William Cox1, Harold M. Van Es1, and Tawainga Katsvairo2. (1) Cornell Univ, Ithaca, NY 14853, (2) Univ of Florida, 155 Research Road, Quincy, FL 32352
The challenge for variable N rate management is to identify specific fields 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 at three sites and manure vs. manure + 55 kg N/ha fertilizer at two sites) for 3 years to determine if corn responded differently to N levels within fields. Spatial yield variability existed at all sites in dry years but at only two sites in a wet year. Spatial yield difference variability in response to N existed at only two of 15 site-year comparisons. Although late-spring soil nitrate concentrations in the upper 30 cm were less than 25 mg/kg 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 non-manured 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 late spring soil nitrate concentrations and yield maps. With this in mind, we evaluated growth, N concentration, and N uptake of the two corn hybrids at the 6th leaf (V6), silking (R1), and physiological maturity (R6) growth stages to try to explain the spatial yield variability that existed at three fields in the last two years of the study. Biomass, N concentration, and N uptake mostly showed no spatial variability at the three growth stages. Plant height at the V6 stage, which showed spatial variability at four of the six site-years that crop measurements were taken, correlated with yields at two sites (r=0.55 and 0.66) in the wet year and at one site in the dry year (r=0.56). Plant height at the V10 stage, which had spatial variability at all sites, correlated with yields at all sites (r=0.55-0.66) in the wet year and at one site (r=0.56) in the dry year. Nitrogen uptake at the R6 stage, which correlated with yields at all sites in the wet year (r=0.25-0.58) and the dry year (r=0.34-0.65) did little to explain the N response within sites. Stalk nitrate concentrations showed no spatial variability, despite spatial variability for residual soil nitrate concentrations at all sites. Residual soil nitrate concentrations, which had distinct zones within a field in the upper 30 cm in the dry year, may provide useful information for development of variable N rate management for subsequent years.