Will variable-rate nitrogen fertilization using corn canopy reflectance sensing deliver environmental benefits?
Within-field variability of corn (Zea mays L.) N need calls for development of precision fertilizer application strategies. One approach many are investigating is in-season canopy reflectance sensing. Justification for this strategy partly rests with the premise it will improve N use and reduce N loss from fields. The objective of this study was to determine the potential environmental benefits using corn canopy reflectance sensing for N fertilization. On 16 field-scale sites, multiple blocks of randomized N rate plots (0–235 kg N ha–1) traversing fields were side-dressed between the V7 and V11 growth stages. Sensor measurements were obtained from these and adjacent N-rich reference strips at side-dressing. Environmental indicators were examined at the determined optimal nitrogen rate (Noptimal) and the nitrogen rate the producer used (Nproducer). A partial nitrogen mass balance (PNB) on response blocks within fields highlighted how variable Noptimal likely resulted in multiple and different N loss pathways. For many fields, Noptimal was less than Nproducer, and the observed trends were as expected: higher yield efficiency (YE), higher nitrogen fertilizer recovery efficiency (NFRE), lower unaccounted for N, and less postharvest inorganic N. For a measurement examining canopy sensor-based N applications, N savings of 10 to 50 kg N ha–1 would be expected, but savings varied by reflectance readings, soil type, and fertilizer and grain prices. In some situations sensor-based N would be greater than Nproducer. Given that sensor information can be processed into an N rate that approximates Noptimal, the results support sensor-based N applications have potential for environmental benefits.