Rice (Oryza sativa L.) is the main staple food for more than half of the world’s population. In 2011, worldwide rice production exceeded 672 million mt. Iran ranked 20th in terms of rice production in the world. Plant spacing has an important role on growth and yield of rice. Optimum plant density ensures the plant to grow properly with their aerial and underground parts by utilizing more solar radiation and soil nutrients. Timing of nitrogen application had a significant role on reducing nitrogen losses, increasing nitrogen use efficiency and avoiding unnecessary vegetative growth.
In order to do this research, an experiment was conducted as split plot based on a randomized complete block design with three replications in a research farm located in Qaemshahr during the years 2013-14 and 2014-15. The experimental factors included three levels of plant density (16, 25 and 33.3 plants per m2) as the main factor and sub-factor was nitrogen splitting in 5 levels (S1: 100% at the beginning of planting, S2: 50% at the beginning of planting + 50% at the beginning of tillering, S3: 50% at the beginning of planting + 50% at the panicle formation, S4: 33.33% at the beginning of planting + 33.33% at the beginning of tillering + 33.33% at the panicle formation and S5: 33.33% at the beginning of planting + 33.33% at the panicle formation + 33.33% at the end of panicle emergence).
The results showed that grain yield of Hashemi and Koohsar cultivars in the second year decreased 9.21% and increased 23.74% respectively, than the first year. Also, the second year compared to the first year, showed a decrease of 11.62% and 30.36% of straw yield, respectively, in Hashemi and Koohsar cultivars. Grain yield and harvest index were increased and decreased with increasing density in Hashemi cultivar, respectively. Straw yield in both cultivars increased with increasing density. The lowest straw yield and the highest harvest index were obtained in both cultivars, with nitrogen application in equal proportions at the basic stages, initial cluster and complete clustering. The interaction between the factors in the cultivars showed that in each two years, the number of cluster per square meter increased with increasing plant density.
The results of this study showed that in the studied cultivars, the highest effect of plant density and nitrogen splitting on yield components was related to the number of panicles per square meter, because ultimately increasing the number of plants per unit area and nitrogen splitting equally in the basic stages, beginning tillering and primary cluster emergence resulted in the highest grain yield per hectare by increasing the number of panicles per square meter.