June, 1958] V. N. AMBLE, K. S. KRISHNAN AND J. S. SRIVASTAVA 47
of the simple average of the mean difference over all daughter-dam pairs corres-
ponding to the bull. In order to achieve a reasonable precision, it is common to
confine progeny testing to bulls with at least six daughter-dam pairs with corres-
ponding records. There were 16 such bulls out of the 52 used in the herd under
study.
Results of the progeny testing for the four characters, lactation yield, yideld per
day of lactation, yield per day of calving interval and lactation period are given in
Table IX. Two other characters examined earlier, viz., calving interval and age
at first calving were omitted for the sire evaluation as the variations observed in
respect of these characters were probably wholly due to non-heritable causes, as
indicated by the heritability studies (vide Section 3.2).
It may be observed from Table IX that sires Nos. 118 (0) and 43 (0) were the
only two bulls whose daughters gave significantly higher lactation yield (by about
900 lb.) than their dams. Daughters of the bull No. 118 (0) also gave 3.5 lb. of
milk per day of lactation period more than their dams (significant at 10 per cent
level). The lactation periods for the daughters of sire No. 43 (0) were on an
average 48 days longer than their dams the difference being significant at 10 per
cent level. The two sires Nos. 56 and 136 (0) may also be considered as having
contributed to the improvement, as the average yields of their daughters were
higher than their dams in respect of all the three yield characters. These
increases were, however, not significant at the conventional 5 per cent level owing
to the large variation in the yields of their daughters.
The daughters of sire No. 255 were substantially poorer than their dams, the
average yield being lower by about 900 lb. per lactation, 1.6 lb. per day of lactation
and 1.5 lb. per day of calving interval. The average lactation period for these
daughters was also significantly lower than for their dams by 37 days. The progeny
of sire No. 144 were also inferior to their dams by about 770 lb. per lactation, signi-
ficant at 10 percent level, along with a shorter lactation period of two months,
significant at 1 per cent level. Bull No. 139 had daughters having average values
for yield per day of lactation and yield per day of calving interval significantly
lower than their dams by about 2 lb. and 1.5 lb. respectively. The average yield
per day of calving interval for the daughters of sire No. 38 was lower by about 1.5
lb. per day, significant at 1 per cent level, even though the fall in the lactation
yield and that in the yield per day of lactation were negligible. The average
lactation period of the daughters of sire No. 282 was lower than that of their
dams by 53 days, significant at one per cent level.
To sum up, sires Nos. 118 (0), 43 (0), 56 and 136 (0) might be considered as having
contributed to the improvement of yield and the sires Nos. 255, 144, 38 and 139
as having depressed it. The other sires did not materially alter the level of
performance.
1.5.2—Sire indices
If a number of sires are to be compared in respect of their transmitting ability,
it is necessary to assign an index to each sire. Such an index can be calculated for
estimating the breeding worth of a sire in respect of each character studied. It has
been shown by one of the authors [1956] that the daughter average index corrected
for the unequal production level of the dams by using regression technique is superior