Milk Genomics – Impact of Genetic Polymorphism on Bovine Milk Composition and Processability

Abstract: Milk genomics is a new area of research that explores the relations between cow genetics and milk characteristics. Identifying the genes controlling compositional and technological traits of milk will give opportunities to select cows for targeted milk production. In this thesis, the impact of different genetic approaches, i.e. quantitative genetic evaluation, single gene effects and genomic selection, on numerous compositional traits and technological properties of milk have been studied. The cows used were elite dairy cows of the Swedish Holstein and Swedish Red breeds with high genetic merit, reflecting the future Swedish cow population. The elite dairy cows were shown to have genetic progress in relation to yields of milk and milk components that are to a great extent contributing to the Nordic total merit of the Swedish breeding objective, whereas the contents of some milk components, e.g. lipid and whey protein contents, had decreased on average. Milk from the elite cows had good rennet-induced gelation properties, but was more susceptible to lipid autooxidation. These findings show clearly that breeding program influences milk characteristics considerably, offering opportunities to alter milk quality through breeding. Selecting for smaller casein micelles and lower milk pH through breeding would enhance gelation properties and may thus improve the initial step in the processing of cheese. Furthermore, it was shown that the majority of the casein micelles have a spherical shape, but a low concentration of larger and elongated aggregates exists, which can be expected to have a large impact on technological properties during renneting of milk. Polymorphisms in the leptin, leptin receptor, ß-casein, κ-casein and ß-lactoglobulin genes were shown to be associated with milk composition traits and cheese characteristics, thus indicating possibilities of using these polymorphisms as markers within genetic selection programs to improve and adjust several parameters and properties. Using genomic selection, relations between direct genomic values for traditional production traits and novel milk traits were found. This shows opportunities of indirect selection of milk composition traits and technological properties important for cheese production that are not registered in the milk recording scheme. In conclusion, this thesis has shown possibilities of changing the quality and functionality of milk through selective breeding. The Swedish breeding objective is today and was previously selecting for milk composition by analyzing the contents and yields of lipids and proteins. This study goes beyond this traditional breeding strategy by analyzing compositional traits and technological properties in detail and has thus contributed to a deeper knowledge of milk quality. This gives opportunities to meet new demands on milk, which certainly will increase the economic output of both the breeding and dairy industries.