INCREASED N-USE EFFICIENCY USING A CHLOROPHYLL METER ON HIGH-YIELDINGIRRIGATED RICE

The chlorophyll meter (SPAD) can be used to monitor leaf N status in r ice (Oryza sativa L.). The objective of this study was to compare N-us e efficiency of SPAD-based N fertilizer management with recommended fi xed-schedule N fertilizer splits applied al key growth stages. Four fi eld experiments with 'IR72' were conducted, three at the International Rice Research Institute (IRRI) and one at the Philippine Rice Researc h Institute (PhilRice). In the SPAD-based N-treatment, N was topdresse d when the SPAD value of the topmost fully expanded leaf fell below 35 (corresponding to about 1.4 g N m(-2) leaf) from 15 days after transp lanting to booting stage. The amount of N applied in each topdressing was determined by expected crop N demand predicted by equations relati ng N uptake to cumulative thermal units. Yields with SPAD-based manage ment were 93-100% of maximum yields achieved by the best fixed-timing treatment but lower total N rates were used in all SPAD-based N treatm ents. Increased recovery efficiency from applied N and greater utiliza tion of the acquired N to produce grain contributed to the significant ly greater fertilizer-N efficiency of the SPAD-based than of the fixed -timing N treatments. The improved congruence of N supply and crop dem and in SPAD-based treatments resulted in fewer unproductive tillers, l ess leaf senescence at flowering stage and comparable or greater crop growth rates after flowering than the best fixed-timing treatment. It is concluded that increased N fertilizer efficiency at high yield leve ls is possible using a chlorophyll meter to monitor leaf N status and guide fertilizer-N timing on irrigated rice.