Rapidity of cortisol transfer between blood and milk following adrenocorticotropin injection

Cortisol concentrations in milk and blood plasma were measured in 12 lactating Holstein cows following administration of 40 IU adrenocorticotropin intravenously (groups 1 and 2) or 4 ml saline (control). Blood and milk samples were collected at 15 or 30-min intervals for 4 h from control and group 1 cows and at hourly intervals for 4 h from group 2. Cortisol concentrations in plasma and milk were increased 15 min after treatment and peaked by 1 h in group 1 but remained unchanged in controls. Group 2 cows were sampled less frequently but showed a pattern for plasma and milk cortisol concentrations similar to group 1. The profile of increased cortisol concentrations and of their decline 1 to 2 h later was similar for plasma and milk. These results suggest that cortisol concentrations in milk reliably indicate cortisol concentrations in blood and may be valuable in monitoring stress in dairy cows, depending upon duration of stress and its timing relative to milk removal.