Evaluation of using half-sibling beef cows to increase growth and carcass uniformity of calf crops

Uniformity of growth and carcass traits of calves resulting from commercial beef cows that were either half-siblings (HS) or similar phenotype but unknown genetic relationship (PS) were determined over 5 calf crops. Residuals (n = 853) of 205-d adjusted weaning weight from a repeated-measures ANOVA model were evaluated for homogeneity of variance between cow groups with Levene’s test in PROC GLM of SAS. Adjusted weaning-weight SD was not different between cow groups (22.4 and 21.6kg for HS and PS cows, respectively; P = 0.56). Lifetime weaning-weight production SD was 320kg for HS cows and 322kg for PS cows (P = 0.95). Healthy calves from each year were finished in a commercial feedlot. Standard deviation of HCW, LM area, and LM area per unit of HCW of calves from HS cows tended to be less (P ≤ 0.15) than SD of calves from PS cows, whereas finishing-period ADG tended to be greater. Observed differences in calf-trait SD were small (approximately 1.5%) and overall were similar to expectations calculated from quantitative genetics theory. However, observed differences in SD fluctuated widely from expectations for any given trait, indicating that very large samples may be needed to reliably observe the small differences that are expected. Therefore, preferentially selecting commercial beef cows so that they have half-sibling genetic relationships, over and above phenotypic selection criteria, appears to have limited ability to reduce phenotypic variability of calf crops, especially for traits with low heritabilities.