Ground-based canopy reflectance sensing for variable-rate nitrogen corn fertilization
Nitrogen available to support corn (Zea mays L.) production can be highly variable within fields. Canopy reflectance sensing for assessing crop N health has been proposed as a technology to base side-dress variable-rate N application. Objectives of this research were to evaluate the use of active-light crop-canopy reflectance sensors for assessing corn N need, and derive the N fertilizer rate that would return the maximum profit relative to a single producer-selected N application rate. 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 eight treatments from 0 to 235 kg N ha–1 on 34 kg N ha–1 increments, side-dressed between the V7–V11 vegetative growth stages. Canopy sensor measurements were obtained from these blocks and adjacent N-rich reference strips at the time of side-dressing. Within fields, the range of optimal N rate varied by >100 kg N ha–1 in 13 of 16 fields. A sufficiency index (SI) calculated from the sensor readings correlated with optimal N rate, but only in 50% of the fields. As fertilizer cost increased relative to grain price, so did the value of using canopy sensors. 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. These results affirm that, for many fields, crop-canopy reflectance sensing has potential for improving N management over conventional single-rate applications.