Calculated milk production losses associated with elevated somatic cell counts in dairy cows: review and critical discussion.

Relationships between somatic cell count (SCC) and variation in milk production at the cow level were reviewed to provide average reference values suitable for the assessment of economic losses due to subclinical mastitis. The literature analysis involved 19 papers, defining milk yield and/or its composition either at test-day level or at the whole lactation level as statistical unit. Within each type of approach, study populations and designs differed. Regression models implemented also showed large differences. At test-day level, the average trend was a loss of 0.4 kg of milk in primiparous cows and 0.6 kg in multiparous, by each 2-fold increase of SCC above 50,000 cells/mL. At the lactation level, the average trend was a loss of 80 kg of milk in primiparous and 120 kg in multiparous, by each 2-fold increase of the geometric mean of SCC above 50,000 cells/mL. Protein content of milk showed a small increase of 0.15 g/kg (at the test-day level) while fat content showed a small decrease of 0.20 g/kg (both at the test-day and at the lactation level), by each 2-fold increase of SCC. The value of further studies was underlined, especially to provide more accurate quantification of the composition changes associated with elevated SCC, and to improve the imperfect knowledge about the effects of parity and stage of lactation on the studied relationships.