Yoshida - 1981 - Fundamentals of Rice Crop Science

GROWTH AND DEVELOPMENT OF THE RICE PLANT 61
1.12.2. Harvest index and grain-straw ratio
A common way of examining rice grain yield is to measure the total dry weight and
dry grain yield, and then compute the ratio of these two, as shown below:
Harvest index (H.I.) =
economic yield
biological yield
= dry grain yield
(1.17)
total dry weight
:. Dry grain yield = H.I. × total dry weight. (1.18)
Equation 1.18 indicates that grain yield can be increased by either increasing total
dry matter production or by increasing the harvest index. The total dry weight is a
measure of a crop’s photosynthetic performance and the harvest index is a measure
of the economically useful fraction of the bioiogical yield. The total dry weight of a
good rice crop is around 10-20 t/ha, depending on variety, management. and
environment. The harvest index is about 0.3 for traditional tall varieties and 0.5 for
improved, short varieties. As a result, grain yield usually ranges between 3 and 10
t/ha per crop unless the crop lodges. The grain-straw ratio, similar to the harvest
index and extensively used in the past, is a ratio of dry grain yield to dry straw
weight:
Grain-straw ratio =
dry grain yield
dry straw weight
(1.19)
The grain-straw ratios of rice crops range from about 0.5 for traditional, tall
varieties to about 1.0 for improved, short varieties.
1.12.3. Yield components
A second method for examining yield performance is to break the yield into its
components:
Grain yield (t/ha) = panicle number/m 2 × spikelet number/panicle
× % filled spikelets × 1,000-grain weight (g) × l0 –5 (1.20)
= spikelet number/m 2 × % filled spikelets
× 1,000-grain weight (g) × 10 –5 . (1.21)
In equation 1.20, the spikelet number includes filled, partially filled, and unfertilized
spikelets. The filled spikelet is called grain. The percentage of filled
spikelets is a ratio of the number of grains to the total number of spikelets. 1 The
1,000-grain weight is the average weight, in grams, of 1,000 grains.
1 There is some confusion in the terminology of yield components. The following are considered
synonyms when yield components are discussed:
spikelet number/panicle and grain number/panicle
total spikelet number/m 2 and total grain number/m 2
% filled spikelets and % filled grains