above-stated genetic conditions apply.

Experiments With Frozen Semen

The practical problem then focused on whether or not similar
events occurred in semen frozen to very low temperatures. Since
the rate of any observed change was expected to be slowed by the
low temperatures employed for semen storage, the period of obser-
vation had to be greatly extended.

Storage at -79 to -88°C. (Solid €02 + alcohol, and mechanical re-
frigeration.[fl

The first experiment with frozen semen (16) was designed to
tell us something about 1) the period of optimum fertility, and 2)
to determine whether or not both sexes were equally responsible for
the observed summer depression of fertility. Two different experi-
ments were conducted, one in which the data were arranged so that
the month of the year breakdown was the month of insemination (sea-
son was associated with the cows insemination) and, a second, in
which the impact of season on the bull was studied, the month of se-
men collection being the basis for the statistical analysis.

The results are shown in Tables 5 and 6. Table 5 shows the fer-
tility and embryonic mortality for Experiment Il by the age of the
semen used. It was similar to the findings for Experiment | which
have been published (16). Fertility increased andembryonic mortal-
ity declined in the first to the second month of frozen semen use.
Fertility was optimum in the second month, remained so for about
three months, and then declined. By six months the fertility had
dropped about 13 percentage units and embryonic losses were on the
increase (16).

Table 6 shows the decrease in fertility from storage of more
than six months for each of the experiments arranged (Experiment 1)
by month of insemination, and (Experiment 11) by month of semen col-
lection. It is clear that the variability in maintenance of fertil-
ity on storage under our conditions is dependent upon the month of
the year in which the semen was collected. That semen collected in
the warm months of the year simply did not maintain its fertility
as well, regardless of the month in which it was bred to cows, as
did that semen collected in the cooler months of the year (16).

17 B 8

—'] am indebted to the directors and management of the Central Ohio
Breeding Association, 1224 Alton Darby Road, Columbus, Ohio 43 228
for permission to conduct and to publish the results of the experi-
ments with frozen semen and to the employees of that organization,
particularly Messieurs Richard Kellogg, Manager, Charles Knoo p,
Laboratory Director, and Donald McKean, Statistician, for the de-
tailed recording of the results of insemination.

192