Effect of milk yield on relationship between bulk milk somatic cell count and prevalence of mastitis

The possible dilution effect of increasing milk yield on bulk milk SCC was studied in a field trial. Data on breed distribution, numbers of cows, average milk yields, average bulk milk SCC, and estimated prevalences of mastitis were available for 15,514 Swedish dairy herds. The overall mean of herd prevalence of mastitis, as estimated by the definition employed, was 26.7%, and the overall mean of the geometric average of bulk milk SCC was 204,000 cells/ml. Correlations between prevalence of mastitis and average bulk milk SCC ranged between .53 and .77, and geometric averages were only marginally more correlated to prevalence of mastitis than were arithmetic averages. The average herd prevalence of mastitis was found to increase, within bulk milk SCC level, as milk production increased. The regression coefficients of average milk yield on bulk milk SCC, estimated conditionally on mastitis prevalence, show that the bulk milk SCC decreased by 11.1% for each increase in the milk yield of 1000 kg of FCM. This implies that part of the decrease in average bulk milk SCC achieved during recent years may be an artifact due to the concurrent increase in milk production.