Predicting Mastitis Resistance Breeding Values from Somatic Cell Count Indicator Traits

A new breeding value estimation procedure for mastitis traits has been implemented in the Dutch/Flemish dairy genetic evaluation. This breeding value estimation uses incidences of subclinical mastitis together with five indicator traits derived from test day somatic cell counts to predict the breeding value for resistance against subclinical and clinical mastitis (SCM and CM, resp.). The mastitis breeding values are combined in an udder health index, using weights reflecting the mean economic losses of occurrence of (sub)clinical mastitis. Results show a marked improvement in reliability of the new udder health index over the previous one. The mean reliability for bulls with 100 or more lactating daughters was 85%, where this used to be around 50%. Correlations between mastitis resistance breeding values, the new udder health index and the old one show that the new index is better suited for breeding for mastitis resistance generally (correlations between index and CM was 0.95, between index and SCM was 0.91), where the old index predominantly emphasised subclinical mastitis (correlations 0.78 with CM and 0.91 with SCM). The genetic trend for mastitis traits shows improvement in the last decade, after somatic cell counts breeding values were introduced and after adding udder health traits in the Dutch total merit index NVI.