An investigation into the effects of clinical mastitis on milk yield in dairy cows

Milk and disease records were used from 1275 lactations of British Friesian cows, belonging to Sonning farm of Reading University, resulting from calvings that occurred between 1984 and 1989. Eight hundred and forty pairs of successive lactations were available from the total lactation recorded. These were used to consider the relationship between the occurrence of clinical mastitis and changes in milk yield, and to develop a statistical technique that enables the estimation of changes between successive lactations associated with clinical mastitis. The average incidence of clinical mastitis was found to be 35.8% per year. A significant correlation between successive lactations of the same animal was observed for total milk yield. Pairs of successive lactations without clinical mastitis were then compared by a least squares multiple linear regression analysis with pairs in which disease occurred in the current lactation but not in the preceding one. Cows that had clinical mastitis in the current lactation yielded significantly less milk: 231 kg per lactation less than those without mastitis in the current lactation. This mean reduction in the total milk yield applied for all cows irrespective of yielding ability. © 1993