w ith identical twins in New Zealand (7), has led to the development of
progeny testing methods in which the contemporaries of a bull's daughters
in the same herd are used as a standard of comparison. If A.l. provided
evidence that genetic differences between herds were rather small, it
certainly will quickly remove any such genetic differences that there
might have been. The groups of herds that have been using the bulls
from a particular A.l. centre for several generations must be  genetic-
ally very similar to one another.

Fears have been expressed that the very widespread use of individual
bulls will greatly reduce the genetic base in a breed using artificial
insemination and lead to an increase in the rate of inbreeding. What
evidence do we have on this point? In Great Britain at least, the effect
of A.l. on the breeding structure has been much less than might have been
expected. Certainly the availability of bulls at A,l. stations for the
breeding of commercial females has greatly reduced the number used for
natural service but it is not to any great extent disrupted the existing
breed pyramid. A social structure of natural service breeding still exists
in parallel with A.l. organisations and, until recently, almost all bulls
used in artificial inseminations were bred by natural service. In other
w ords, in breeding terms the A.l. organisation was an appendage to the
existing structure rather than an integral part of it. | have myself always
felt that the tremendous possibilities of progeny testing given to us by
A.l. wi!! not be properly utilized until the majority of bulls being
brought into the system were themselves bred by sires which had been
t ested there. This is still not the case in my own country though it does
represent the situation which has existed for some years past in Denmark.

In fact we do not have much evidence on the effects of A.l. on the
rate of inbreeding. In Great Britain the effect on the breeding structure
of pedigree Friesian cattle has been looked at (8). This breed has been
dominated by the importation of bulls from the Netherlands, the last
m ajor importation having been in 1950. The conclusion from the pedigree
analysis was that "by 1960 A.l, had not affected the hierarchical structure
of the breed and had not therefore narrowed its base. Although the 1950
importation of stock was as large as importations in previous years (1914
and 1936) it was interesting that ten years after the importation its in-
fluence on the breed in terms of contributions to females born in 1960
w as very much less than the previous two importations." The point was
m ade (which we will return to later in the discussion of the situation in
D enmark) that the development of progeny testing methods in A.l. had
allowed a much more accurate identification of the inferior imported
sires and had led to the bulls being used in a much more rational manner.
The situation in Denmark is of great importance, as in this country the
domination of the breeding programme by the use of A.l. - there are
very few bulls used for natural service even in the well-known breeders'
herds - makes this a very interesting case for analysis, especially because

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