 + 0.87 + 1.74 kg ae ha⁻¹ at 1, 3, and 5 weeks after planting (WAP) provided 82 to 85% control in 2009, but only 23 to 30% control in 2010, a hot and dry year. Glyphosate applied after preemergence (PRE) herbicides improved control to 90 percent. Programs that included s-metolachlor + metribuzin applied preemergence and followed by glyphosate + fomesafen applied postemergence provided the best control (93%) at 8 WAP. Glufosinate based herbicide programs provided greater than 85% control when applied alone, and control increased to 95% when preceded by PRE herbicides. Many conventional control systems integrating different modes of action provided more than 80% control at final evaluation of Palmer amaranth in 2009 and 2010. In soybean, the most consistent and effective program was flumioxazin applied PRE followed by chlorimuron + thifensulfuron, which provided 99 and 82% control at final evaluation in 2009 and 2010, respectively. Cotton fields were heavily infested with Palmer amaranth, but control with glyphosate had historically been good. Glyphosate applied early postemergence, late postemergence, and late post-directed provided more than 95 percent control at final evaluation of Palmer amaranth. Preemergence applications of fomesafen, fluometuron, or pendimethalin + fomesafen provided 77 to 99 percent early-season control and control was complete with an additional postemergence glyphosate application. Glufosinate applied at 0.45 kg ha⁻¹ at 1 and 3 WAP or applied at 0.45 kg ha⁻¹ following a preemergence herbicide provided greater than 95% control. Greenhouse experiments confirmed Palmer amaranth resistance in a population collected from Greensville County, Virginia. In the first experiment, the resistant biotype's I₅₀ value (rate necessary for 50% inhibition) for dry weight was 1.47 kg ae ha⁻¹, which is 4.6 times greater than the susceptible biotype and 1.7 times the recommended use rate of glyphosate. For fresh weight, the I₅₀ value of the resistant biotype was 1.60 kg ae ha⁻¹, 4.7 times that of the susceptible biotype of 0.34 kg ae ha⁻¹. In the second experiment, the I₅₀ value for the susceptible population could not be determined because all glyphosate rates resulted in complete control. However, the resistant population required 1.01 and 1.30 kg ae ha⁻¹ of glyphosate to reduce the fresh and dry weight by 50%.