Similarly, the SPAD value of LYP9 was about 2 units
higher than that of SY63 at the same N rate. For precise N
management aiming at high grain yield and fertilizer-N use
efficiency, the optimal SPAD threshold was about 36 for SY63
and 38 for LYP9 grown under subtropical conditions. The
relationship between grain yield and average SPAD value from
15 days after transplanting to heading also supports that
different SPAD threshold should be used for SY63 and LYP9 in determining the timing of N application (Fig. 6). Based on this
information, we also increased SPAD thresholds by two units in
FTNM for LYP9 in 2005. Peng et al. (1996) found that the
SPAD threshold for optimizing N management was 35 for
indica varieties grown under tropical conditions.