Grain Sorghum and Late-emerging Palmer amaranth Response to Sorghum Density and Nitrogen Rate in an Irrigated Environment

Palmer amaranth germination and emergence occur throughout the growing season; however, little is known about the impact of late-emerging Palmer amaranth on sorghum, a major crop in Kansas. Field studies were conducted in 2016 and 2017, to measure grain sorghum and late-emerging Palmer amaranth’s response to sorghum population density and nitrogen rate. Treatments comprised weed-free and weedy sorghum as main plots, three sorghum population densities as sub-plots, and three nitrogen rates as sub-sub-plot treatments laid in a randomized complete block design with a split-split-plot arrangement. Weedy sorghum consisted of late-emerging Palmer amaranth only. Weed-free sorghum out-yielded its weedy counterpart by 68 and 45% in 2016 and 2017, respectively. At high sorghum population density (296,400 plants ha-1), applying 112 kg N ha-1 did not improve sorghum grain yield or decrease Palmer amaranth number and height but increased sorghum head number and height, and reduced Palmer amaranth biomass by 65%. Altogether, our findings suggest that while there is an opportunity to maintain grain sorghum yield and achieve some Palmer amaranth control with the integration of high sorghum population density (296,400 plants ha-1) and nitrogen rate (224 kg ha-1) in an irrigated environment, late emerging Palmer amaranth can still cause significant yield reduction (>55%).