Clinical mastitis in Norwegian cattle: frequency, variance components, and genetic correlation with protein yield.

Records of clinical mastitis in first lactation Norwegian Cattle from 1978 onward were analyzed. Variance components for clinical mastitis were estimated with a linear sire model using records of more than 1.2 million cows from 2043 sires, resulting in heritability estimates of 0.035. Different strategies for extracting data gave very similar results, and estimated heritability for mastitis was the same with univariate and bivariate (with protein yield) analyses, which indicates that selection bias caused by correlated responses from other traits in the breeding goal is not a problem with this data set. The estimated genetic correlation between clinical mastitis and protein yield is 0.25.