June, 1958] V. N. AMBLE, K. S. KRISHNAN AND J. S. SRIVASTAVA 43
The results of the comparisons of the cows belonging to different generations
with their dams are given in Table VI for all the six characters mentioned
earlier. The salient features of these comparisons are given below :
(i) The yield per day of lactation for the cows belonging to the first farm-bred
generation was higher by about 1.8 lb. over their dams. This increase was statisti-
cally significant. For the same set of cows increases of about 560 lb., per lactation
and 46 days in calving interval over their dams were observed, both of which only
just failed to attain the conventional 5 per cent level of significance.
(ii) No material change from the first-generation cows to the second-generation
cows was observed in respect of any of the characters studied. The increase of 48
days in the calving interval was slightly smaller than the 5 per cent critical
difference.
(iii) The third-generation cows showed significant increases over their dams of
2.5 months in the age at first calving and 47 days in the calving interval. There were
decreases of about 370 lb., 0.8 lb., and 0.9 lb. in the average lactation yield, the yield
per day of lactation, and the yield per day of calving interval respectively but all the
differences were less than the respective critical difference at 5 per cent level of
significance.
(iv) The average performance of the fourth-generation cows was not significantly
different from that of their dams for any of the six characters examined.
(v) There was a significant decrease of 43 days in the average lactation
period of the cows in the fifth-generation compared with their dams. There was a
consistent decrease in respect of all the other characters excepting age at first calving,
though none of them was significant. The magnitude of the fall in the lactation
yield was, however large, being of the order of 600 lb.
1.4.3—Generationwise comparisons adjusted for annual variations
The first lactation records of 290 cows belonging to the six farm-bred generations
examined in the preceding sections were distributed over a period of 28 years [1924-51].
The changes in the feeding, management and other environmental factors which are
likely to have occurred during this period would have been partly responsible for the
variation observed in successive generations. In order to remove, to the extent
possible, this source of variation from the comparison of the cows belonging to
different generations, the method of analysis of variance by fitting constants was
adopted. As the number of records made in each year was few, the records were
grouped into five periods. Each period consisted of five years except the first
and the last periods which contained records of seven and six years respectively. A
preliminary analysis indicated a marked variation from period to period ; but the
variation from year to year within periods was not significant.
The records of four cows belonging to the sixth generation were omitted as these
were too few to draw any inference about the change brought about in this gene-
ration. The records of 29 cows whose ages at first calving were either earlier than
32 months or later than 52 months were also excluded from the analysis since very
early or much delayed first calving appeared to have adverse effect on milk produc-
tion in this herd (vide Sec. 3.3.4.). These cows belonged to the different generations
as follows : 6 in F 1 , 7 in F 2 , 9 in F 3 , 5 in F 4 and 2 in F 5 . In view of the distribution