Precision agriculture research has been directed toward enhancing the efficiency of N inputs by quantifying in-field variability. Remotely sensed indices such as normalized difference vegetation index (NDVI) can determine in-field N variability in maize (Zea mays L.). One method of determining NDVI is through the use of ground-based active crop canopy sensors. Several crop canopy sensors determine NDVI, however, climatic and management variables may affect NDVI readings. Our objectives were to compare two ground-based active crop canopy sensors (Crop Circle amber and GreenSeeker red) across plant growth stage, wind, crop row spacing, sensor movement speed, and N fertilizer rate under greenhouse conditions. Results show that wind had no effect on the NDVI readings of either sensor. Nitrogen rate and growth stage did affect the NDVI of both sensors with NDVI values generally increasing with increased N rate and advancing growth stage. For both sensors the V8 NDVI r 2 with N rate were lower than those observed at V10 and V12. However, the GreenSeeker (red sensor) had much lower r 2 values at V8 than the Crop Circle (amber sensor). Sensor speed had an effect on red sensor NDVI values while the amber sensor was not affected by sensor movement speed. The amber and red sensors distinguish plant N status and growth stage differences in maize in a greenhouse environment. However, the red sensor had more variability in NDVI readings and was affected by movement speed. The amber sensor shows no such limitations and therefore performed best under greenhouse conditions.