Marker assisted selection of heifers improved milk somatic cell count compared to selection on conventional pedigree breeding values

Abstract. Although mastitis in cattle is an important factor for dairy economy and animal welfare and although udder health parameters have a substantial genetic variability, in many countries there is little or no improvement of udder health in the conventional commercial breeding programs. Strategies implementing information about Quantitative trait loci (QTL) via genetic marker information seem to offer new prospects to improve this situation. In a proof-of-principle approach, we show that selection of German Holstein heifers prior to first calving based on marker information regarding a confirmed QTL affecting somatic cell score (SCS) on bovine chromosome 18 (BTA18) (MAS strategy) indeed enabled prediction of halfsibs with a high (q) or a low (Q) number of somatic cells in milk already early in the first lactation. Compared to a strategy relying on conventional breeding values only (CON strategy), selection including marker information resulted in a stronger discrimination between and a higher uniformity within the MAS-Q and -q groups compared to the CON-Q and – q groups selected by conventional selection strategies. The selected heifers, which are clinically unaffected, however genetically different in their capacity of mammary gland protection against pathogens, will serve as tools for a comprehensive expression analysis to highlight molecular processes underlying a different genetic predisposition to mastitis susceptibility. Thus, functional mechanisms resulting in a decreased susceptibility of the Q individuals can be identified to further improve selection on udder health in cattle.