Management of rice production systems to increase productivity

efficiency (Cassman and Harwood, 1995). It is therefore important to increase
fertilizer‐N recovery and internal N‐Utilization efficiency (NUE) in rice
production systems through cultivar improvement and better crop
management (Ying et al., 1998). It is reasonable, therefore, to expect increases
from low‐input rice varieties to make up for the incremental rice yield needed
to feed the increasing population.
Fertilizer‐N recovery efficiency can be estimated by the ratio of
increased plant N that results from N application to the amount of applied N
(Novoa and Lomis, 1981). Nitrogen‐utilization efficiency may be defined as
the amount of grain produced per unit N acquired by the crop. The amount of
N uptake and the NUE of crops depend on the yield level and environmental
conditions (Ying et al., 1998). Fertilizer‐N recovery equal to 50% to 70% of
what is applied can be achieved when N is applied in the proper amount, in
the proper form, and at the proper time (Peng and Cassman, 1998).
Grain yield increase can be achieved either by increasing biomass
production or harvest index, or both (Yoshida, 1981). It is controversial as to
which component should be emphasized to further improve yield potential of
current cultivars. Comparisons between modern and traditional cultivars of
major cereal crops attribute improvement in yield potential in many cases to
the increase in harvest index rather than in biomass production (Evans et al.,
1984). When comparisons were made among modern cultivars, however,
high yield was achieved by increasing biomass production (Amano et al.,
1993). Hybrid rice cultivars have reportedly about 15% greater yield than
inbreds mainly due to an increase in biomass production rather than in
harvest index (Yamauchi, 1994). The number of spikelets per unit land area,
or sink size, is the primary determinant of grain yield in cereal crops grown in
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