During the last decade, Serer farmers of the Sine region in the central and western part of Senegal have started to grow again the sanyo millet (Pennisetum glaucum), a long cycle (110-140 days) and photoperiodic traditional variety that had disappeared for 30 years from this region due to the rainfall decrease which has affected the Sahelian and Sudano-Sahelian zones starting from 1970, leaving only the short-cycle (90 days) souna millet in the fields. We made the assumption that the reintroduction of the sanyo millet could be an agronomic “marker” of the increase in rainfall observed in Senegal since the mid-1990s (Salack and al., 2011) attesting to the capacity of farmers to adapt to the evolution of their environment. We wanted to check, however, whether this necessary climatic opportunity was sufficient to explain farmers choices. We investigated how important was the sanyo comeback in local farming systems, its geographical diffusion, and its biophysical, economic, social and cultural drivers. We carried out (a) simulations of souna and sanyo annual development during the 1950-2013 period using the SarraH©Cirad model; (b) several Focus Group Discussions with farmers; (c) a large survey by questionnaire on farming systems among 1,061 farms in the 30 villages of the IRD human and health observatory zone of Niakhar (monitored since 50 years); and (d) a rapid survey in 240 villages of the region located between Bambey and Diourbel in the North and Fatick in the South (about 1000 km2) on whether the sanyo millet was grown and its date of reintroduction. The SarraH©Cirad crop model was parametered according to previous works. Survey data were carefully analysed using relevant statistics to assess the factors underlying sanyo reintroduction. Crop model simulations confirmed that sanyo reappearance is due to the recent rainfall improvement which now allows getting again grains with this variety whereas since 1970 grains production was only possible with souna. But simu