The use of sensors that allow the application of nitrogen fertilizer at variable rate has been widely used by researchers in many agricultural crops, but without success in sugarcane, probably due to the difficulty of diagnosing the nutritional status of the crop for nitrogen (N). Active crop canopy sensors are based on the principle that the spectral reflectance curve of the leaves are modified by N level. Researchers in USA indicated that in-season N stress in corn can be detected using a crop canopy sensor and N rate algorithms could be developed to make in-season N rate applications. The objective of this study was to evaluate the nutritional status of sugarcane, fertilized with different nitrogen rate applied at different periods after harvest, using a crop canopy sensor in order to generate parameters to establish the most sensitive growth stage to detect N deficiency in order to guide N application. The experiment was installed in a commercial area of sugarcane (first ratoon), cultivated with the variety IACSP95-5000. The experimental design was randomized blocks in 5x5 factorial design with four replications, with five nitrogen rates (0, 50, 100, 150 and 200 kg ha^-1 N) applied at five different periods after sugarcane harvest (immediately after harvest, 30, 60, 90 and 120 days after the harvest - DAH). During the crop growing (30, 60, 90, 120 and 150 DAH) the N nutritional status of the crop was assessed through leaf laboratory analysis and the results compared with canopy reflectance reading using an active optical sensor (ACS-430, Holland Scientific Crop Circle, Lincoln, NE, USA). In all evaluation periods, crop biometric parameters (tillers m^-1, height and diameter of tillers) were measured. Pearson’s correlation shows that the vegetation index presented the highest correlation with tiller height (R² = 0.46 – average 0.194 m) and the number of tillers (R² = 0.63 - average 20 tillers m^-1) when evaluated at 90 DAH, indicating that the most appropriate period for use of active optical sensor in the field to predict N deficiency in sugarcane is around 90 DAH with 16-20 tillers m^-1 and average height 0.190 m.