EFFECT OF DRAINAGE DATE ON YIELD AND DRY-MATTER PARTITIONING IN IRRIGATED RICE

Double cropping of rice is constrained by tight cropping calendars. In mechanized production systems such as in the Senegal River delta of A frica, drainage date is important for yield, maturity and field traffi cability. Randomized complete-block field experiments were conducted a t the WARDA research station at Ndiaye in Senegal during the hot-dry a nd wet seasons to determine effects of drainage date on soil drying, g rain yield and assimilate partitioning. The soil was a slightly acid, shrinking clay. pre-germinated seed of I Kong Pao rice was sown broadc ast and the plots kept flooded until drainage at four different dates after flowering (treatments). Topsoil and grain moisture content and t he dry weight of panicles, stems, green leaves and dead leaves were me asured on six dates between Ist flowering and 2 wk after maturity. Soi l moisture content decreased more rapidly under a green canopy than wi th a mature crop. Grain yield was reduced by 34 to 36% in both seasons when plots were drained 3 to 4 days after flowering (DAF) (P < 0.01, compared to drainage at maturity) but were not affected when drained 1 2 to 14 DAF or later. Grain yield reduction was due one-third to poor grain filling and two-thirds to unfilled spikelets. Maturity (20% grai n moisture content) was advanced 10 to 11 d by early drainage (3 to 4 DAF), compared to drainage at maturity. Apparent mobilization of reser ves based on dry weight decrease of vegetative organs after flowering was 32 to 42% and constituted 46 to 84% of panicle growth, depending o n treatment. Early drainage reduced dry matter production during ripen ing but did not proportionally affect grain yield because of compensat ory mobilization of stem reserves. Models are proposed on the bufferin g effect of reserves if assimilation during grain filling is variable.