Vle CONG. INTERN. REPROD. ANIM. INSEM. ARTIF.,, PARIS, 1968, VOL. I

of the high concentration on a few bulls in the decades before the use of
artificial insemination. We have the additional advantage here that the
population is closed. Measurements of the absolute amount of inbreeding
in Red Danes have been made recently with the surprising result that (at
least in the samples taken) the absolute amount of inbreeding was less for
the animals most recently born than it had been for previous groups. Thus,
not only had A.l. not produced an increase in the rate of inbreeding, it
had in fact slowed it down. How had this come about?

The concentration of blood which occurred in the days before A.l.
w as widely used was due to the establishment of reputations by compara-
tively few sires at any time. Animals containing these ancestors in their
pedigree were widely sought after and the desirability of an animal might
w ell be increased by having a fashionable bull as a great-grandfather.
The pyramidal structure of the breed also made the spread of genes from
individual animals throughout the whole breed very rapid. For instance,
the bull having the greatest contribution to the British Friesian breed at
the moment, significantly increased his genetic contribution from 5% to
8% between 1945 and 1960, though he himself was born in 1921, Thus
not only did few bulls make reputations but they retained them for a long
time. | would suggest that the Danish results are a consequence, not of
the A.l. structure itself, but of the ability of the Danes, mostly through
the use of progeny testing stations,to-judge the bulls in A.l. adequately
and quickly and to use the best of them for bull breeding. About fifty
Red Danish bulls are so tested each year. Thus several bulls may establish
reputations in a given year but these are liable to be dimmed fairly rapidly
as new bulls make their names in the years ahead. In this situation, a
bull's influence cannot be exerted at such a distance through a pedigree
as it could in the past. Then the sons and grand-sons of a very famous
bull would acquire a reputation almost by reftection. At present in Den-
mark, a bull may be very well known and have many sons in A.l., but
these sons are going to be evaluated as individuals. If they do not breed
satisfactorily, their father's influence will not spread widely. The bulls
w hose progeny do well are used very extensively to breed sons. Looking
at the annual report of A.l. in Denmark in 1966, we find that of the 636
Red Danish bulls in use 69 were by one sire, 49 by another and that in all
eight sires had more than 20 sons in A.l. All of these sires have of course
done well themselves in progeny testing stations. Though 69 sons would
seem to be a very large contribution to the breed, the fact that there are
so many bulls contributing sons at any time would suggest that the rate of
inbreeding is not going to be particularly high. | mentioned earlier the
m odel of a cattle population in terms of the number of sires from which
sons are bred. The effective number of sires of bulls in use at any time in
the Red Danish breed in Denmark seems to be of the order of 25. If we
accept this figure as a basis for a prediction for the rate of inbreeding, this
would suggest a rate only about 1/3 as fast as held in most breeds in the

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