Economic and environmental benefits from canopy sensing for variable-rate nitrogen corn fertilization

Nitrogen (N) available to support corn production can be highly variable within fields. Canopy reflectance sensing for assessing crop N health has been proposed as a technology on which to base top-dress variable-rate N application. The objective of this research in Missouri and Nebraska was to evaluate the economic and environmental benefit of active-light crop-canopy reflectance sensors for corn N rate decisions. In Missouri, a total of 16 field-scale experiments were conducted over four seasons (2004-2007) in three major soil areas. Multiple blocks of randomized N rate response plots traversed the length of the field. Each block consisted of 8 treatments from 0 to 235 kg N ha ^-1 on 34 kg N ha^-1 increments, top-dressed between V7-V11 vegetative growth stages. Canopy sensor measurements were obtained from these blocks and adjacent N-rich reference strips. A sufficiency index calculated from the sensor readings correlated with optimal N rate, but only in 50% of the fields. While soil type, fertilizer cost, and corn price all affected our analysis, a modest ($25 to $50 ha^-1) profit using canopy sensing was found. Fertilizer savings of 10 to 50 kg N ha^-1 could be expected in most situations, but savings also varied by reflectance readings, soil type, and fertilizer and grain prices. In the Nebraska studies, canopy sensing for one site allowed 39% savings in N applied compared to the traditional N management strategy, while producing similar grain yields. These results affirm using crop-canopy reflectance sensors for detecting corn N fertilizer needs that vary spatially within fields.