Calving ease and growth rate of Simmental-sired calves. III. Direct and maternal effects.

Field records from the American Simmental Association were used to estimate genetic parameters for direct and maternal effects of traits associated with dystocia. Estimates of the additive genetic variances of direct and maternal effects and of the additive genetic covariances of direct and maternal effects were calculated by equating the sire variance component, maternal grandsire variance component and covariance component between sire and maternal grandsire to their biological causal components. Independent variables in the mixed-model, least-squares analyses were herd, sire or maternal grandsire of the calf, sex of calf and percentage Simmental in the calf. The dependent variables were calving ease (score), assisted births, birth weight and gestation length. Sire and maternal grandsire were significant sources of variation for all traits studied. Maternal grandsire accounted for two to three times more variation in calving ease and assisted births than did sire, whereas site accounted for a larger percentage of the total variation in birth weight and gestation length. Heritability estimates for the maternal effects were .20, .13, .10 and .09 for calving ease, assisted births, birth weight and gestation length, respectively. Genetic correlations between the direct and maternal effects were -.53, -.55, -.24 and -.38 for calving ease, assisted births, birth weight and gestation length, respectively. The effect of these correlations on selection is discussed.