Exploring Coagulation Properties in Bovine Milk Using Milk Genomics Approaches

Abstract: Swedish Red (SR) dairy cattle is the second most common breed in Sweden and hence SR milk quality is crucial for the nutritional value and the processability of the milk delivered to the Swedish dairies. This thesis will give an insight into the quality of milk from SR cows and it will illuminate the possibilities for improvements of both breeding programs and the quality of different dairy products, focusing on cheese characteristics. As part of the Swedish-Danish Milk Genomics Initiative, SR milk was compared with milk from two other Nordic breeds, Danish Holstein (DH) and Danish Jersey (DJ). The present thesis showed that the variation in milk composition, and consequently milk gelation properties, could be explained by genetic variation. This genetic variation could be observed between different dairy cattle breeds (SR, DH and DJ), but also within the breeds. Milk from SR cows had poorer milk gelation properties than milk from DJ cows and to some extent also than milk from DH cows. Furthermore, 18% of the SR cows produced non-coagulating milk. However, large variations were found within the SR breed showing that there is room and possibilities for improvement of gelation properties. The variation in gelation properties was best explained by variations in ionic calcium concentration, total calcium content and casein micelle size. Heritability estimates were found to be moderate to high for the rennet-induced milk gelation properties and compositional traits. The high heritability for gelation properties suggests that this trait can be improved through breeding and that the problem with non-coagulating milk could be eliminated. Composite casein genotypes were found to affect rennet-induced gelation and the most common composite genotype in SR was associated with poorer coagulation properties than five other composite genotypes present within this breed which was suggested to be due to variations in casein micelle size. The study showed that composite αS1-β-κ-casein genotype BB/A1A2/AB and BB/A1A1/AE could be used as markers for good gelation properties in SR milk. Furthermore, new quantitative trait loci (QTL) affecting milk gelation properties have been identified which were not only situated in the casein gene cluster on chromosome 6 but also on other chromosomes. These QTL can be used to pin point new genetic markers for rennet-induced milk gelation properties which can be incorporated into genomic breeding schemes. With knowledge on how cow genetics affect milk composition and milk gelation properties, both using the well-known genetic protein variants as genetic markers and using the new haplotypes found in this thesis, the best cows and bulls can be chosen for future breeding. This will be economically beneficial for farmers, breeding companies and the dairy industry