June, 1958] V. N. AMBLE, K. S. KRISHNAN AND J. S. SRIVASTAVA 65
3.—SEPARATION OF THE COMPONENTS OF VARIATION
The variation observed among the animals in respect of milk yield and other
heritable traits such as lactation yield studied in the earlier sections is the resultant
of heritable and non-heritable causes. The heritable component arises due to genetic
makeup and indicates the scope for improvement through selective breeding.
Studies on the non-heritable components due to environment are helpful in making
suitable adjustments for these factors in order to assess properly the changes brought
about in the genetic constitution of the herd.
3.1—REPEATABILITY
Among the environmental factors influencing milk production some may be of a
temporary nature affecting the milk yield in a particular lactation, while others may
be of a permanent nature, such as for example, the incidence of mastitis which affects
the yield in all the subsequent lactations. A measure of the overall variation due to
heritable causes together with the environmental factors of the latter type is the
repeatability coefficient which is estimated as the intraclass correlation between
the records of the same cow in her different lactations. Repeatability coefficients
were worked out for both the Sindhi herds for the five characters, viz. the lactation
yield, the yield per day of calving interval, the lactation period and the calving in-
terval. These are presented in Table XXI along with the corresponding 95 per cent
confidence intervals. In obtaining these coefficients the later records used were
converted to the first lactation basis by using the lactation correction factors given
in section 3.3.1.
It may be seen from Table XXI that of all the characters studied the lactation
yield was repeatable to the greatest degree, the coefficients of repeatability being
0.61 and 0.54 respectively for the Hosur and the Bangalore herds. At the other end
the repeatability coefficient of calving interval was as low as 0.21 for the Hosur herd
while for the Bangalore herd it was not significantly different from zero.
The repeatability coefficients serve to assess the valae of early records as selec-
tion criteria for improving later performance of the selected individuals. Thus, it
would appear that selection based on the whole lactation yield or the yield per day
of lactation of the early records, is likely to lead to a better average performance
in later lactations in respect of these characters. Early selection based on lactation
period is also likely to lead to some improvement in this character in later lactations
but very little success can be expected in adopting selection based on the calving
interval in early lactations with a view to reducing the calving interval in the herd.
3.2—HERITABILITY
The additive component of the heritable variation present in a herd is measured
by the heritability coefficient. An efficient estimate of this coefficient is obtained
by doubling the intrasire regression of the daughter's performance on that of its
dam. Such estimates based on first lactation records are presented along with their
standard errors in Table XXII for both the Sindhi herds under examination.
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