High residue cover crops alone or with strategic tillage to manage glyphosate-resistant palmer amaranth (amaranthus palmeri) in Southeastern cotton (gossypium hirsutum)

Glyphosate-resistant (GR) Palmer amaranth (Amaranthus palmeri S. Wats) is redefining row crop weed management in the Southeast due to its widespread distribution, high competitive ability, copious seed production, and resilience to standard weed management programs. Herbicides alone are failing to provide adequate control of Palmer amaranth; thus adding cultural practices to the weed management program were evaluated to determine if they would provide additional weed suppression in cotton (Gossypium hirsutum L). A replicated, on-farm research and demonstration project was initiated in the fall of 2009 in Alabama, Georgia, South Carolina, and Tennessee and continued in certain states for two additional years to address valid concerns that conservation tillage systems were at risk. In the Southeast, use of tillage is increasing in efforts to gain control of GR Palmer amaranth. In this project, two high residue cover crop management systems and a standard grower system of using conservation tillage without a cover crop were compared at each location. The cultural systems contrasted were a high-residue cover crop conservation tillage system, fall inversion tillage followed by planting a high-residue cover crop conservation tillage system, and winter fallow conservation tillage system. Cover crop biomass yields varied from 570 to 6,790 kg ha(-1) depending on location, environment, and type of cover crop. Fall tillage increased cover crop yields. The effects of tillage on GR Palmer amaranth density varied with Palmer amaranth density across locations. Where GR Palmer amaranth densities were relatively low, (approximately 1,000 plants ha(-1) or fewer), there were few differences in GR Palmer amaranth escapes among treatments. Where GR Palmer amaranth densities were relatively high (18,000 plants ha-1 or greater), winter fallow systems had higher Palmer amaranth densities escaping weed management programs compared to either cover crop system. For sites with two years of data, no year by treatment interaction was detected, indicating that GR Palmer amaranth densities persisted for two seasons regardless of treatments. Among locations, trends in cotton yields were the opposite of those for Palmer amaranth escapes. High residue cover crops tended to suppress Palmer amaranth and increase cotton lint yields. However, no cultural management system consistently netted greater net returns than did other systems across locations and years.