Growth and yield of rice in the Murrumbidgee Valley as influenced by climate, method of sowing, plant density and nitrogen nutrition

The climate of the Murrumbidgee Irrigation areas varies only slightly regionally; the long summer days with high solar radiation allow a growing season of 190-200 days and are conducive to high yields. Nevertheless, appreciable variations in annual yields occur, these being associated with low temperatures at sowing and particularly at flowering. A high proportion of sterile florets are induced with minimum temperatures of less than 15°C, this effect being accentuated by high concentrations of nitrogen. Two series of field experiments were made at two sowing dates over two seasons. In one series the varieties Calrose, Kulu and Baru were sown either by combine or aerially with three rates of nitrogen fertiliser. In the other the same three varieties were compared at two densities and two rates of nitrogen supply. Detailed growth analyses were made from fortnightly harvests. Aerial sowing gave the most rapid early growth leading to a large leaf area, especially with high nitrogen. Lodging later in the season was encouraged and the yield was frequently reduced below that of combine-sown rice with an intermediate supply of nitrogen, the yield reduction being reflected by a high degree of floret sterility and low grain weight. These and other differences between the varieties and the.treatments explored were analysed in detail throughout the season. One interesting effect was an apparent increase in growth rate late in the season. Low density resulted in slow early growth but differences between densities narrowed during grain-filling. Nitrogen had a larger effect than density. Calrose appeared to be the best adapted variety, tillering more profusely than the thick-stemmed Baru with its large panicle. Although Baru had the highest potential for yield, this was rarely realised mainly because of high floret sterility. Kulu tillered too profusely and was also prone to floret sterility. A reasonable detailed analyses of the nitrogen balance was made. The rate of nitrification during fallow was about O.46 ppm d ̄ ¹ and after adding sulphate of ammonia this rose to 2.2 ppm d ̄ ¹during the germinating period prior to permanent flood. Denitrification is extremely rapid after permanent flood. Ammonium ions are not lost when reducing conditions are induced; hence the most effective use of nitrogen is obtained with aerial sowing or applying it just before permanent flood after combine sowing. Then 30.6% of that applied was recovered in the crop compared with 16% when combined in with the seed.