Top down preselection using marker assisted estimates of breeding values in dairy cattle

Top down preselection of young bulls before entering progeny testing has been proposed as a practicable form of marker-assisted selection (MAS), especially in dairy cattle populations with large male paternal half-sib families. Linkage phase between the superior (Q) and the inferior (q) QTL alleles of heterozygous sires (Qq at the QTL) with informative markers is established within each paternal half-sib family and may be used for selection among grand-progeny. If, additionally to sires, bulldams are also genotyped and data from consecutive generations are used, then a marker-assisted best linear unbiased prediction (MA-BLUP) model can be employed to connect the information of all generations and families of a top down design, and to select across all families. A customized 'augmented' sire model (with sires and dams of sires as random effects) is introduced for this purpose. Adapted formulae for the mixed model equations are given and their equivalence to a corresponding animal model and to a certain variant of previously proposed reduced animal models is shown. The application of the augmented sire model in MA-BLUP estimation from daughter-yield deviations and effective daughter contributions is presented.