Late-Season Corn Measurements to Assess Soil Residual Nitrate and Nitrogen Management

Evaluation of corn (Zea mays L.) N management and soil residual NO3-N late in the growing season could provide important management information for subsequent small grain crops and about potential NO3-N loss. Our objective was to evaluate the ability of several late-season corn measurements, which have been advocated to assess N management, to identify sites with elevated soil residual NO3-N. These crop-based measurements were collected at three reproductive phases and included normalized difference vegetative index (NDVI) at 10 site-years and green-leaf number and chlorophyll (SPAD) meter readings at six of these sites. The corn stalk nitrate test (CSNT) and postharvest soil residual NO3-N were measured at all sites. Four levels of N were applied, ranging from N deficient (0 or 67 kg N ha(-1)) to excessive (269 kg N ha(-1)). The CSNT was positively (p < 0.001) correlated with residual NO3-N, although residual NO3-N was not always low at CSNT values <2.0 g NO3-N kg(-1), where drought reduced production. Drought stress was a major factor influencing excess N supply and residual soil NO3-N. Canopy measurement values at growth stages R3-R4, including NDVI, which can be measured remotely, were effective indicators of drought stress. Across sites, relative canopy readings best predicted relative grain yield when collected at R3-R4, underscoring the importance of reference strips. Use of remotely measured NDVI would allow policymakers to identify drought sites in the late summer and target them for cover crop planting, thus reducing potential winter NO3-N losses in humid regions.