SEASON AND PLANT TYPE AFFECT THE RESPONSE OF RICE YIELD TO NITROGEN-FERTILIZATION IN A SEMIARID TROPICAL ENVIRONMENT

Production of flooded direct-seeded rice (Oryza sativa L.) in semi-ari d tropical regions of northern Australia would be enhanced by increasi ng the efficiency of fertiliser nitrogen (N) use. Short-statured and e arly-maturing genotypes have replaced the taller and later genotypes i n northern Australia, and they may respond differently to N. This pape r reports the results of 4 experiments comparing the response of 3 ric e genotypes differing in maturity and stature to 5 rates of applied ni trogen (0, 70, 140, 210, and 280 kg/ha) over 4 seasons (2 wet and 2 dr y seasons) in the Burdekin River Irrigation Area, northern Australia. Grain yield varied among seasons and was negatively correlated with av erage daily mean temperature during the 30-day period before anthesis. The response of yield to N fertilisation was generally higher in the dry season. Panicle number was correlated with grain yield in both sea sons, yet responded to N fertilisation only in the dry season. In 3 of 4 experiments, grain yield responded to the application of up to 70 k g N/ha, yielding about 750 g/m(2). In only 1 dry season experiment did grain yield respond to the application of 140 kg N/ha, yielding about 930 g/m(2). In this experiment, the response of grain yield to N rate also varied among genotypes such that yield in the early-maturing gen otypes (Newbonnet and Lemont) was more responsive to N rates above 70 kg/ha than in the late-maturing genotype (Starbonnet). Of the 3 genoty pes examined, highest yields were attained in Newbonnet (early-maturin g, medium-statured) by combining high total dry matter production with high harvest index, indicating that this plant type may have an advan tage in northern Australia. Yields in Lemont (early-maturing, short-st atured) and Starbonnet (late-maturing, tall-statured) were limited by dry matter production and harvest index, respectively. There is some e vidence that increased dry matter production in Newbonnet compared wit h Lemont was related to increased stem length. The evidence linking hi gh harvest index with increased earliness in Newbonnet compared with S tarbonnet is less compelling.