Optimal use of nitrogen fertilizer at panicle initiation

A field experiment was conducted on puddled soil during the wet season of 2014 to determine the effect of early use of N fertilizer on the optimal use of N fertilizer at panicle initiation (PI). This study also examined whether or not tiller number, leaf area index (LAI), and leaf color can be used to adjust N fertilizer rate at panicle initiation for increased rice yield with higher N use efficiency. The treatments were arranged in a randomized complete block design with four replications. Ten treatments of control and nine treatments of N fertilizer were made. The nine treatments were three N rates at early N (low is 30 kg/ha, medium is 50 kg/ha, and high is 70 kg/ha) and three N rates t panicle initiation (low N rate is 10 kg/ha in UE and 0 kg/ha in UY; medium N rate is 30 to 40 kg/ha in UE and 15 to 25 kg/ha in UY; and high N rate is 50 to 70 kg/ha in UE and 30 to 50 kg/ha in UY). The present results showed that high N rate at early stage during WS are important in increasing the number of tiller, LAI, LAR, and plant N content at PI in both sites. Nitrogen rate at PI revealed good correlation with LCC and SPAD but weak correlation with LAI, TDM, and SLA. Also, the results showed that LAI, LCC, SPAD, PN, and solar radiation can be used to determine the effects of early N application on the optimal use of N panicle initiation. Medium N rate at PI have increased grain yield of 0.2 Mg/ha, medium AE sub N (17 kg/kg) in both sites, low RE sub N in UE site (36%) , and high RE sub N in UY site (59%). Solar radiation has an important role in this study, that is, the medium N fertilizer recommendation (40 kg/ha in UE site 15 kg/N ha in UY site) increased grain yield (reached 71% in UE site and 97% in UY site of yield potential), TDM, and CGR when supported by high N at early and enough solar radiation during PI to harvest (mean 14.6 MJ/day).