Field experiments were conducted at the village Kasiadihi, Dhenkanal district, Orissa, India during wet seasons 2001, 2002 and 2003 under non water-stressed conditions (0–25 cm standing water) to assess variability in N uptake and utilization by medium and late duration rice varieties. The N rates were 0, 40, 80 and 120 kg N/ha applied as urea in four equal splits at transplanting, active tiller initiation, panicle initiation and flowering stages. The grain yield response was up to 80 kg N/ha. The optimum grain yield attainable by the efficient medium duration varieties was 4·5 t/ha. The N efficient late duration varieties produced optimum grain yield of 5·8 t/ha. The relationship for total dry matter and grain yield production between N fertilized (40, 80 and 120 kg N/ha) and non-fertilized treatments were all significant, suggesting cultivar selection under optimum N fertilized conditions. The difference in optimum yield of the medium and late duration varieties was due to the differences in the amount of N uptake and its use efficiency by the plant for grain production. There was a curvilinear relationship between grain yield and N use efficiency for grain production. The relationship between N use efficiency for grain production and N contents of leaf, stem and grain at maturity was quadratic. The optimum plant N use efficiency of medium duration varieties was 49 kg grain/kg N uptake, achieved with leaf, stem and grain N contents of 10, 8 and 14 g/kg, respectively, at maturity. For late duration varieties, the optimum plant N use efficiency was 68 kg grain/kg N uptake and it was maintained with leaf and stem N content of 4·0 g/kg each and grain N content of 9·0 g/kg at maturity. The N content in plant organs could be the selection guide used to obtain efficient rice varieties.
There is a need to increase the rice production in nonwaterstressed rice‐growing areas during the wet season in Asia by the use of a suitable combination of a medium‐ or long‐duration variety and appropriate nutrient management strategy. The crop growth simulation model ORYZA 1N was used for variety selection and N optimization in nonwaterstress conditions. Selection was made from 12 released rice varieties of 115 to 150 d duration. The model was calibrated using input data from field experiments that were conducted during the wet season (June–November) of 2001 at the Central Rice Research Institute, Cuttack, India. In the medium‐duration varieties (115–130 d), there was good agreement between simulated and observed leaf area index, biomass, and grain yield. The simulated biomass of long‐duration varieties (135–150 d) showed large deviation from observed biomass at flowering. In the wet season of 2000, the model accurately predicted the grain yield, biomass, and leaf area index of medium‐ and long‐duration varieties. When the ORYZA 1N model was used to simulate the effect of planting dates on rice yield, it predicted the decline in yield with late planting. It was recommended that farmers consider planting new variety Ranjit in the favorable lowlands that do not have water stress during the wet season and should apply 80 kg N ha−1 in four equal splits at transplanting, active tillering, panicle initiation, and flowering. Technology verification trials of this practice conducted during the wet season of 2002 produced 5.51 Mg ha−1 of rice, compared with 4.36 Mg ha−1 grown with the conventional practices of area farmers.
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