For a long period cattle breeders have considered the average gestation period of all breeds of cattle to be in the neighborhood of 283 days. Lush (1937) made a study of 27,810 gestations and found the average to be 282.1 days, which is close to the generally accepted average. Rife, et al. (1943) report a marked difference in the gestation periods of Hereford and Aberdeen-Angus cows maintained in the Ohio Experiment Station herds. The gestation periods of the same cows when cross-bred were approximately half way between. The Hereford cows when mated to a Hereford bull had an average gestation period of 289.0 days; the Aberdeen-Angus cows when mated to Aberdeen-Angus bulls had an average of 272.8 days; and when cross-breeding was practiced in both the Hereford and Aberdeen-Angus cows had an average of 281.4 days
The West Virginia workers assumed that such variations were indications that the gestation period varied in accordance with earliness of maturity among breeds and is a heritable variation.
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