Source: Gutiérrez-Reinoso MA, Aponte PM and García-Herreros M (2022). A review of inbreeding depression in dairy cattle: current status, emerging control strategies, and future prospects. Journal of Dairy Research 89, 3–12. https://doi.org/10.1017/ S0022029922000188
Dairy cattle breeding has historically focused on relatively small numbers of elite bulls as sires of sons. In recent years, generation intervals were reduced and more diverse sires of sons could have been selected, because with genomic selection a much larger number of individuals are being analyzed. 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.
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.