Incidence of clinical mastitis in a herd of Jersey cattle.

Incidence of clinical mastitis in 1278 lactations of 390 Jersey cows was studied for effects of lactation number, quarter of udder, month of calving, sire, and milk yield. Results for quarter did not conflict with the usual finding that incidence is higher in rear quarters. A mastitis index based on stage of lactation at which infection occurred had less variation, greater variation among sires, and a continuous distribution, in contrast to the traits porportion infected and number of cases. Mastitis increased with advancing lactation number subsequent to second lactations. Fall calvers may have less mastitis since they would be dry or in late lactation during summer. Incidence of mastitis increased at a decreasing rate as total milk yield in the lactation affected increased.