vapour freezing method of Jondet. For the first time standard
products with a simple method of preparation were available, as
was a simple standard method of freezing.

Since 1966, 337,069 first inseminations have been carried
out in the North of England at a 30/60 day non-return rate of
77.1% and this compares with a liquid semen non-return rate of
77.7% for 361,816 cows for the previous comparable period. Soon
it is expected that 350,000 cows per annum will be inseminated
in a completely liquid nitrogen service.

Several techniques are involved in the overall result and
these will be discussed and the results shown on the adjoining
tables.

Another factor in the results has been the greatly inc-
reased demand for nomination services since the advent of the
liquid nitrogen service in straws and 20% of services are carried
out with nominated bulls at an enhanced fee. This figure
compares with 3/4% of nominations prior to the liquid nitrogen
service and demonstrates the reliability of the service, although
it has been found that nominated services give a lower non-return
rate than commercial services. There are reasons for this, for
which space does not permit discussion.

Initially semen was collected p.m., diluted and cooled to
59.C., then glycerolised slowly over a period of two hours and
frozen a.m., the following day. During this period 30,176
firsts were carried out giving a 30/60 day non-return rate of
77% and this compares with a non-return rate on 138,117 firsts of
76.9% when semen was glycerolised quickly and allowed a similar
equilibration period - fig. a.

Next, comparisons were made between semen glycerolised by
the two methods and with a 6/7 hour equilibration interval;
37,462 firsts with the slow method gave a non-return rate of
77.1% whilst 36,519 firsts with the quick method gave a non-
return rate of /7.4% - fig. b.

A further analysis showed that with the quick method glyc-
erolisation and short equilibration period a higher percentage of
e jaculates were frozen, the rejection rate prior to freezing was
0.2% and after freezing 3.5%; whilst for the quick method of
glycerolising with long equilibration period the rejection rate
prior to freezing was 2.5% and after freezing 7%. Finally the
slow method of glycerolising and long equilibration gave a
re jection rate prior to freezing of 2.5% and after freezing of

13%.

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