Innovative tools such as the chlorophyll or SPAD meter offer a new strategy for optimizing fertilizer nitrogen (N) application in rice (Peng et al 1996, Balasubramanian et al 1999). A simple and portable device, the SPAD meter provides a nondestructive and accurate measurement of rice leaf N status in situ in the field. It thus helps assess the N topdressing requirements of rice crops. Earlier studies indicated that critical SPAD values could be established for different varieties and growing seasons, depending on local growing conditions (Balasubramanian et al 1999). This study was conducted to determine the critical values for optimizing grain yield and N use efficiency for different rice varieties and growing seasons in the Cauvery Delta of Tamil Nadu.
On-station trials were conducted at the experimental farms of SWMRI in Thanjavur (representing the New Cauvery Delta area) during the 1996, 1997, and 1998 dry seasons [DS (kuruvai): June to September] and 1997-98 first wet season [WS-I (samba): August to January] and second wet season [WS-II (thaladi): October to February]. Different SPAD threshold levels (29, 31, 33, 35, 37, and 39) were evaluated along with the local N recommendation of 125 (dry season) or 150 (wet season) kg N haÐ1 and a zero-N control. Treatments were arranged in arandomized complete block design with three replications. Rice varieties used for different seasons and locations are given in the table.