Incidence of clinical mastitis in a herd of Holstein cattle.

Incidence of clinical mastitis in 760 lactations of 331 Holstein cows was studied for effects of sire, parity, and month of calving. Incidence of clinical mastitis was higher in hind quarters than fore quarters. Positive correlations between number of cases in one quarter with number in other quarters and between proportion infected in one quarter and proportion infected in one quarter and proportion infected in others indicated that occurrence of mastitis in one quarter was associated with above average frequencies of mastitis in the other quarters of the same cow. Clinical mastitis increased with increase in lactation number up to fourth lactation. Cows calving in January and February more frequently had mastitis than those calving in March and April.