Variety-specific nitrogen fertilizer recommendation for lowland rice

Nitrogen (N) is the most limiting nutrient for rice in tropical Asian soils, and almost every farmer has to apply the costly N fertilizer to get a desirable yield of rice. Modern high yielding rice varieties may have differences in accumulating and using N from soil and applied fertilizer. An experiment with 10 rice genotypes was conducted to estimate their efficiency to use inherent soil N, applied fertilizer N, and to predict optimum N input for a desirable yield. The genotypes were grown at two N levels: 0 and 130 kg N ha-1. Under the N control plots, the grain yield of rice varied from 3.26 to 4.63 t ha-1, and the yield range in the N treated plots was 4.69-6.24 t ha-1. The tested rice genotypes varied in straw N concentration (0.45%-0.73%) and grain N concentration (0.81-1.53%). Therefore, the N requirement among the tested rice genotypes varied from 9.02 to 13.78 kg t-1 in the N control plots and from 10.93 to 15.66 kg t-1 in the N-treated plots. Despite their modern plant characteristics, the tested genotypes significantly varied in N use efficiency and apparent N recovery. The BR4839-17-5-2-2-HR5 gave agronomic N use efficiency of 18.38 kg grain/kg applied N, whereas in BR6209-16-3 it was only 5.15 kg/kg. The apparent N recovery of the BR4839-17-5-2-2-HR5 was 34.85%, which was as low as 11.25% in BR6209-16-3. The calculated N fertilizer dose for the observed yield varied from 78 to 113 kg ha-1, compared to a flat soil test-based recommended dose. For a target yield of 6.0 t ha-1, the required N fertilizer for the advance line BR4839-17-5-2-2-HR5 was 91 kg ha-1, compared to 109 kg N ha-1, which was required for the variety BRRI Dhan 29. The study shows that the N fertilizer recommendation for rice would be more cost-effective if the genotypic efficiency in N use of rice at biochemical level is taken into account.