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Relation of Various Factors to the Breeding Efficiency of Dairy Animals and to the Sex Ratio of the Offspring

Bureau of Dairy Industry, Agricultural Research Administration, U. S. Department of Agriculture

ABSTRACT

The breeding records of the Bureau of Dairy Industry herd at Beltsville, Md., have been studied from the standpoint of breeding efficiency and sex ratio of offspring as influenced by the ages of the cows and bulls and by the season of the year.

The sires showed a gradual lessening of breeding efficiency with advancing age, with the exception of the 7-year-old group. No explanation can be offered for the decrease that occurred in number of services per conception at this age.

After the first gestation, age had little apparent effect upon the breeding efficiency of the cows. Heifers being bred for the first time required more services than the older cows.

Bulls over 5 years of age showed a distinctly higher number of services per conception than did young bulls when bred to heifers being bred for their first gestation. Young bulls that were bred exclusively to virgin heifers proved even more efficient than the whole group of young bulls.

Breeding efficiency of the cows was not appreciably affected by length of calving interval.

The most noticeable effect of season on breeding efficiency was the relatively large number of services required for conception during midsummer, followed by a sharp decrease in the fall.

Data showing the yearly variation in breeding efficiency indicate that there was a tendency for the years of lowest breeding efficiency to follow years when the percentage of abortions was highest.

A tabulation of the complete breeding life histories of 725 cows in the Beltsville herd indicates the probable breeding losses in a dairy herd from various causes.

There is no evidence that any of the factors studied influenced the sex ratio of the offspring.