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Fertility of Bull Semen as Influenced by Dilution Level, Antibiotics, Spermatozoan Numbers and the Interaction of these Factors

American Foundation for the Study of Genetics, Madison, Wis.

ABSTRACT

Two controlled, factorial experiments involving 6,407 services from 32 collections from nine bulls were conducted to determine whether or not there was a different rate of decline in breeding efficiency when dilution of semen was increased from 1:100 to 1: 300 with yolk-citrate-sulfanilamide-penicillin-streptomycin than without the antibiotics. There was a slight and insignificant difference. Therefore, the decline in breeding efficiency with increase in dilution level can be assumed to be the same whether or not antibiotics are used.

Although there was a very highly significant difference in non-return rate for 1:100 and 1: 300, this difference was no longer significant when the non-return averages were adjusted to the same spermatozoan numbers by means of covariance. These results indicate that spermatozoan numbers are much more important than dilution effect in influencing breeding efficiency at the dilution levels studied.

Addition of antibiotics caused a significant improvement in breeding efficiency over samples not containing them. There was evidence that some bulls were benefited more than others, but such bull differences could not be definitely established.

Non-return rates from three experiments representing 10,745 services were calculated for different numbers of spermatozoa per milliliter of diluted semen. The data for 4 million or more spermatozoa per milliliter were linear. The regression coefficient for this range was 0.6. As spermatozoan concentration was reduced below 4 million, the decline in breeding efficiency was much more rapid. The regression coefficient was 3.6. An average non-return rate of 53 per cent was obtained with samples containing 1.5 to 1.9 million spermatozoa per milliliter.

Field data from 72,336 services were studied in a similar manner. These data as a whole and also those for 4 million or more spermatozoa per milliliter were curvilinear. There was a gradual decline in breeding efficiency as spermatozoan numbers were reduced from 12 to 8 million, a more rapid decline as numbers were reduced from 8 to 4 million, and a very rapid decline as numbers were reduced below 4 million. An average non-return rate of 49 per cent was obtained with semen containing 2.4 to 2.9 million spermatozoa per milliliter.