Bovine udder quarter milk in relation to somatic cell count : focus on milk composition and processing properties

Abstract: The dairy industry requires raw milk of high quality in order to produce milk products of high quality and quantity. Mastitis is one of the most prevalent and economically important production diseases in dairy cattle. It causes increased somatic cell count (SCC), deteriorated milk composition and consequently altered processing properties of milk. Altered milk composition due to mastitis often occurs in only one of the four udder quarters of the cow. Milk with high SCC and deteriorated milk composition can be excluded at cow level. A future option will be to detect and exclude milk at udder quarter level, which can be achieved in automatic milking (AM) systems. This thesis examined alterations in milk composition in individual udder quarters in relation to elevated SCC. The studies were mainly carried out on Swedish Red Breed cows with low to moderate SCC, all supplying milk to a commercial dairy. It was found that around 30% of the cows with low cow composite SCC (below 100 000 cells/ml) had one or more udder quarters with elevated SCC and altered milk composition. Furthermore, individual udder quarters with deteriorated milk composition and signs of reduced blood-milk barrier were found in cows with low to moderate levels of composite SCC. In addition, cows with elevated SCC had one or more udder quarters with low SCC and unaffected milk composition. It was also observed that the relative day-to-day variation in milk components in the four udder quarters of a healthy cow is similar, which suggests that using repeated measurements of milk composition at udder quarter level could be a suitable tool for detecting alterations in milk composition and health. Milk composition was affected by duration of storage and content of bacteria, as well as SCC. Separating the milk from individual udder quarters was shown to affect the content of total protein, whey protein and lactose at cow level. The main conclusions from this thesis are that individual udder quarters with deteriorated milk composition can be found in cows with fairly low SCC, and that exclusion of the milk from these udder quarters can improve overall milk quality. These results are important when discussing whether exclusion of some milk at udder quarter level should be introduced to improve milk quality.