Sire association with intramammary infection and clinical mastitis in a Holstein-Friesian × Jersey crossbred pedigree herd

This research paper describes a test of association of sire with susceptibility to mastitis, using a custom-bred population of dairy cattle. We hypothesised that sire daughters ranked as more resistant to intramammary infections in their first two lactations would be more resistant to an intramammary challenge in their third lactation. Mastitis phenotypes were generated for a Holstein-Friesian × Jersey crossbred research herd of 864 cows, bred from six defined sires and managed as two cohorts in a seasonal calving system in New Zealand. Naturally occurring new intramammary infections (IMI) and clinical mastitis (CM) were monitored in their first two lactations from herd records, milking staff observations and bacteriology of quarter milk samples collected at four time-points during each lactation. The animals retained to their third lactation were then exposed to a single intramammary challenge with <i>Streptococcus uberis</i>. We used a relative risk (RR) analysis to rank performance of sire daughters for pathogen-specific phenotypes for new IMI and CM, and somatic cell count (SCC) traits, and their clinical outcomes to the challenge. Generally, daughters of sire B had the highest RR for new IMI or CM by a major pathogen, whereas daughters of sires A and C had a consistently lower risk. The RR for sires E, D and F were intermediate and inconsistent across major pathogens. Daughters of sire B ranked highest for all CM cases and SCC traits whereas sires A and C ranked lowest. Following intramammary challenge, daughters of sires A and C were more likely to develop CM, whereas daughters of sire B and F were less likely to develop CM. Thus, the hypothesis was rejected. The results revealed strong associations between sire and pathogen-specific mastitis phenotypes, and validated use of SCC and CM traits in sire selection and breeding programmes to improve mastitis resistance.