Inbreeding effects in dairy cattle populations - Part 1: Analyses using pedigree data

Increasing levels of inbreeding with the loss of genetic diversity have been observed since years and are therefore the subject of scientific research for many decades and the resulting consequences for the economically important performance, health and functional traits in dairy cattle breeding. The aim of this study was therefore to review methods and previous findings on inbreeding effects in dairy cattle populations based on pedigree data from the literature. Negative effects of inbreeding (inbreeding depression) are well documented for breeding traits in livestock populations. An increase in the inbreeding coefficient based on pedigree data by 1% was associated with a decrease in the phenotypic trait value by 0.13% of the mean or by 0.59% of the phenotypic standard deviation. A reduction of inbreeding depression (purging) can occur when, as a result of selection, the individuals affected by stronger inbreeding depression in their survival and reproductive rate are increasingly impaired or used less frequently in breeding and thus alleles with negative effects on fitness are lost in the population. If sufficiently deep pedigree data are available, it becomes possible to show effects of reduced inbreeding depression using ancestral inbreeding coefficients. Across all studies, the estimated value for the ancestral inbreeding depression F-a_Kal according to KALINOWSKI et al. (2000) was -1.272% of the phenotypic standard deviation, for the new inbreeding F-a_New according to Kalinowski et al. (2000) -0.866% of the phenotypic standard deviation and the interaction between F and F-a_Bal (Ballou, 1997) -7.063% of the phenotypic standard deviation. This shows that purging of the inbreeding depression by selection was not detectable for all traits investigated, as the inbreeding depression was higher for ancestral inbreeding than for new inbreeding or the inbreeding depression associated with the interaction of the classical inbreeding coefficient with F-a_Bal. For the 305-day milk yield traits of the cows, there was clear evidence of purging of inbreeding depression in the Holsteins in the Netherlands and a reduction of inbreeding depression in the Holsteins in Ireland. However, for fertility and longevity traits, there was no reduction in inbreeding depression due to selection. It is therefore important to minimize the increase in ancestral and new inbreeding. In this way, a reduction in selection progress through inbreeding depression can also be prevented.