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STATISTICAL STUDIES ON BREEDING DATA [Vol. XXVIII, Part II,
respect of these characters in these two herds. Variation in calving interval and
age at first calving in the two herds apparently depended largely on feeding, manage
ment and other non-heritable causes. A study of the Gir herd at Indian Dairy
Research Institute and the Kangayam herd at Hosur (I.C.A.R. unpublished re
ports) indicated a similar situation.
The superiority of the selected parents measured by the phenotypic selection
differentials obtained in sections 1.3 and 2.3 will not be entirely reflected in their
progeny since milk yield is affected by non-heritable causes also. The genetic
component of the superiority of the selected parents can be estimated by making
use of the coefficients of heritability and repeatability. The genetic selection
differential for cows was worked out by using the formula ∆G = Σl i h ni
2 (y ni — y) / Σl i
where l i and n i are respectively the total number and the number of normal lacta
tions by the cow, h 2
ni the heritability coefficient corresponding to n i records
of a cow, y ni the average of the n i normal records each converted to the first lacta
tion basis and y the average of y ni over all the cows. The genetic selection differ
entials for cows in the Hosur and the Bangalore herds were respectively 118 lb. and
192 lb. which worked out to 3.2 per cent. and 7.8 per cent. of the average lactation
yields in the two herds.
The genetic selection differential for bulls was worked out similarly by replac
ing in the formula used in the last paragraph, y ni by the corrected sire index of the
bulls, h ni by a similar factor appropriate to the number of daughter's records on
which the index was based and 1 i by the number of progeny born to the bull. The
genetic selection differentials obtained for the Hosur and the Bangalore herds were
—109 lb. and 210 lb. respectively. The negative value of the estimate for the Hosur
herd was due to the wider use of bulls having sire indices lower than the herd average.
The change expressed as a percentage of the average index of the bulls was —2.9
for the Hosur herd and 7.6 for the Bangalore herd.
The expected genetic advance per year in the herd due to the selection policy
adopted can be worked out by dividing the sum of the genetic differentials for the
two parents by the sum of the two corresponding generation intervals. The expect
ed genetic advance in the Hosur herd was 0.7 lb. per year (0.02 per cent. of the average
lactation yield) and that in the Bangalore herd was 29 lb. per year (1.1 per cent.).
The above estimates of the genetic advance based on the number of lactations
completed by a cow and on the number of progeny born to a bull reflect the genetic
changes due purely to the selection of the parents for getting the progeny. Rendel
and Robertson [1950] and others have, however, used the number of progeny retain
ed till maturity as the weighting factor. Such an estimate of genetic advance would
include the effect of the selection of the progeny itself from birth to maturity. The
estimates of genetic advance per year calculated on this basis worked out to 43 lb.
for the Hosur herd and 35 lb. for the Bangalore herd. In terms of the average
lactation yield, the genetic advance per year was 1.2 per cent. for the Hosur herd