Genetic parameters of pathogen-specific incidence of clinical mastitis in dairy cows

Data from 274 Dutch herds recording clinical mastitis (CM) over an 18-month period were used to quantify the genetic variation for overall and pathogen-specific CM. Analysed pathogens were Staphylococcus aureus, coagulase negative staphylococci (CNS), Escherichia coli, Streptococcus dysgalactiae, Streptococcus uberis and other streptococci. The data set contained 47 563 lactations of 28 695 cows of different parities. Cases of overall and pathogen-specific CM were treated as all-or-none traits. Variance components for the sire, maternal grandsire and permanent environmental effect were estimated using generalized linear mixed models with a logit link function for the binary traits. Average number of days at risk and in milk on trial was 198 days. The estimated heritability for overall CM was 0.04, and similar heritabilities for the pathogen-specific CM were estimated. Repeatability across lactations was low for overall and pathogen-specific CM (0.10 to 0.14). Genetic correlations with milk yield and somatic cell score (SCS) differed according to pathogen. For instance, the incidence rate of clinical E. coli mastitis was slightly unfavourably correlated with milk yield at 150 days (0.13) but stronger with SCS (0.74). Whereas, the genetic correlations with clinical Str. dysgalactiae mastitis were 0.70 and 0.16, respectively. The expected correlated responses showed that current selection practices (using milk yield and SCS) will be effective in reducing the incidence of E. coli and CNS but less effective in reducing the incidence of S. aureus and Str. dysgalactiae, even with a large relative weight for SCS in the selection index.