Genetic (co)variances among birth weight, 200-day weight, and postweaning gain in composites and parental breeds of beef cattle.

Genetic and environmental (co)variances for birth weight, adjusted 200-d weight, and postweaning gain were estimated in nine parental and three composite populations of beef cattle. The parental breeds were Angus (A), Braunvieh (B), Charolais (C), Gelbvieh (G), Hereford (H), Limousin (L), Pinzgauer (P), Red Poll (R), and Simmental (S). The composites were MARC I (1/4 B, 1/4 C, 1/4 L, 1/8 H, 1/8 A), MARC II (1/4 G, 1/4 S, 1/4 H, 1/4 A), and MARC III (1/4 R, 1/4 P, 1/4 H, 1/4 A). Heritabilities of additive direct genetic effects for birth weight (.50) and postweaning gain (.49) were greater than for 200-d weight (.32). Heritabilities of additive maternal effects of .09 for birth weight and .10 for 200-d weight were much smaller than direct effect heritabilities. Heritabilities were larger in composites than in parental breeds for additive direct effects of all three traits but smaller for maternal 200-d weight. Correlations were high and positive for direct genetic effects of the three weight traits and higher in composites than in the parental breeds. Correlations between direct and maternal genetic effects for both birth weight and 200-d weight were near zero. Some differences in variances among populations were correlated with differences in weight and milk yield. Heavier populations had larger variances, supporting the use of logarithmic transformation of weights to stabilize variances among genetic groups. Increased average milk yield was correlated with decreased phenotypic variance of 200-d weight. Average milk yield was also implicated in the expression of direct and maternal genetic effects for 200-d weight and their covariance. Comparison of univariate and multivariate estimates of genetic variances suggested that it is important to include birth weight in multivariate analyses of all weight traits to account for increased preweaning mortality of calves with extremely heavy or light birth weights. Based on heritability estimates, within-herd selection in composites should be at least as effective as in purebreds. Some differences among populations in genetic parameters were indicated, especially maternal 200-d weight and its correlations with other traits.