Optimal nitrogen fertilizer rate for corn can vary substantially within and among fields.  Current N management practices do not address this variability.  Crop reflectance sensors offer the potential to diagnose crop N need and control N application rates at a fine spatial scale.  Our objective was to evaluate the performance of sensor-based variable-rate N applications to corn, relative to constant N rates chosen by the producer.  Fifty-five replicated on-farm demonstrations were conducted from 2004 to 2008.  Sensors were installed on the producer’s N application equipment and used to direct variable-rate sidedress N applications to corn at growth stages ranging from V6 to V16.  A fixed N rate chosen by the cooperating producer was also applied.  Relative to the producer’s N rate, sensors increased partial profit by $42 ha-1 (P = 0.0007) and yield by 110 kg ha-1 (P = 0.18) while reducing N use by 16 kg N ha-1 (P = 0.015).  This represents a 24 to 26% reduction in the amount of N applied beyond what was removed in the grain, thus reducing unused N that can move to water or air.  Our results confirm that sensors can choose N rates for corn that perform better than rates chosen by producers.