Palmer amaranth (Amaranthus palmeri) control affected by weed size and herbicide spray solution with nozzle type pairings

Palmer amaranth can grow 4.2 mm in height per degree day; hence, delays of a few days in weed control deployment can result in applications to larger than labeled weeds. Therefore, it is critical to understand the impact of plant size at the time of application in conjunction with herbicide spray solution and nozzle type pairings on the effectiveness of weed management programs in the Enlist E3 and XtendFlex production systems. Field experiments were conducted in 2020, in no-crop conditions, at the Milo J. Shult Agricultural Research & Extension Center in Fayetteville, AR, and the Jackson County Extension Center near Newport, AR, to evaluate the influence of Palmer amaranth size on its control with glufosinate, dicamba, and 2,4-D applied alone and in mixture with specific nozzle pairings as mandated by label requirements. Also, a laboratory experiment was conducted at the Lonoke Extension Center (Lonoke, AR) to evaluate the droplet size and velocity of the spray solutions and nozzles used for the field experiment. A 5- and 10-percentage point reduction in control was observed when applying dicamba (66%) and 2,4-D (63%) alone, respectively compared to each mixed with glufosinate (71% and 73%, respectively). Palmer amaranth density increased to 55, 73, 100, 115, and 140 plants m-2 when 15-, 25-, 41-, 61-, and 76-cm tall plants were sprayed, respectively, compared to when 5-cm tall plants were sprayed (9 plants m-2). Nozzle types did not impact weed control and density. The addition of glufosinate to 2,4-D increased the percentage of driftable fines compared to 2,4-D alone. Effective short- and long-term chemical control of Palmer amaranth will require growers to be timely with their weed management programs, overlay residuals, and expect the need for sequential applications.