Yoshida - 1981 - Fundamentals of Rice Crop Science

PHYSIOLOGICAL ANALYSIS OF RICE FIELD 239
The constant 1,000-grain weight of a given variety does not mean that individual
grains have the same weight per grain. Individual grain weights vary to
some extent but the mean value is constant (Fig. 7.4). The term ripened grains has
been used in Japan to distinguish between commercially useful grains and chaff.
The ripened grains are those that are heavier than salt water with a specific gravity
of 1.06 (Matsushima 1970). For indica rices, specific gravity values of 1.10–1.12
are tentatively suggested (Osada 1974).
7.4. COMPARISONS OF GRAIN YIELDS ACHIEVED BY DIFFER-
ENT LEVELS OF TECHNOLOGY
7.4.1. Yield changes associated with technological changes — the Japanese
experience
In 900 AD, the average yield of brown rice in Japan was approximately 1 t/ha. In
1868, the year of the Meiji revolution, which was the dawn of modern Japan, it
was about 2 t/ha. Thus, it took about 1,000 years to double the average yield
(Ishizuka 1969). The introduction of irrigation systems is believed to have contributed
to higher yields and to have laid the ground work for further progress in
agricultural technology after the Meiji Revolution. The yield increased to about 3
t/ha before chemical fertilizers were introduced around 1930 (Fig. 7.5); compost
and other kinds of organic matter were the major sources of added plant nutrients
up to that time.
In 1954, the yield increased to 4 t/ha, i.e., the yield doubled in less than 100
years. Heavy applications of nitrogen fertilizers after World War II caused many
changes in variety, plant protection, soil improvement, and cultural management.
The high national average yield in Japan today has been realized through the
integration of technology.
7.4.2. Grain yield comparison among national average, experiment station’s
record, and the highest yield
From 1949 to 1968, Japan’s No. 1 rice yield contest was held to encourage
Japanese farmers to produce more rice. In 1960, 13.2 t/ha was recorded in this
contest. It was the highest yield ever achieved in Japan (Agricultural Policy Study
Commission 1971). National average yields increased from 3.88 t/ha in 1949 to
5.51 t/ha in 1968.
Long-term weather-assessment field trials have been conducted for many years
at agricultural experiment stations in Japan. These experiments assess the effects
of yearly weather fluctuations on rice yields, using the same cultural practices. A
comparison of rice yield from various sources, summarized in Figure 7.6, indicates:
• The national average yield is very similar to the weather-assessment trial
yields recorded at agricultural experiment stations.
• The highest yields achieved by farmers are more than two times greater than
the national average regardless of weather conditions.