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

increased 1.7 percentage points in the low fertility bull group,
increased 0.9 percentage points for the medium fertility bull
group, and declined 1.5 percentage points in the high fertility
bull group.

We can only speculate about the reason for the difference
in the behavior of the three bull groups. Perhaps the confus-
jon that already exists in the literature in regard to optimum
fertility and motile sperm concentration can be explained some-
what by these data and observations. With liquid or frozen
semen, the fertility of inseminating doses approximating 5 x 106
motile sperm has been found to be inferior to or essentially
equal to higher concentrations.

This is not the first time data have been obtained such as
these determined for the group of bulls of high fertility. Dunn,
et al. (1), similarly observed lowered fertility with the high-
est sperm cell concentrations in two of three replications when
5, 12, and 30 x 10° total cells per dose of frozen semen were
compared. In a less comprehensive frozen semen field trial by
American Breeders Service in 1957, the 60-90 day non-return
rates observed with 20, 30, and 40 x 106 total cells were T2
75, and 72% (P <0.10) respectively. Recent data published by
the Milk Marketing Board (2), comparing 5, 10, and 15 x 10 cells
per dose in a liquid semen field trial totaling 6,000 first ser-
vices showed 30-60 day non-return rates of 84.3, 8k4.3, and 83.Th.

In summary, bulls of differing fertility levels react dif-
ferently to changes in sperm cell concentration. It may be ad-
visable for an A.I. organization to rank bulls according to their
fertility and then assign them desired sperm concentrations so
calculated as to achieve optimum fertility.

REFERENCES.

(1) Dunn, H.O., Larson, G.L., and Willett, E.L., 1953. Prelimi-
nary Breeding Results With Frozen Semen. J. Dairy Sci.,
36:578.

(2) Milk Marketing Board. Report of the Breeding and Produc-
tion Organization (1966/67), 17:123.

(3) sullivan, J.J., Elliott, F.I., Bartlett, D.E., Murphy, D.M.,
and Kuzdas, C.D., 1966. Further Studies on Use of Polymyxin
B Sulfate with Dihydrostreptomycin and Penicillin for Con-
trol of Vibrio fetus in a Frozen Semen Process. J. Dairy
Scith 49:1569.

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