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Abstract Assessment of crop N requirements is necessary to develop production systems with optimal N input. A field experiment with six maize ( Zea mays L.) hybrids grown at three N fertilizer rates (0, 100, and 200 kg N ha −1 ) was conducted on a well‐drained sandy loam of the Grenville series (coarse‐loamy, mixed, mesic Typic Eutrochrepts) on the Central Experimental Farm at Ottawa, ON, in Canada (45°23' N, 75°43' W) for 3 yr (from 1991 to 1993) to evaluate whether canopy reflectance and greenness can measure changes in maize yield response to N fertility. Canopy reflectance, leaf area and greenness were measured on 11 dates from 4 wk before to 4 wk after anthesis. Grain yield at harvest was also measured. Direct radiometer readings at the 600‐ and 800‐nm wavelengths or a derived normalized difference vegetation index NDVI = (800 nm − 600 nm)/(800 nm + 600 nm) best differentiated N and hybrid treatments at most sampling dates. Canopy light reflectance was strongly correlated with field greenness at almost all growth stages (field greenness being a product of plant leaf area and leaf greenness measured with a chlorophyll meter, in this case a SPAD‐502). Both canopy light reflectance and field greenness measured preanthesis were correlated with yield at harvest. Light reflectance measured after anthesis differentiated hybrid differences in leaf senescence. Our data suggest that light reflectance measurements prior to anthesis may predict grain yield response and provide in‐season indications of N deficiency.
L. et al. (Fri,) studied this question.