Spatial growth and nitrogen uptake variability of corn at two nitrogen levels

Crop measurements that predict spatial yield variability and correlate with yields may facilitate development of variable N rate management. We evaluated growth, N concentration, and N uptake of two corn (Zea mays L.) hybrids at two N rates (130 vs. 185 kg ha(-1) at two sites and manure vs. manure + 55 kg N ha(-1) at another site) in 2000 (wet) and 2001 (dry) to help explain corn spatial yield variability that existed at five of six site-years. Biomass, N concentration, and N uptake mostly had no spatial variability at the sixth leaf stage (V6), silking (111), and physiological maturity (R6). Plant height at V6, which had spatial variability at four site-years, correlated with yields at two sites (0.55 and 0.66) in 2000 and at one site (0.56) in 2001 Plant height at V10, which had spatial variability at all sites, correlated with yields at all sites (0.50-0.64) in 2000 but at only one site (0.69) in 2001. Nitrogen uptake at R6, which correlated with yields at all sites in 2000 (0.25-0.58) and 2001 (0.34-0.65), did little to explain the N response within sites. Stalk NO3-N concentrations showed no spatial variability, despite spatial variability for residual soil NO3-N concentrations at all site-years. Residual soil NO3-N, which had distinct zones of high concentrations in the upper 30 cm in 2001, may provide more useful information than crop measurements for development of variable N rate management.