Intramuscular injections of 200 IU of adrenocorticotrophin (ACTH) for 4 consecutive days failed to elicit detectable increases in the somatic cell content of milk from normal quarters despite large and sustained increases in the concentration of circulating leukocytes. Neither exercise nor stage of lactation had any detectable effect on the milk somatic cell response to ACTH. During midlactation (86 days in milk, 26 kg daily production) milk somatic cell counts for uninfected and Corynebacterium bovis-infected quarters averaged 28 and 77 × 103 cells/ml of milk, respectively, and during late lactation (302 days in milk, 5-kg daily production), 28 and 58 × 103 cells/ml of milk. The data suggest that neither ACTH nor late lactation will increase the concentration of milk somatic cells in quarters free from mastitis pathogens. These results support the use of the somatic cell concentration of milk as a criterion on which to assess milk quality.

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