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

The chlorophyll meter (SPAD) can be used to monitor leaf N status in rice (Oryza sativa L.). The objective of this study was to compare N-use efficiency of SPAD-based N fertilizer management with recommended fixed-schedule N fertilizer splits applied at key growth stages. Four field experiments with ‘IR72’ were conducted, three at the International Rice Research Institute (IRRI) and one at the Philippine Rice Research Institute (PhilRice). In the SPAD-based N-treatment, N was topdressed 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 transplanting to booting stage. The amount of N applied in each topdressing was determined by expected crop N demand predicted by equations relating N uptake to cumulative thermal units. Yields with SPAD-based management 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 treatments. Increased recovery efficiency from applied N and greater utilization of the acquired N to produce grain contributed to the significantly greater fertilizer-N efficiency of the SPAD-based than of the fixed-timing N treatments. The improved congruence of N supply and crop demand in SPAD-based treatments resulted in fewer unproductive tillers, less 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 levels is possible using a chlorophyll meter to monitor leaf N status and guide fertilizer-N timing on irrigated rice.