Contemporary Groups for Genetic Evaluations

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Abstract
Contemporary groups are used to remove biases from genetic evaluations due to differential effects such as management associated with the grouping. Numerous groups, however, can result in small numbers of records per subclass with associated loss of effective number of daughters for sire evaluation and increased prediction error variance. Thus, in practice, mean square error, bias squared plus prediction error variance, may be more meaningful than bias alone or prediction error variance. Considering contemporary groups as fixed removes bias due to association between effects corresponding to contemporary groups and sires. If contemporary groups are considered random, then effective number of daughters is increased at the expense of possible bias. Various compromises may be effective for increasing genetic gain. Arbitrary definition of contemporary groups can include herd-year-season of freshening, lactation number, registered or nonregistered, sampling or postsampling daughters, and special treatments among others. The assumption of homogeneous genetic and residual variances is likely to be incorrect. Alternative methods include simple transformations, a two-step transformation, and multiple trait modeling. Multiple trait analyses may include the assumption of genetic correlations of unity, common genetic and heterogeneous residual variances, and joint estimation of genetic values and variances.