MANAGEMENT OF FERTILIZER NITROGEN IN DRY-SEEDED, DELAYED-FLOOD RICE

Approaches taken in management of fertiliser nitrogen (N) in dry-seeed , delayed-flood rice (Oryza sativa L.) in the United States of America vary considerably. A 2-year study was conducted in Louisiana to evalu ate N fertiliser efficiency and accumulation when applied in single or multiple applications. The cultivars Lemont and Tebonnet, representin g semi-dwarf and conventional tall plant types, respectively, were gro wn on a Crowley slit loam (Typic Albaqualfs soil). Urea N was applied using 1 of 3 strategies: in a single (1-way) preflood (PF) application of 134 kg N/ha; in a 2-way split consisting of 100 kg N/ha PF and 34 kg N/ha at panicle initiation (PI); or in a 3-way split consisting of 66 kg N/ha PF, 34 kg N/ha at PI, and 34 kg N/ha at panicle differentia tion. Microplots were established within each whole plot and fertilise d with 2 atom% N-15 urea. Fertiliser N application time had no influen ce on plant accumulation of N-15 urea or biomass production. Plant acc umulation of N-15 urea was significant across year and cultivar. Tebon net accumulated more N-15 urea than Lemont. Accumulation was also sign ificantly higher in 1992. Method of application of fertiliser N signif icantly affected grain yields. The highest average grain yield of 7905 kg/ha occurred with a single PF application. Yields were reduced by 3 78 kg/ha with a 2-way split application and by 1074 kg/ha with a 3-way split application. Method of application of main crop fertiliser N ha d no effect on ratoon crop yields. Total grain yield (main + ratoon) w as significantly reduced with the 3-way split. This study demonstrates the importance of supplying adequate N to the rice plant before perma nent flood establishment. It also suggests that midseason N applied at specific growth stages is less efficiently utilised, especially when insufficient N is applied at PF.