Palmer amaranth influences selection of crop production practices such as irrigation, nitrogen (N) application, and weed control. The objectives of this research were to determine if Palmer amaranth was more responsive to applied N than corn and if this differed under dryland and irrigated conditions in Kansas. Field experiments were conducted near Manhattan, KS, in 2005 and 2006 to evaluate the influence of N rate and Palmer amaranth densities when grown with corn in two soil moisture environments. A very drought-stressed environment and a well-watered environment occurred in 2006, while both environments in 2005 were intermediate. Dryland weed-free corn yields were 46.5% of irrigated corn yields at the high N rate across years. Irrigated corn yields responded to increasing N rates. In the presence of Palmer amaranth, parameter estimates I and A for the yield loss relationship were not different across N rates for each environment and year except 2006 where 100% yield loss was estimated in dryland compared to 62.5% loss in irrigated environment at high N rates. In three of four environment-years, N rate did not affect the corn yield loss relationship with weed density. In 2006 irrigated environment, greater N rates had less corn yield loss caused by Palmer amaranth. By corn anthesis, weed-free corn biomass was 167.5% greater in irrigated than dryland environments in 2006. Palmer amaranth with no corn increased its biomass by 373 and 361% as N rate increased in 2005 and 2006, respectively. Nitrogen concentrations in plant tissues of corn or weed increased similarly as N rates increased from 0 to 224 kg N ha⁻¹, thus highlighting that both corn and Palmer amaranth responded similarly to increasing N. In general, soil moisture environment was most critical when determining potential corn yield, followed by Palmer amaranth density and N rate.

Nomenclature: Palmer amaranth, Amaranthus palmeri S. Wats. AMAPA; corn, Zea mays L. “DKC60-19RR”.