Yoshida - 1981 - Fundamentals of Rice Crop Science

PHYSIOLOGICAL ANALYSIS OF RICE YIELD 247
The water table of the field used in this experiment was 80 cm below the soil
surface in winter and 18 cm in summer. The rice grain yield is usually around 4.5
t/ha.
The researchers found that a low number of sunshine-hours during ripening
resulted in a low percentage of filled spikelets. To overcome this problem, they
shifted the crop season 1 month earlier. Component technologies included drainage,
deep plowing, and application of 19–56 t compost/ha (Table 7.6). AS a
consequence, applied nitrogen exceeded 400 kg/ha. There was not much difference
in grain yield between the treatments when the field was poorly drained.
When the field received adequate drainage, grain yield increased from 6.6 to 10.0
t/ha.
Another example of high yielding technology is the intensive use of topdressing.
Nitrogen topdressing is normally done at panicle initiation or flag-leaf stage
in Hokkaido, Japan. Shiga and Miyazaki (1977) studied the nitrogen absorption
process of a high yielding crop and simulated that process by topdressing. Nitrogen
topdressing was given 5 times between panicle initiation and heading, and
the amount of nitrogen applied each time was increased stepwise from 5 kg N/ha at
panicle initiation to 10 kg N/ha 5 days later, to 15 kg N/ha after another 5 days after
that, to 30 kg N/ha at flag-leaf stage (15 days after panicle initiation), and 30 kg
N/ha at heading. By this technique, grain yield was increased from 3.8 t/ha without
fertilizer nitrogen to 9.7 t/ha with a simulated topdressing. The simulated topdressing
produced high yields of more than 8.8 t/ha over 5 years.
7.6. FACTORS LlMlTlNG THE PRESENT YIELD PLATEAU
Improving grain-filling is one way to increase grain yield. The filled-spikelet
percentage is about 85%, even under favorable conditions, which means that grain
yield might be increased by 15%. Of the 15% unfilled spikelets, however, around
5–10% are unfertilized, and difficult to eliminate. An increase in photosynthesis
(source activity) relative to spikelet number (sink size) would probably increase
filled-spikelet percentage by only 5–10% and, hence, yield by a similar degree.
Thus, the yield to be achieved by improving gain-filling would be much lower
than the estimated potential yield.
It is possible to increase yield by increasing the number of spikelets per unit of
land area. In other words, it is important to increase yield capacity when yield
capacity is defined as the product of spikelet number and potential weight per
grain. To examine this possibility, it must be known whether grain filling, as
determined by the amount of solar radiation during ripening, is already limiting to
yield. The question implies that the number of spikelets per unit of land area may
be increased through improved cultural practices or by genetic manipulation, but a
considerable portion of the spikelets produced may remain unfilled because of
limited solar radiation.
In Japan. the percentage of ripened grains decreases when the number of
spikelets per square meter is increased (Matsushima 1970, Wada 1959). Thus,