Relationships between clinical mastitis, somatic cell score, and production for the first three lactations of Finnish Ayrshire

Data on 23,196 cows were extracted from the Finnish system for recording health data and merged with information on SCS and 305-d milk production to study 1) the genetic and phenotypic correlations of clinical mastitis (within 150 d postpartum) and SCS across the first three lactations and 2) the genetic relationships between the traits for individual lactations. (Co)variance components were estimated using linear multitrait REML and the expectation-maximization algorithm. Heritability estimates for separate lactations were distinctly higher for somatic cell score (0.14 to 0.19) than for clinical mastitis (0.02 to 0.05). Genetic correlations of the same traits among different lactations were positive and moderate to high, suggesting that, in practice, clinical mastitis and SCS can be considered as the same traits for different lactations. Genetic correlations of clinical mastitis and SCS varied from 0.37 for first lactation to 0.68 for third lactation, implying that clinical mastitis and SCC monitor different aspects of udder health. A clear, antagonistic genetic association existed between clinical mastitis and milk production, but the genetic correlation of SCS and milk production did not differ from 0.