Genetic influences on clinical mastitis and its relationship with milk yield, season and stage of lactation

Milk yield and clinical mastitis records were collected from 1672 cows in 24 dairy herds in southern England. The incidence of mastitis in each of the first two lactations was statistically analysed for herd, 305-day milk yield, length of lactation, age at first calving, season of calving and stage of lactation. A subset of the data, relating to 890 cows, the daughters of 34 sires, was used to obtain estimates of heritability of the incidence of mastitis. Genetic correlations between milk yield and mastitis were also calculated. The average incidence of mastitis was found to be 16% in the first lactation and 20% in the second. Variation in the incidence of mastitis could, for a large part, be attributed to variation amongst herds. The incidence of mastitis in sire groups varied significantly in the first lactation, but not in the second. Averaging over both lactations, the heritability estimate for incidence of mastitis was 0.12±0.09. There were significant positive correlations among herds between the incidence of mastitis and 305-day milk yield, but genetic correlations were, on the whole, small. The level of mastitis incidence did not change with increasing milk yield in cows which lactated for at least 305 days, but it did increase slightly with increasing milk yield in cows which lactated for shorter periods. Season of calving had little effect on the overall incidence of mastitis during a lactation, although cows which calved in winter tended to have mastitis earlier in lactation than those which calved during the other half of the year. About two fifths of cases occurred on average before 30 days post-calving. The incidence of mastitis in the second lactation seemed to vary according to age at first calving particularly in those cows calving for the first time in summer. © 1984.