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Dairy cattle breeding has historically focused on relatively small numbers of elite bulls as sires of sons. In recent years, even if generation intervals were reduced and more diverse sires of sons could have been selected, genomic selection has not fundamentally changed the fact that a large number of individuals are being analysedanalyzed. However, a relatively small number of elite bulls are still siring those animals. Therefore inbreeding-derived negative consequences in the gene pool have brought concern. The detrimental effects of non-additive genetic changes such as inbreeding depression and dominance have been widely disseminated while seriously affecting bio economically important parameters because of an antagonistic relationship between dairy production and reproductive traits. The figure below gives the milestones in unravelling the inbreeding depression in dairy cattle.
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In a review the following inbreeding depression in milk yield traits was found per every percent increase in genomic inbreeding and pedigree based inbreeding, respectively, for milk yield -36.3 and -19.7 kg in 305 days milk yield, for somatic cell count 0.86 and 0.52 CFUs, for fat yield -2.42 and -0.96 kg in 305 days and for protein yield -1.20 and -0.69 kg in 305 days.