Optimal nitrogen (N) fertilizer rate for corn can vary substantially within and among fields. 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 relative to a constant N rate chosen by the producer. Fifty-five on-farm demonstrations were conducted from 2004 to 2008. Sensors were installed on the producer’s nitrogen application equipment and used to direct variable-rate sidedress nitrogen 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 and yield while reducing N use. Our results confirm that sensors can choose N rates for corn that perform better than rates chosen by producers.