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Genetic diversity in Sahelian rice systems is very limited with more than 90% of the irrigated rice area planted to only three varieties. Recent liberalization and intensification of production require varieties that are better adapted to climatic constraints associated with evolving rice-rice and alternative rice-based cropping systems. Forty nine locally grown or introduced genotypes were characterized for their photothermal response in the field using a model described in a previous paper. Varietal constants Tbase, Topt, Tsum, BVP and CPP characterized thermal and photoperiodic effects on flowering and were validated using literature and field data from independent experiments. Photothermal constants were associated with season-specific, agronomic performance. Short-duration types with high plasticity of duration performed best in the dry season, and medium-duration types with high plasticity in the wet season. Short-duration types with low plasticity of duration performed well in both seasons. Locally grown varieties were mostly medium duration and comparatively thermo- and photoperiod-insensitive. Researchers’ selections were markedly different from locally used materials. We conclude that photothermal response of phenology is an important character for breeding improved irrigated rice in arid environments.