September, 1951]
R.
N.
DESAI
187
The reduction in the total sum of squares due to the variations in the ewes
within lines is 37.94 per cent. Such a large reduction indicates that the selection
of the individual ewe is of very great importance to increase fertility in sheep.
If this could be reduced by careful selection, a 26.94 per cent increase in the genetic
gain can be expected. Under average farm conditions it is very difficult to make
very close observations on individual ewes so as to facilitate the selection, still a
part of it could be reduced to make some progress.
The reduction due to age is 6.21 per cent but due to the negative effect of the
years, it is reduced to 3.7 per cent. If such year to year changes in the environment
could be controlled conveniently the age effect would be pronounced to its maximum
level of 6.21 per cent. To keep the flock at the maximum productive level, it
appears that any ewe showing a tendency for lowered fertility after the fifth year may
be immediately culled out and at the same time very rigid selection of the young
ewes may be made on the basis of their first and second lambings and type of birth,
etc.
Inspite of the fact that the sire effect on the lambing percentage has been esti-
mated on the records of the selected ewes, the reduction due to this factor is quite
large. This would indicate the possibility of increasing fertility further through
the selection of suitable sires for the flock.
Next in order of importance is the maternal age effect on the performance of
daughters. The findings for this factor are controversial. If they could be taken as
conclusive ones, the procedure of selection would be slightly changed. As a matter
of fact, in absence of proper explanation and some more evidence in that res-
pect, they cannot be taken as such. It can only be suspected that possibly such an
effect may be due to some physiological factors influencing the growing foetus in the
intra-uterine stage or during the period of nursing the young ones. According
to these data, the reduction in the sums of squares is 3.64 per cent and selection for it
would bring about 1.87 per cent increase in the genetic gain.
The effect of the type of birth on fertility accounts for only 1.53 per cent reduc-
tion of the total variations and indicates the possibility of .77 per cent increase in
the genetic gain. This effect is also estimated on the records of the selected stock. It
would be evident from the numbers in the classifications that the single born group
has been very rigidly selected. In absence of such selection, the differences bet-
ween these two groups might have been widened and the effects of twin born group
on fertility would have been possibly more pronounced. In addition to this, the
number of still births and other deaths at the time of lambing are significantly more
in the twin born group. There may be various causes for this mortality. If some
of the environmental causes could be controlled the inheritance of the multiple births
would be more effective.
SUMMARY AND CONCLUSIONS
The effects of age and year on fertility of sheep were studied from 2,696 lambing
records from 962 ewes. So also the effects of sires, the maternal age effect of the
performance of daughters, and the effect of the type of birth with their relative
importance in selection were studied from 1,368 lambing records of 412 ewes.