54 SELECTION FOR BUTTERFAT PER CENT IN DAIRY HERDS [Vol. XXIII, Part I,
have been complicated however, by the fact that culling for sterility, abortion, etc.
tends to alter the general picture. The, values for the later lactations were based
on small numbers to be given much weight.
In the absence of progeny testing in dairy cattle, the method of Rendel and
Robertson [1950], was used to estimate the genetic rate of improvement. In this
method, the process of selection is divided into two parts: (a) the selection of cows
whose daughters will be incorporated into the herd, and (b) the selection of cows
whose sons will be used in the herd or sold to another herd. The effect of the inten
sity of selection in sires on future cows, is considerably more than when the selec
tion is made for heifers. This is due to the fact that, while the male progeny could
be bred from the top five per cent of the herd [Lush, 1945], the female offsprings
could not be derived from less than 75 per cent of the total breeding stock in order
to provide sufficient replacement. The phenotypic selection differential of the dams
of cows was computed by finding the mean of all first lactations in the population
and subtracting it from the production record of the dams. But as the basis of
selection may be more than one lactation, the improvement is calculated on one
or the average records on which the merit of the dam was judged. Due weight
was given to the number of daughters left by each dam, and the genetic superiority
becomes:
Σih 2 sn
N
where (h 2 s) is the heritability on s records, (i) is the selection differential, and (n)
the number of daughters for each dam, and (N) the total number of daughters.
The corresponding gains due to selection of dams of bulls were also estimated
by using the same formula, but in this case the dams records were weighted accord
ing to the number of daughters sired by their dam. Separate estimates were obtain
ed for sires that were retained in the herds in which they were bred and for those
that were sold to other pedigree dairy herds.
From the genetic superiority thus calculated it was possible to estimate the
rate of genetic gain per year by dividing it by the total generation lengths [Dickerson
and Hazel, 1944]. Rendel and Robertson [1950], have shown that the total genera
tion lengths ΣL in dairy cattle is the sum of the four components: Lee, LCB,
LBB, and LBO, where,
Lcc is the mean age of dams when the heifer calves are born.
LOB is the mean age of dams when the bull calves are born.
JBB is the mean age of sires when the bull calves are born.
LBC is the mean age of sires when the heifer calves are born.
Values of EL have been calculated in the present study.
In this study, the method adopted to determine the number of daughters left
by each sire and dam was carried out as follows: It is possible to find the daughters
left by sires and dams as long as they remain in the herd in which they were bred.
But when one wishes to trace the sires that were sold to other pedigree dairy herds;
it becomes necessary to study the herd books of the different Breed Associations,
and go through every female entry in every herd book and list those sires that were
relevant to this study. The number of daughters left by these sires were noted
down simultaneously. The heritabilities used are given in Table II.