Peter Scharf 1
Nitrogen fertilizer has a tremendous impact on crop growth and is essential for feeding the 7.4 billion people on Earth. It is also the most energy-intensive input to crop agriculture, has a proclivity to escape from ag systems, and has negative off-site impacts when it escapes. For all of these reasons, efficient use of N fertilizer is essential. Crop sensors are a promising approach to optimize N fertilizer application rate and timing. Three separate experiments with maize (corn) helped to define the N efficiency gains to this approach. One experiment group involved 55 field-scale experiments in which the farmer’s N rate was compared to variable-rate N based on crop sensors. System efficiency (N removed in grain/[N applied as fertilizer + manure]) was 0.68 with the farmer’s chosen rate, and increased to 0.78 with sensor-chosen N rates. A second experiment was initiated in 2007 to compare N fertilizer rate and timing decision systems. For 2007-2014, the most profitable pre-plant N rate (200 kg N ha-1) gave system N efficiency of 0.43, while sensor-based N rate gave system N efficiency of 0.74. The third experiment was initiated in 2012 and compared a pre-plant N rate of 155 kg N ha-1 with sensor-based variable-rate N. System efficiency for pre-plant N was 0.51, and for sensor-based N was 0.57. In the latter two experiments, pre-plant N treatments had low efficiency in years with high spring rainfall. Timing of N probably improved N efficiency more than improved N rate.