Both FTNM and RTNM were effective in achieving high
grain yield and fertilizer-N use efficiency in SY63 and LYP9.
The optimal SPAD threshold for guiding N management was
2 units higher in LYP9 than in SY63 due to the difference in leaf
thickness between the two varieties as reflected by a greater
SLW in LYP9 than in SY63. The two varieties did not show a
clear difference in fertilizer-N use efficiency. They required a
minimum total N rate of 120–150 kg ha1 for producing
maximum grain yield. The difference in maximum grain yield
was very small between the two varieties, although LYP9 had a
potential to produce higher grain yield than SY63. The two
varieties responded differently to high fertilizer-N input with
more yield reduction at excessive N rates in SY63 than in LYP9.
This was due to the difference in lodging resistance between the
two varieties at these N levels. Our study suggests that it may be
relatively easy for farmers to manage N for LYP9 because
LYP9 is less sensitive to excessive N inputs than SY63, but, on
the other hand, there is a greater possibility for farmers to apply
excessive N to LYP9 than to SY63.