Vie CONG. INTERN. REPROD. ANIM. INSEM. ARTIF., PARIS, 1968, VOL. II

THE MINIMUM NUMBER OF SPERMATOZOA
PER FROZEN SEMEN INSEMINATION
COMPATIBLE WITH NORMAL FERTILITY
IN CATTLE

D.L.STEWART and G.H.BENNETT
Ministry of Agriculture, Fisheries and Food,
Cattle Breeding Centre, Shinfield, Reading,
England.

The object of the cattle artificial insemination services
is the improvement of livestock by extending the use of
superior sires. This is achieved by the dilution or extension
of semen and its subsequent storage before use. In this
context the minimum number of spermatozoa per insemination
consistent with optimum fertility is of interest. The
literature has been reviewed (1) and (2) in respect of liquid
semen and it was considered that about 10 million spermatozoa
per insemination was in fact the minimum. More recently it was
reported that satisfactory non-return rates with as few as
2.5 millién spermatozoa per insemination could be obtained using
semen extended in the "Caprogen" diluent (3). Less extensive
studies have been made with frozen semen. The consensus of
opinion has considered that 30 to 45 million is the optimum
number although there are a few satisfactory reports on the use
of 20 million per insemination.

This paper gives the results to date of a series of field
fertility trials designed to study this subject in relation to
the use of semen frozen in French straws and to determine at
which number of spermatozoa fertility shows a definite decrease.

METHODS .

In the initial trial the three treatments were 30 x 106,
20 x 109 and 10 x 10° total spermatozoa respectively per 0.5 ml.
straw. Two semen collections from each of three bulls were used
and the treatments balanced between bulls, collections and the
six inseminators engaged in the work.

Immediately after collection the number of spermatozoa per
ml. was assessed by the use of a photo-electric absorptiometer
then the ejaculates, after primary dilution, were split. The
final composition of the diluent was 25 parts egg yolk, 65 parts
3.6 per cent. sodium citrate (Nag CgHg07.2H50) solution and 10
parts glycerol to which was added 500 pg streptomycin sulphate
per ml.

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