Synchronizing nitrogen (N) application with crop demand and soil N supply is one strategy for improving N use efficiency (NUE) in rice. Using a variable N rate application based on observed chlorophyll meter readings at critical growth stages may help optimize grain yield and NUE at the same time. The chlorophyll or SPAD meter can accurately measure leaf chlorophyll content that is related to leaf N status. It could help develop need-based N fertilization in rice (Peng et al 1996). One disadvantage of the SPAD technique, however, is the need for weekly observations to monitor crop N status, which could sometimes lead to many (3Ð6) split N applications (Balasubramanian et al 1999).
Field experiments were conducted during the 1997-98 dry (DS, kuruvai: June to September) and wet (WS-II, thaladi: October to February) seasons at the SWMRI farm in Thanjavur, India. The objective was to determine whether chlorophyll meter readings taken only at critical crop growth stagesÑearly tillering, active tillering, panicle initiation, and 10% flowering Ñwere sufficient, and to apply variable rates of N based on a range of SPAD threshold values (Table 1) rather than on a single critical SPAD value. The effect of variable rate N application, based on a range of chlorophyll meter readings, on grain yield and NUE was measured in these trials. Treatments included different ranges of SPAD threshold values: <32, 33Ð35, and 36Ð38. Rice varieties ADT42 (115 d) and ADT38 (135 d) were used during the DS and WS-II, respectively. Treatments were arranged in a completely randomized block design with seven replications.