Grain sorghum and late-emerging Palmer amaranth response to sorghum planting density and nitrogen rate in an irrigated environment

Abstract

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 trials were conducted in 2016 and 2017 to measure grain sorghum and late-emerging Palmer amaranth’s response to sorghum planting density and nitrogen rate. Trials were comprised of weed-free and weedy sorghum as main plots, three sorghum planting 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 was infested with late-emerging Palmer amaranth only. Weed-free sorghum outyielded its weedy counterpart by 42.2%. At the high sorghum planting density (296 400 plants ha−1), applying 112 kg N ha−1 did not improve grain yield or decrease Palmer amaranth’s number, height, and biomass, but increased sorghum head number and height. Altogether, our findings suggest that increasing sorghum planting density and nitrogen rate in an irrigated environment did not facilitate Palmer amaranth control. Strategies for long-season Palmer amaranth control are needed to protect sorghum yield from weed competition.