LOWLAND RICE YIELD AS AFFECTED BY TIMING OF WATER DEFICIT AND NITROGEN-FERTILIZATION

Water deficit and insufficient N supply are important constraints to the productivity of rainfed lowland rice (Oryza sativa L.). To determine the effects of water deficit and urea application timing on yield and N accumulation of lowland rice, a field experiment was conducted for 2 yr on an Andaqueptic Haplaquoll in the Philippines. Water regimes imposed in the dry season were continuously flooded, not flooded or irrigated from 15 to 35 d after transplanting (DT), and not flooded or irrigated from 41 to 63 DT. Urea treatments in factorial combination with the water regimes were no applied N, early N (80 kg N ha-1, one-half basal incorporated and one-half broadcast at 37 DT), and late N (80 kg N ha-1, one-half broadcast at 11 DT and one-half broadcast at 65 DT). Water deficit at 15 to 35 DT significantly reduced mean grain yield (26%), straw yield (21%), grain N accumulation (31%), and straw N accumulation (31%). Water deficit at 41 to 63 DT, which ended before panicle emergence, decreased yield and N uptake, but less than water deficit at 15 to 35 DT. Nitrogen application consistently increased grain yield even when the rice crop was exposed to water deficit. Mean grain yields were greater (P < 0.05) for early N (5.7 Mg ha-1) than late N (3.7 Mg ha-1) in 1986 but not in 1987 (4.1 and 3.8 Mg ha-1, respectively). Water-by-N interactions were not significant for grain or straw yields and grain or straw N accumulations. The N timing most effective for continuously irrgated rice was also most effective for rice exposed to moderate water deficit before flowering.