Submitted to: Journal of Dairy Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: July 20, 2013
Publication Date: October 25, 2013
Citation: Wilcox, C.S., Schutz, M.M., Rostagno, M.H., Lay Jr., D.C., Eicher, S.D. 2013. Repeated mixing and isolation: Measuring chronic, intermittent stress in Holstein calves. Journal of Dairy Science. 96:7223-7233. Interpretive Summary: During chronic stress, glucose production in response to stress hormones is not sufficient to support glucose demand. This is evidenced by increased blood glucose in calves after transport. In animal well-being studies, determining glucose consumption may be a non-invasive way to substantiate that stress was induced. Molasses is an energy dense food (glucose source). To study whether a drive to replace glucose by molasses consumption would occur with chronically stressed calves, calves were socially stressed by a day of isolation followed by regrouping with new calves. Control calves were housed singly, but with tactile, auditory, and visual access to other calves. The traditional measure of stress, increased cortisol (stress hormone), was detected over the entire period. After a challenge to determine if the calves were chronically stressed (ACTH challenge), cortisol concentrations were dramatically increased in the control calves and low in the stressed calves. This showed that stressed calves’ ability to respond had been suppressed, a sign of chronic stress. Change from negative to positive for fecal shedding of Salmonella was much greater at the first move than in succeeding moves. This suggests that the calves had habituated to some degree to the moves. Additionally, behavior was changed by the stressor. Stressed calves spent more time standing, took longer to lie down, and consumed more molasses (all suggesting an activated state) than control calves. However, after the ACTH challenge, control calves became more active for the first 4 h following the injection than stressed calves, but were less active by the afternoon observation. This illustrated that control calves were more affected by the ACTH challenge, and stressed calves could not respond to the ACTH stimulus. Together, these results demonstrated that a chronic stress condition was reached within a 2-wk period and that increased molasses consumption by dairy calves can be used to confirm that a social stress condition has occurred.
Technical Abstract: Objectives of this study were to determine physiological effects of psychological stress applied to dairy calves and to test if molasses consumption could be used to validate that a stressed condition was achieved. Repeated intermittent applications of acute stressors, isolation and regrouping, over 14 d were implemented to create stress in dairy calves. Treated calves were subjected to 5 cycles of 24 h of isolation followed by regrouping for 48 h in a new group pen and finally an ACTH challenge (0.1 IU /kg of body weight). Twenty 3 wk-old male calves had jugular catheters inserted, and were randomly assigned to stress or control treatments, four pens of four for the stress groups and 4 singly housed controls. Blood samples were collected for cortisol analysis via a jugular catheter 15 and 0 min prior to the calves being moved and then 0, 15, 30, 60, 90, 120, and 300 min after the calves had been moved. Blood samples were taken in a similar manner after the ACTH challenge. An additional blood sample was also taken prior to each move for immunological analysis. Fecal samples were collected prior to and after each move. Behavior in group and control pens was recorded by digital video recorder and molasses consumption was measured once per cycle. Peak (P < 0.001)and total cortisol (P < 0.001) were greater for stressed calves until the ACTH challenge. On d 14 the control calf cortisol increased and the stressed calves’ cortisol continued to decrease, suggesting adrenal fatigue. Number of calves that became positive for fecal shedding of Salmonella after the acute stress of being moved and number of calves that were positive after the move decreased with each move. Fifty six percent of stressed calves changed from negative to positive for shedding after the first move compared to 18.75% of stressed calves after the third move. Difference in fecal shedding of Enterobacteriaceae from samples taken before and after moving calves on d 6 was less (P < 0.05) than on d 2, d 3 and d 5. Molasses consumption was greater (P < 0.05) for treated calves on d 2 and 11. The latency to lie after eating also increased as the study progressed requiring more time for stressed calves to lie on d 12 than on d 3 and was greater than controls on d 4, 8, 12, and 14 (treatment by d interaction, P = 0.04). Stressed calves also stood more (P < 0.05) than control calves in the 4 h afternoon period of d 4, 5, 7, and 14. However, during the 4 h morning observations on d14 (ACTH challenge), control calves stood more (P = 0.046) than stressed calves. This model induced chronic stress as characterized by adrenal fatigue, which was confirmed by molasses consumption and behavior changes. These results show that molasses consumption may be used to confirm a stressed condition has been obtained, similarly to sucrose consumption in some mouse models.