Spatial yield response of two corn hybrids at two nitrogen levels

The challenge for variable N rate management is to identify specific field areas that respond to specific N levels. We evaluated two corn (Zea mays L.) hybrids at two N rates (110-130 vs. 165-185 kg ha(-1) at three sites and manure vs. manure + 55 kg ha(-1) at two sites) to determine if corn responded differently to N and hybrids within fields. Spatial yield variability existed at all sites in dry years (1999 and 2001) but at only two sites in a wet year (2000). Spatial yield difference variability in response to N existed at only two of 13 site-year comparisons. Although late-spring soil NO3-N concentrations in the upper 30 cm were <25 mg kg(-1) on 15 to 25% of the manured fields in the wet year, spatial yield difference variability in response to N did not exist. At a nonmanured site, spatial yield difference variability in response to N existed with temporal yield stability across dry years (r = 0.96). Surprisingly, corn responded to the higher N rate on 25% of this field where yields were least, but not where yields were greatest. Apparently, variable N rate management of corn requires more information than soil NO3-N concentrations and yield maps. Spatial yield difference variability between hybrids existed at only four of 15 site-year comparisons, despite hybrid interactions with sites. Adoption of variable hybrid selection is unlikely if hybrids that show interactions with sites do not show spatial yield difference variability within sites.