Influence of split nitrogen fertilization on grain density of modern rice varieties

High density (HD) grains are those with specific gravity greater than 1.20. Grains with HD is very important since it increases milling recovery and improve head rice yield which is the final marketable product. The improved quality therefore can command higher market price of rice. This study was conducted under field and screenhouse conditions to assess the influence of split N fertilization on the proportion of HD grains of IR58 and IR70. Moreover, it aimed to understand some physiological factors that promote HD grain production as affected by N split application. Split N application improved the number of HD grains. HD grains increased from 10% to 24% in IR58 and from 37% to 45% in IR70 under field condition. The application of N at the middle growth stages (between PI [panicle initiation] and heading) was found necessary for grain development and production of HD grains. Further application of N beyond heading is not necessary for the production of HD grains. IR70 had more HD grains than IR58. However this depends on the fertilization scheme and growing condition. Reduction of tillers of both IR58 and IR70 did not improve HD grains but increased the number of good grain (sp gr 1.20). The improved grain number and weight of detillered plants signify that less but large tillers can improve assimilate distribution among the grains in the panicle. Grain yield and yield components were not influenced by split N application especially under field condition. However, yield was more related to grain number and good grain index. Leaf area was enhanced by heavy N fertilization at early growth stage. However, late N application at heading or 15 days beyond heading prolonged the duration of green leaves. On the other hand, applying 50% of the recommended rate of N basal and adding the 50% at PI was enough to sustain longer leaf area duration.