Effect of Adrenocorticotropin Injection and Stress on Milk Cortisol Content

Abstract

Whether cortisol content of milk might objectively measure stress in lactating dairy cows was studied. A procedure was developed for measuring cortisol in milk samples by a combination of solvent extraction, chromatography and competitive protein-binding procedures. Cortisol is normally in cow''s milk, and injection of 200 IU adrenocorticotropin into 5 cows caused an increase in concentration of cortisol in milk from 2.5 to 8.7 ng/ml. The stress of shipping 39 cows by truck from one dairy to another also increased the concentration of cortisol in milk and decreased milk production. The concentration of cortisol in milk was correlated with the average concentration in blood during the interval of milk secretion. Under control conditions and after injection of adrenocorticotropin to increase circulating cortisol, the regression equations describing the transport of cortisol from blood to milk are nearly identical. The mammary gland may be integrating the circulating concentration of cortisol, and the cortisol concentration in milk may reflect the average concentration in blood during the interval of milk synthesis. Because the secretion of cortisol into milk is one example of a more general phenomenon, a model is presented to describe the diffusion of materials across the mammary secretory cell and into milk.