Sesame was grown in four seasons in which differences in the distribution of the rains and treatments of pre-sowing flooding, irrigation during crop growth and surface drainage resulted in contrasts in soil water content and crop development. In wet years pre-sowing flooding, absence of surface drainage and premature irrigation gave 32-37% decreases in yield. In a season of low rainfall prevention of surface run-off and one later irrigation gave, in the absence of pre-sowing flooding, a 43% increase in yield over that from plots irrigated only at sowing and with surface drainage. Agronomic applications are discussed.Poor emergence and susceptibility to drought and waterlogging can make sesame (Sesamum indicum L.) a difficult crop on heavy clay (Tothill, 1948). The extreme variability of yields under rain at the Kenana Research Station on the heavy clay of the central rainlands of the Sudan (M. A. Mahmoud, F. A. Salih, unpublished;Salih, 1979) suggested that there might be a positive though probably limited role for irrigation. The present paper examines data from irrigation experiments collected in four consecutive seasons with different rainfall distributions and discusses modifications of cultivation practices.
METHODSThe field layout at the Kenana Research Station and plant sampling methods previously described (Hack, 1976a) were employed in the four years 1965-68. The cultivar was Wad en Nail Heavy Black. A selected strain with greater uniformity, A/5/13, was provided by M. A. Mahmoud in 1967 and 1968. In the experiments from which growth data are reported seeds were sown on ridges 60 cm apart and seedlings thinned to 15 cm in the row. In other experiments seed was sown on beds separated by furrows. Plots were 100 m 2 and surrounded by bunds a little higher than the ridges.