Some Factors Affecting Gestation Length and Birth Weight of Beef Cattle

Data are presented on the influence of breed, age of dam, sex of calf, sire, years, and interaction of sex and breed on length of gestation in beef cattle. The average gestation lengths for the three breeds studied were: Angus, 281.7 days; Hereford, 286.1 days; and Shorthorn, 284.3 days. The differences between the mean gestation lengths for the breeds were statistically highly significant. Bull calves tended to be carried longer than heifer calves. Within breeds the difference between the sexes was significant only for the Angus breed. The influence of age of dam, sire, years, and breed-sex interactions on the length of gestation was not significant. Data are also presented showing the effect of gestation length, breed, sex, age of dam, sire, years, and heredity on the birth weight of the calf. The birth weight of the Hereford calves was significantly larger than those of the Angus and Shorthorn calves. The difference between the Angus and Shorthorn birth weights was not significant. The regression of birth weight on gestation length was +.376 and was highly significant. A highly significant difference between the birth weights for the sexes was found. About 10 percent of the sex difference was due to differences in gestation length. The data indicate that a definite relationship exists between age of dam and birth weight of calf and that the maximum birth weight is not reached until the cows are nine to ten years old. The regression of birth weight on age of dam was +1.04 and highly significant. The effect of sires on birth weight was insignificant as were the year effects. The paternal half-sib regression gave an estimate of heritability of 0.22 for birth weight.