VIe CONG. INTERN. REPROD. ANIM. INSEM. ARTIF., PARIS, 1968, VOL. I

INFLUENCE UPON FERTILITY OF BULL SEMEN OF
STORAGE TIME IN LIQUID NITROGEN

B. STROM
Malmen A,.I. Society,
Hallsta, Eskilstuna, Sweden

More and more attention is payed to the influence upon fer-
tility of storage time of deep frozen semen as new breeding
plans include beneficial calculation for long time storage
of large amount of frozen bull semen.

Field trials have been reported by Rowson (1956) and by
MacPherson (1956). They found no effect on fertility after
periods of four and three years respectively on very small
numbers of inseminations, Mixner and Wiggins (1960) reported

a drop from 66.2 per cent after two years storage to 56.1

per cent after four years of storage, the difference being
just significant. Approximately 100 inseminations were made

at each test period. In another trial by the same authors
(1964) the fertility of frozen semen stored for periods through
two years approximated that of the fresh control semen. The
fertility of semen stored for four and eight years was lower
than fresh semen and semen stored for periods through two
years. In a long term storage Stewart (1961) found no signifi-
cant loss in the fertilizing capacity of frozen semen stored
one, two and four years (approximately 300 inseminations at
each test period). In another long term storage trial Stewart
(1964) reported a drop in non-return rate from 66.5 per cent
to 58.0 per cent, when the semen was stored from 1-4 weeks

to 9 years., The value of the regression coefficient was -0.8
indicating a loss of 0.8 per cent per year ot storage.

The present investigation on the effect of storage on ferti-
lity of bull semen, comprising a total number of 55,378 in-
sem.nations from 66 bulls, is based on the routine material
from a middle Swedish A,I. Society, Malmensemin.

The semen is normally collected all over the year and stored
for longer or shorter times in pellets in liquid nitrogen.

For each 4-week period of the year for the use of the semen,
the average 56 days” non return rate has been determined for
each 4-week period that the semen has been stored. For example,
in calendar weeks 16-20 semen of 1-4, 5-8, and so on, weeks

of age has been inseminated and the average 56 days’ non re-
turn rate has been determined for each one of these age-classes
of the semen,

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