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Active sensor reflectance assessments of corn (Zea mays L.) canopy N status are advocated to direct variable N applications and improve N use efficiency (NUE). Our goals were to determine: (i) growth stage and (ii) sensor vegetation index with greatest sensitivity in assessing N status and grain yield. Variable crop N was generated by supplying N at different amounts and times in three field studies. Chlorophyll meter (CM) and sensor data were gathered at two vegetative (V11 and V15) and two reproductive (R1 and R3) growth stages, using the Crop Circle sensor that measures reflectance in visible (590 nm) and near infrared (NIR) (880 nm) bands. Sensor data were converted to the normalized difference vegetation index (NDVI590) and chlorophyll index (CI590) values. Grain yields were also determined. Sensor indices were more highly correlated with CM readings for vegetative vs. reproductive growth (r2 of 0.85 vs. 0.55). The CM vs. CI590 slope was over twice the NDVI590 slope value, indicating CI590 was more sensitive than NDVI590 in assessing canopy greenness. Indices did not diff er in ability to distinguish yield variation. Results indicate sensor CI590 values collected during vegetative growth are best suited to direct variable N applications.