Birth Weight as a Criterion of Selection in Beef Cattle

Abstract

Birth weights of calves in the beef Shorthorn herd at Beltsville, Maryland tended to increase at the rate of 0.2 pound per month of increase in the age of the dam until the dams were six years old, after which there was no effect of age of dam. When birth weights were adjusted to the basis of six-year old dams, male calves averaged 4.2 pounds heavier than females. Weight of dam was found to be related to birth weight of calf to about the same extent as was the age of the dam. The multiple correlation between both age and weight of dam and birth weight was 0.56, or very little higher than the simple correlations. A significant but relatively low positive correlation was found between the length of the gestation period and birth weight. With the use of half-sib correlations, heritability of the original birth weight was 29 percent, while for birth weights corrected for age of cow, sex, and dam's index (average birth weight of all calves from a given dam) the heritability was 11 percent. Of the calves which were fed out as steers, the largest ones at birth and those with the highest prenatal growth rates tended to reach 500 pounds (weaning weight) and 900 pounds (slaughter weight) the soonest. There was little relationship between birth weight or prenatal growth rate and the length of the feeding period from weaning to slaughter. Correlations between birth weight or prenatal growth rate and economy of gain during the feeding period showed there was practically no association. The results of this study indicate that birth weight should be given some consideration in selection, in view of the fact that it had an influence on the time required for the animals to attain a slaughter weight of 900 pounds. The results also indicate that since the dam had a considerable maternal influence on birth weight as well as on the subsequent development of the calf, greater care should be taken in the selection of the dams and in the evaluation of their influence in progeny tests of sires. Copyright © . .