Milk production and somatic cell count in Michigan dairy herds

The established association between milk production and SCC in dairy cattle is increasingly used to estimate lost production due to mastitis. Such cost estimates are used to make decisions regarding cost effective mastitis prevention and control. It is therefore important to verify the relationship between SCC and milk production using data from different areas of the country and by using different analytical methodology. Our study used the 1985 to 1986 Michigan DHIA data base and analyzed daily milk production records rather than lactation summary records as used in the past. One advantage to our approach was that it did not give equal weight to all lactations, regardless of their duration. Also, it enabled inclusion of cows that had incomplete lactations caused by culling, or had other reasons for removal from the herd. A statistical model was constructed to predict milk production on the basis of herd, cow within herd, stage in lactation, month of calving, lactation, and SCC. The data base contained 397,172 milk test records obtained from Michigan DHIA from 504 Holstein herds in Michigan's lower peninsula. Our final model predicted 78% of the variation in milk production. Prediction of milk loss for each herd was highly correlated (r = .98) with the prediction model adopted by most DHIA organizations. Our model predicted that the mean herd lost a mean of 1.17 kg of milk/cow per d associated with SCC.