Co-segregation of quantitative trait loci (QTL) for milk production traits and length of productive life with QTL for left-sided displacement of the abomasum in German Holstein dairy cows

Left-sided displacement of the abomasum (LDA) is a commonly observed disease in Holstein dairy cows, previously shown to be genetically correlated with milk performance traits and length of productive life (LPL). The objective of this study was to detect joint quantitative trait loci (QTL) among milk production traits and LDA using a paternal half-sib design including only LDA affected daughters. Joint QTL for LDA and milk production traits in LDA-affected daughters may indicate genetic correlations among these traits due to linkage disequilibrium or even pleiotropic genes. We genotyped 14 paternal half-sib groups including 328 LDA affected daughters and eight sires for 302 microsatellites. Linkage analyses were performed for milk performance traits, somatic cell score (SCS), and LPL. In total, we identified seven genome-wide and further 30 chromosome-wide significant QTL for milk performance traits, SCS, and LPL. All four QTL for LPL, two of the QTL for fat yield, and one QTL for protein percentage overlapped with QTL identified for LDA and twelve further ones overlapped with or were adjacent to family-dependent QTL for LDA. Furthermore, polymorphisms located within the DGAT1 and ABCG2 genes, which were previously reported to be associated with milk performance traits were analysed for all cows. The DGAT1 SNP was associated with milk yield, fat yield, fat percentage, and protein percentage, but showed no association with LDA, while the ABCG2 SNP was monomorphic. We can conclude that genetic correlations among LDA and LPL may arise from these joint QTL. Therefore, selection for longevity should even lower LDA incidence in German Holsteins. A lower co-incidence for LDA-QTL locations was found for fat yield and protein percentage. This study is a step towards better understanding of genetic correlations of LDA with milk performance traits and identification of possible side-effects due to selection for milk production in dairy cows.