Evaluation of Sires in Herds Feeding Differing Proportions of Concentrates and Roughages
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To study the effect of a potential shift from rations high in concentrates to high forage on sire evaluation, separate sire evaluations were calculated from records from groups of herds in the Northeast which differed in proportion to concentrates fed. These sire effects were computed from first lactations for each group as though they were fixed effects in a model which included effects of herd year-season. Components of variance for sires and covariance between sire effects in different groups were used to compute genetic correlations. The covariance between solutions for the same sires in two groups is an unbiased estimate of the genetic covariance. Sire variances seemed to be underestimated by Henderson's Method 3, so they also were estimated from the covariance between two sets of sire solutions from splitting each group. The correlations were similar for all combinations of groups and did not decline as the groups compared became more separated in proportion of concentrates fed. Therefore, the fraction of net energy provided by concentrates does not need to be considered in sire evaluations.