80
THE DEEP FREEZING OF SEMEN
[ Vol. XXIV, Part II,
the ampoules are transferred to their permanent container, which is a heavily in-
sulated box containing trays of alcohol at —79°C. Bull semen has been stored in
this way for a year and still shows good activity on thawing and pregnancies have
been obtained with semen seven months old which is the oldest tried so far. The
limit of successful storage at this temperature is not known, but it may well be
many years, or even many tens of years.
The practical applications of the use of this technique are obvious.
In the first place it will enable artificial insemination centres to provide a com-
plete nominated service whereby any breeder can select whatever bull he likes for
use in his herd. There is no wastage of semen at all as every single dose can be
used up and this will allow greater use of proven sires. It will provide a valuable
method of progeny testing as banks of semen can be established for use long after
a bull is dead. In countries such as New Zealand with an extremely short breeding
season, stores of semen can be laid down during the quiet period so that the bulls
are not overworked at the peak time. The fact that activity is almost stopped on
freezing means that whatever the age on the semen at the time of use it is really
only about one day old biologically and if used soon after thawing, this point
should be reflected in an improved conception rate. The international transport
of semen will be made a simple process, for shipments could now be sent by sea,
the time factor being unimportant. In addition it provides a method of quarantine,
for where semen is coming from countries where foot and mouth disease exists, it
can be held in store until the incubation period for the bull from which it came, has
passed, and provided this bull is still free from disease, then it would be safe to use the
semen in the country to which it has been sent.
Theoretically, it would also be possible to cover enormous numbers of cows
from a single central station if only frozen semen was used. Banks of stored semen
could set up at all sub-centres so that in the event of the main unit being knocked
out by foot and mouth disease, the sub-units had several months' supplies of semen
in hand which could give the main centre time to find replacements and yet still
allow the services to continue. Questions of inbreeding, etc. would have to be
closely watched in such a case.
This technique is still new and its possibilities have not yet been fully explored
but it will undoubtedly be of great benefit to the artificial insemination movement
as a whole, and particularly to the development of the international exchange of
semen.
Preliminary experiments using semen from other species have been carried
out but some difficulties have been encountered particularly with the boar and
stallion. Boar sperm has, however, been kept alive for five months at —79°C.
although a large number of sperms die off during the initial period of glycerol pene-
tration. The same features also apply to the stallion but there is little doubt that
as time progresses, techniques will be developed for the successful storage of these
species also.