116
Sire Index for Milk Production in Cattle
factors affecting milk-yield in order to reduce
sufficiently the environmental errors. This is
usually done by standardizing (1) the order of
location, (2) the length of lactation and (3) the
number of milkings per day.
The effect of selection of dams is to under-
estimate the value of the sire i.e., to make the
sire appear as if he does not possess a high
measure of the transmitting ability, when in
fact he does. The means by which we can
allow for this effect, at least partially, is to use
a slightly larger level of probability in tests of
significance than is conventionally done. This
will be explained later.
It is usual to include first lactation yields in
calculating the idex even though they are
not an absolute index to the performance in
later lactations. The reason for their use is that
a breeder wants to have the earliest indication
regarding the value of a sire and cannot wait
until all the later lactation yields are available.
It is, of course, advisable to recalculate the
index, as later lactations are recorded, but care
should be taken to avoid the bias that is likely
to be introduced by the selection of dams and
daughters that may have taken place in the
meantime. Usually, the later lactation yields
are reduced to their equivalent first lactation
yields with the help of approved conversion
factors before recalculating the index. No such
factors have, however, been worked out for use
in India. The appropriate course is to work out
a regression of yield on the -order of lactation
from the material locally available and use it
to convert the later lactation yields to their
equivalent first lactation yields before recalcul-
ating the index. This would secure the com-
parability of one index figure with another.
The procedure can be used whether the lacta-
tions of dams and daughters are the correspond-
ing lactations or not. Where the order of lacta-
tion of all mates of a sire is not known, though
records of milk-yields are available, the proce-
dure is only slightly different.
No standard
method can be suggested in either case, but
statistical technique to suit the requirements of
the data can be devised.
Length of lactation is another important
factor influencing the yield of milk. If the
lengths of lactation of dams and daughters are
unequal, sire index based on lactation yields
may give a misleading idea about the value of
the sire. Table VI showing the dam-daughter
differences for yield and the length of the first
lactation, gives an idea of the extent to
which the length of lactation may influence
the yield of milk. In calculating the index, it
is usual to allow for the difference in the length
of lactation by considering the milk-yields of
the first 300 days. In India just now, it might
be possible to. provide for the reporting of the
first 300 days' yield in registers maintained on
some farms, but it will be sometime before this
can become general. In the meantime, the
only course available is to recalculate the yields
from what records there are, but these may not
suffice, and at any rate, the procedure would
involve considerable time and labour. As an
alternative, the lactation yields may be adjusted
to the standard length of 300 days by the use
of regression. Logically, this is actually a more
appropriate method, since it uses complete in-
formation on lactation yields. Moreover, it has
an added advantage of minimizing the sampl-
ing error of the sire index. In this method, the
observed
relationship
between
the yield
and the length of lactation is used to adjust the
yields to the standard length of lactation.
Details of the method are illustrated later in
Tables II and III.
The number of milkings per day is another
factor influencing the yield of milk. Milk-yields
obtained under twice-a-day milking conditions
are usually used in calculating the index.
Where the number of milkings is more than
two, the records are usually corrected to twice-
a-day milking condition with the help of approv-
ed conversion factors. In India, twice-a-day
milking is a general, and consequently, the use
of such factors would be only rarely called for
in practice. Where, however, they are needed,
they may be worked out from the available
past records.
Even when the milk-yield records of dams and
daughters are equalized, the differences between
them will still be affected by errors which are
partly systematic and partly random. In so
far as the systematic errors are concerned, one
can merely make a close study of the environ-
ment and make allowance such as one thinks
to be the fairest for departure from normal
conditions, but beyond that, there is little that
one can do, e.g., differences arising from the.
practice of weaning in mothers and daughters.
Random errors are easier to deal with. They
cancel out in the process of averaging. Conse-
quently, when the number of dam-daughters
pairs is sufficiently large, the sampling errors of
the average milk-yields is small, and the index
can be relied upon to give an accurate indica-
tion of the breeding worth of a sire.
The concept of sampling error is relatively
new to breeders, consequently, it is not yet
usual to calculate the sampling error as a guide
to the reliability of the index. It is, however,