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STATISTICAL STUDIES ON BREEDING DATA [Vol. XXVIII, Part II,
of health, etc. Besides these, climatic variation due to season may also have an
influence. Studies made to evaluate the effects of these factors to the extent possible
from the available data are discussed in the following sections.
3.3.1—Order of lactation
An exact study of the effect of the order of lactation on the lactation yield and
other related characters such as lactation period, yield per day of lactation, etc.,
is not possible from the present data in so far as some culling of cows on the basis
of the earlier lactation performance has been practised. Consequently, the cows
which have completed on the farm a large number of lactations, say six or more,
should be considered as a set of selected cows and the conclusions based on the
records of these cows may not strictly hold for an average cow of the herd.
An approximate method in the circumstances, suggested by Sanders [1927], is
to compare the average yields in successive lactations over all available pairs of
records. It is common to employ the technique of fitting an orthogonal polynomial
of a suitable degree in order to minimise the effect of random fluctuations. This
procedure was adopted for evaluating the effect of the order of lactation on all the
principal characters examined earlier. The trends observed are indicated in the
chart. The fits obtained for the three yield characters and for the lactation period
were satisfactory. Linear trend was indicated for the lactation period in the Hosur
herd while addition of a second degree term was necessary to get a better fit of the
steeper fall in the lactation period in the later lactations made in the Bangalore herd.
For the three yield characters, quadratic curves fitted well for the Bangalore herd
while third degree terms improved the fits in the case of the Hosur herd. The
trends in calving interval were inconsistent for the two herds and even within each
herd were erratic. The trends in the different characters are discussed in detail
in the following paragraphs :
For the Sindhi herd at Hosur, the three yield characters, viz. the lactation yield,
the yield per day of lactation and the yield per day of calving interval, increased
from the first to the third lactation and steadily decreased thereafter ; but the rate
of increase as well as the rate of fall differed for the three characters, being the
lowest for the lactation yield and the highest for the yield per day of calving interval.
Between the third and the fourth lactations fitted values for all the three characters
showed very little difference. The peak values for the three characters, lactation
yield, yield per day of lactation and yield per day of calving interval, were respectively
about 8 per cent, 15 per cent and 20 per cent more than the values of the first lacta-
tion. The performance in the eighth lactation was less than that of the first
lactation by about 9 per cent, 1 per cent and 2 per cent respectively for the three
characters in the same order.
For the Bangalore herd, the lactation yield and the yield per day of calving
interval showed continuously increasing trends from the first to the fourth lactation
and steady decrease from the fourth to the eighth. For the yield per day of lacta-
tion the trend was similar except that the peak was reached in the fifth instead of
the fourth lactation, but the difference between the fourth and the fifth lactations
was negligible. The peak yields in the case of the lactation yield, the yield per day