Corn Plant Density Effects on Grain Yield and Palmer Amaranth Response to Dicamba

Palmer amaranth (Amaranthus palmeri S. Wats.) is primarily found in fallow and cultivated fields throughout Kansas (KS). Previously, we reported that the effective dose of dicamba for 50% control (ED50) of a 10-30 cm tall A. palmeri accession from KS (KSP) during the fallow-year of a wheat-corn-fallow rotation (WCF) ranged between 40.4 to 283.5 g ae ha-1 Here, we investigated corn (Zea mays L.) plant density effects on grain yield and the KSP response to dicamba in the corn-year of the WCF rotation. The experiments used a randomized complete block design with four replicates and a split-plot arrangement of treatments. Main plots consisted of corn planted at five densities (49,400; 61,700; 74,100; 86,400 and 98,800 plants ha-1) and sub-plots consisted of six doses of dicamba [(70, 140, 210, 280, 420, and 560 g ae ha-1) applied near V6], a weedy-check, and a weed-free check. In general, acceptable grain yield and KSP control with dicamba doses < 560 g ae ha-1 were only achieved when corn was planted at a density ≥ 74,100 plants ha-1. Based on the ED50 estimates, the KSP required 4.4-8.2% less dicamba for height and biomass reduction as corn plant density increased from 49,400 to 98,800 plants ha-1. Conversely, the amount of dicamba required for 50% reduction of KSP density increased 45.1% from 144.4 to 209.6 g ae ha-1 as corn plant density increased from 49,400 to 98,800 plants ha-1, respectively. Taken together, this study shows that integration of high-density corn planting with dicamba is a cost-effective tool for controlling A. palmeri. However, resistance mitigation and a season-long control of this species cannot be achieved without the integration of other more diversified and robust strategies that include the use of preemergence fb postemergence herbicide programs that overlap residuals with cultural, biological, and mechanical weed control tactics.