Fig. 9 Variability maps for destructive measurements of total dry matter at harvest compared to normalized difference vegetation index (NDVI) values taken 36 d before harvest on a millet field (trial 2) at Sadoré, Niger. Note that the darker areas in the NDVI image represent areas of low plant density. The four low vegetation E-W strips visible on map C correspond to the tracks of the linearily moved wheels of the irrigation system