Author : Grisana Linwattana

Summary : Direct-seeded rice cultivation is continuously expanding due to lower labor costs, increasing need to intensify cropping, and availability of effective herbicides for weed control. But shifting to direct seeding may affect stand establishment because of non-availability of field tested nitrogen management technology as is known for the transplanted rice. This study was therefore, conducted for two wet and one dry season to determine the suitable splitting proportions and time of N application that could enhanced grain yield of direct-seeded rice at Ubon Ratchathani, Thailand. Regardless of season and rice variety used, 60kg N/ha applied in three splits, i.e. 15 kg N/ha at 15 and 30 days after emergence (DAE) and the remaining 30 kg N/ha at 45 DAE consistently gave the highest grain yield. The crop that received this type of splitting proportion and time of N application had the tallest plants, highest number of tillers, highest shoot biomass, higher leaf area index (LAI), higher N content of above ground plant parts and higher nitrogen uptake, and thus, the highest grain yield. Plant height, number of tillers, and N uptake were positively correlated with grain yield. Conversely, when half of the total amount of N rate was applied 5 DAE, and one-fourth at 20 and 35 DAE, grain yield and almost all other agronomic/growth parameters were lower. The results indicate that in direct-seeded rice, greater rsponse to nitrogen application can be obtained when the nitrogen application was done at a time close to the reproductive stage, e.g. 45 DAE. There is greater demand for N at this stage. Land suitability evaluation for N management of direct-seeded rice was carried out based on six factors such as annual rainfall, soil texture, cation exchangeable capacity (CEC), climatic drought occurrence, farmers' practices and maximum depth of water using the Geographic Information Systems as a tool.