Season and plant type affect the response of rice yield to nitrogen fertilisation in a semi-arid tropical environment

Production of flooded direct-seeded rice (Oryza sativa L.) in semi-arid tropical regions of northern Australia would be enhanced by increasing the efficiency of fertiliser nitrogen (N) use. Short-statured and early-maturing genotypes have replaced the taller and later genotypes in northern Australia, and they may respond differently to N. This paper reports the results of 4 experiments comparing the response of 3 rice genotypes differing in maturity and stature to 5 rates of applied nitrogen (0, 70, 140, 210, and 280 kg/ha) over 4 seasons (2 wet and 2 dry seasons) in the Burdekin River Irrigation Area, northern Australia. Grain yield varied among seasons and was negatively correlated with average 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 seasons, 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 kg 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 genotypes (Newbonnet and Lemont) was more responsive to N rates above 70 kg/ha than in the late-maturing genotype (Starbonnet). Of the 3 genotypes examined, highest yields were attained in Newbonnet (early-maturing, medium-statured) by combining high total dry matter production with high harvest index, indicating that this plant type may have an advantage in northern Australia. Yields in Lemont (early-maturing, short-statured) and Starbonnet (late-maturing, tall-statured) were limited by dry matter production and harvest index, respectively. There is some evidence that increased dry matter production in Newbonnet compared with Lemont was related to increased stem length. The evidence linking high harvest index with increased earliness in Newbonnet compared with Starbonnet is less compelling.