Genetic evaluation of clinical mastitis traits in Holstein cattle

The results obtained from different models for predicting breeding values for clinical mastitis (CM) in Holstein cattle were compared. CM was recorded in 30 882 lactations of 12 793 cows in 8 herds from 1996 to 2016. CM was considered either as an all-or-none binary trait (0 - absence of mastitis; 1 - at least one case of CM per lactation) or as the number of cases. CM is recorded in the first 150 days of lactation or throughout the entire course of lactation. Breeding values were predicted with a single-trait repeatability model and with a bivariate animal model, where CM during the 1st lactation and CM during the 2nd and later lactations were considered as two different traits. Estimated heritability ranged from 0.06 for CM as a 0/1 trait during the first 150 days of lactation in the repeatability model to 0.12 for the number of CM cases during the 1st lactation in the bivariate model. Ranking of the sires with 15 or more daughters and of the cows was performed according to their breeding values, and Spearman correlation coefficients were calculated. Rank correlations of breeding values from the repeatability and bivariate models were stronger for the same parts of lactation (150 vs 305 days; 0.88-0.96) and for the repeatability model and the bivariate model for the 2nd and later lactations (0.95-0.98). Trends of average male and female breeding values according to their birth year were used to assess genetic changes in the population. The average breeding values declined slowly for cows born since 2003 but stayed above neutral value, thus indicating permanent genetic deterioration of mastitis resistance.