possibility of non-random seasonal effects. The effect of sea-
son per se is the subject of another paper to be presented at
this Congress.

Storage in liquid nitrogen prior to distribution varied
from approximately three months to one year. Each of the three
treatment concentrations related to any one season of use was
treated the same, however.

The semen was extended in an egg yolk-sodium citrate-glyc-
erol extender containing antibiotics as described by Sullivan,
et al. (3). One ml glass ampules were used and coded for treat-
ment identification. The inseminating technicians recorded the
collection code at the time the semen was used along with all
other pertinent information on the breeding receipts. First
services were accumulated. Inseminations which took place out-
side the intended season were not used in the final tabulation.
Second services were accumulated on a 60-90 day basis.

RESULTS. The mean concentrations of progressively motile sperm
were 5.19 x 100 (low), 10.06 x 100 (medium), and 14.91 x 10
(high). On a total of 57,130 first services, the mean 60-90 day
non-return rates for the low, medium, and high concentrations
were 73.3, 76.2, and 76.3% respectively (Fig. 1). The difference
between the low concentration and the medium and high concentra-
tions was highly significant (P <0010

In an attempt to explore the possible relationship between
the non-return rate as affected by the concentration of progres-
sively motile sperm and the fertility level characteristic of
different bulls, the data were partitioned. The bulls were
placed in groups of three according to their over-all 60-90 day
non-return rate in this study. The low group had a mean non-
return rate of 72.6%, the medium group 75.4%, and the high group
77.9%. The difference between the low and medium group was
highly significant (P <0.01) and the difference between the
medium and high group was significant (P <0.05).

When the concentration was raised from 5 x lO6 to 10 x lO6
progressively motile cells per ampule, the non-return rate in-
creased in all bull groups, but to a lesser degree with the bull
group of higher fertility. That is, the non-return rates were
raised 3.1, 2.8, and 2.1 percentage points respectively for the
low, medium, and high fertility bull groups (Fig. 1), When the
concentration was increased from 10 x 10° to 15 x lO6 progres-
sively motile cells the response in non-return rate differed
(P €0.10) among the bull groups. The non-return rate was

1308