Effect of nitrogen rate and its application schedule on leaf area development and crop growth of local and modern rice varieties

The experiments were carried out from 1996 to 1998 at the Bangladesh Rice Research Institute in Gazipur. Four rice varieties- Nsail and BRRI dhan31 (BR31) in the wet season (WS) and HB6 and BRRI dhan29 (BR29) in the dry season (DS) were tested. Nsail and HB6 were local, whereas BR31 and BR29 were modern varieties. Four N rates-0, 50, 100 and 150 kg N ha-1 were given. The rates of 50, 100 and 150 kg N ha-1 were given in three application schedules- two, three and four splits. The pattern of leaf area index (LAI) and crop growth rate (CGR) differed among the varieties. LAI and CGR of DS varieties were very low during early growth stages compared to WS varieties. LAI increased gradually and reached its peak around PI stage in Nsail, whereas for the other varieties, peaks were around heading stage. HB6 always showed a lower LAI than the offer varieties. The yield-LAI relationship showed that the optimum LAI of the modern variety for achieving the highest yield ranged from 4 to 5, whereas the optimum range of the traditional DS variety was between 3 and 3.5. The LAI of the lodging-prone variety Nsail was above optimum throughout the growing season and the relationship between LAI and yield was negative. Nitrogen rate had a profound effect on leaf area development. Two-split treated plots of BR31 and 3-split treated plots of HB6 and BR29 showed the highest LAI around PI and heading stages in most of the cases. In Nsail, the CGR peaked before PI stage, whereas for BR31 and HB6 the peaks were reached around heading stage and declined thereafter. For the modern DS variety, an almost steady growth rate was observed. In most cases, the 3-split treated plots of BR29 showed higher CGR at PI and heading stages compared to the other split arrangements. Higher CGR after heading for the modern DS variety BR29 compared to other varieties may have led to a higher grain yield in BR29.