December, 1956]
K.
S.
KRISHNAN
167
ascribed wholly to random environmental causes. The lower lactation lengths are
at least in part due to poorer genotypes. The actual lactation yield—rather than
the yield adjusted for 300 days—should, therefore, be considered as reflecting the
milk potentiality of the progeny. Cases where shorter lactation periods are due
to the result of known abnormalities such as death of calf or diseased condition of
the cow, should be omitted rather than corrected. For cows having a lactation
period longer than 300 days also the yield corrected to 300 days by using regression
technique does not appear to be a suitable substitute to the actual yield obtained
during that period, as the regression results generally in an over-correction. This
is on account of the fact that the yield in the tail end of a lactation is generally
lower than the average yield per day over the entire lactation period. It is, there-
fore, suggested that the yield in the first 300 days of a lactation should be taken
for the lactations longer than this period. A good reason for this practice is that
in well organised dairy farms, it is desirable to provide for annual calvings
and a dry period of about two months to help the cow maintain her health. For
lactations completed in less than 300 days, the complete lactation record seems to
be the appropriate one. Wherever data on the yield in the first 300 days are not
available, it is preferable to carry out the studies on the unadjusted yield rather
than on the yield corrected to 300 days using the regression technique.
Another factor influencing the lactation yield of a cow is the order of lactation.
The effect of this factor can be eliminated by confining the study to the first lactac-
tion records only. The first lactation records are preferable to the later ones as
they will be available earlier and will be influenced to a lesser extent by selection.
The extent of gain that can be achieved by using the later lactation records in
addition, requires further investigation.
An important consideration in planning a systematic breeding programme
providing for sire evaluation is the number of daughter-dam pairs required to prove
a sire. An answer to this question depends on how sure of his proof one wants to
be and on the order of variability among the daughters' yields after correcting for
the inequalities in the dams' performances. With the conventional 5 per cent
level of significance and a coefficient of variation of the order of 40 per cent for
lactation record observed in the case of the six herds already referred to, the superio-
rity of a bull whose corrected index is 20 per cent higher than the herd average can
be detected with 12 daughter-dam pairs. With the same number of pairs, it will
be possible to distinguish between bulls whose corrected indices differ by more than
32 per cent at the same level of significance. Sukhatme [1944] suggested that in
breeding problems the 10 per cent level of significance may be used as an aid to
possible retention of superior breeding material which is difficult to select with
greater certainty. If this level of significance is adopted, a difference of the order
of about 28 per cent or higher will be revealed as significant. The corrected index
for a sire calculated on 12 pairs of records is expected to be determined with a
standard error of the order of 11 per cent.
The results indicate that, while there is no harm in using as low a number of
pairs as five or six for getting the first indication of the breeding worth of a sire
and for discarding inferior bulls having lower indices than the herd average, the