Estimation of variance components and prediction of breeding values for scrotal circumference and weaning weight in Limousin cattle.

Variance and covariance components were estimated for yearling scrotal circumference and weaning weight from Limousin field data. Records of 8.226 bulls were used to evaluate 584 sires and 653 maternal grandsires. Data included all herdbook records of bulls with a recorded scrotal circumference and their weaning contemporaries. Analyses were performed by restricted maximum likelihood techniques employing the expected maximization algorithm and fitting both single- and two-trait models. Scrotal circumference was first fitted in a single-trait, sire model to obtain starting values for variances for a later analysis. Likewise, weaning weight was fitted in a single-trait, sire-maternal grandsire model to obtain priors for (co)variances for a later analysis. Scrotal circumference and weaning weight were then fitted together in a two-trait model to estimate variance components. Estimates of variance components were calculated by equating (co)variances obtained from the models to their expectations. Estimates of heritability of scrotal circumference, direct weaning weight, and maternal weaning weight were .46, .25, and .19, respectively. Estimates of genetic correlations between yearling scrotal circumference and direct weaning weight, scrotal circumference and maternal weaning weight, and direct weaning weight and maternal weaning weight were .14, -.22, and -.44, respectively. The estimate of the environmental correlation between scrotal circumference and weaning weight was .61. Genetic parameters obtained were then used in two-trait, reduced animal mixed-model equations for a maternally influenced trait to predict breeding values for animals.