Influence of Sex and Sire on Weaning Weights of Southwestern Range Calves2

Abstract

Sex and sire influences upon the weaning weights of Southwestern range calves were evaluated, and heritability estimates for weaning weight were computed for each sex. Records on 329 bull calves and 332 heifer calves produced by 11 sires on two ranches over a period of 6 years were used. The calves were purebred Herefords weaned at an average age of about 270 days. The weights were adjusted to a standard weaning age of 270 days and to a constant age-of-dam basis preparatory to analysis. Bull calves were significantly heavier than heifer calves (P<.01). The sex differences in weaning weights ranged from 44 to 99 pounds and from 72 to 99 pounds on ranch I for the periods 1949 through 1954 and 1952 through 1954, respectively. Sex differences ranged from 53 to 77 pounds on ranch II for the period 1952 through 1954. The sex x sire interaction within ranches and years was not significant. Because of significant sex x ranch and sex x year interactions, the use of a mean sex difference in weaning weights as a sex adjustment factor on various ranches or even over a period of years on the same ranch was deemed inadequate. The difference between the mean weaning weights within ranches and years, after the weights are adjusted to a standard weaning age and to a common age-of-dam basis, should provide a more realistic adjustment factor for large herds. Highly significant differences in weaning weights among years and between ranches and a significant ranch x year interaction (P<.05) support a recommendation that weight comparisons be confined to single ranches and specific years in commercial selection for improvement of weaning weight. Sires had a significant influence on the weaning weight of both bull (P<.05) and heifer calves (P<.01). Within-sex estimates of the heritability of weaning weight by the paternal half-sib method, with approximate 95 percent confidence intervals, were 0.28 ± 0.32 and 0.57 ± 0.41 for bulls and heifers, respectively. The difference between the two estimates as considered nonsignificant, but the higher estimate derived from the heifer progeny demonstrated a phenomenon also reported in other studies when within-sex estimates were calculated for several traits. Copyright © . .