SOMATIC-CELL COUNT AS A SELECTION CRITERION FOR MASTITIS RESISTANCE IN DAIRY-CATTLE

The purpose of this study was to investigate whether it is possible to increase the resistance to clinical mastitis by selecting for bulls siring daughters whose milk has a low somatic cell count. Data from the animal disease recording scheme, including all veterinary treatments for clinical mastitis, were used in the analysis. A total of 1462 progeny tested Swedish Red and White bulls (SRB), and 911 Swedish Friesian bulls (SLB), with nearly 750 000 effective daughters, were included. Effects of alternative selection criteria on response to clinical mastitis were studied by using selection index theory. Correlations between relative breeding values for clinical mastitis and somatic cell count were 0.45 for SRB bulls and 0.41 for SLB bulls. Corresponding genetic correlations were indirectly estimated to be 0.79 and 0.71. The relationships were found to be linear. It was concluded that it is possible to improve resistance to clinical mastitis by selecting for a low somatic cell count. Due to the higher heritability of the somatic cell count selection based on this was more efficient than selection directly on clinical mastitis to improve mastitis resistance when only small progeny groups (less than 100 daughters) were available. However, a combination of both measures was about 20% more efficient than the former alone. There were no significant differences in relative breeding values for clinical mastitis between bulls differing in their proportion of Holstein-Friesian genes.