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

structure for the breeding programme, making allowance for the amount
of inbreeding and its probable effect on production. He then suggests
that, for populations of the size of several hundred thousand cows, the
input of two new tested sires each year into the bull breeding nucleus
would give the maximum rate of improvement. This would be equivalent
to about 10 sires to breed males every generation and might give a rate
of inbreeding of the order of that usually in breeds using natural service
in the past. It would, however, be very necessary in such s¢hemes to
make sure that a bull which acquired a particularly high reputation was
not used too much. It might also be necessary to make special matings
of such bulls to their daughters to detect previously unrecognised harmful
recessives. There are some geneticists who believe that this will "come
out in the wash" by the harmful recessives appearing in the normal course
of the breeding programme. | myself feel that this is somewhat too late
and that some special test matings should be carried out.

In several countries in which breeding programmes dependent on A.l.
are now being developed, such as in Great Britain, the existence of a
parallel breeding programme using natural service presents at the same
time a danger and a safeguard. The danger is that there will always be
so much dilution of the genes in the A.l. sector of the population from
the other that the possibilities of improvement in A.l. will not be utilized
to the full. On the other hand the existence of the natural service sector
does provide a safeguard against mistakes in the breeding programme in
the A.l. sector. Looking even further ahead one might speculate as to
what extent it might be possible to develop within a country (or perhaps
within a particular breed type) a number of separate and distinct genetic
strains. Selection within these strains could be based on the progeny
testing of the bulls in A.l. but strains could be kept distinct in the pro-
duction of new males. In breeding the commercial cattle of the country,
it would then merely be necessary to make sure that two males from a
given strain never succeeded one another in the same pedigree. One
might then eventually get into a situation in which selection pressure was
being exercised by means of progeny testing but in which the commercial
cattle of the country would not be increasing their homozygosity. | feel
myself that some such breeding structure making use of hybrid vigour will
eventually be used in dairy cattle as it probably will in all other breeds
of livestock. It will probably come later in dairy cattle than other
species and it may be very difficult to set up because of the inevitable
tendency for a very good sire to be used throughout the breed, but | sus-
pect that in the long run some breeding programmes like this will be used.

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