Effects of Penned vs. Pasture Feeding Techniques on Cortisol Levels in Weaned Angus Bulls

Abstract

The effects of feeding in open pens versus pasture on blood cortisol levels in response to exogenous administrations of adrenocorticotropic hormone (ACTH) was measured in 30 weaned Angus bulls. Twenty-four bulls were allotted to eight 24.4 X 6.1 m pens, and the remaining six were kept in pastures. Threats, butts or physical combat that resulted in a subordinate yielding space to an aggressor (yields), and feeding location were recorded in 2 wk intervals during feeding periods in order to classify penned bulls as being either dominant or submissive. Adrenal response to ACTH treatment was quantified at the beginning, middle and end of the study (approximately 56 d apart). During each sampling period, 8 dominant, 8 submissive penned bulls and 6 pasture bulls were haltered and blood was collected via jugular cannula every 20 min for 2 h, followed by injection of 100 IU of ACTH via jugular cannula and then blood samples were taken every 20 min for 4 h. Dominant bulls delivered more butts (P<0.05) over the entire period than did submissive bulls. Submissive bulls exhibited more instances of yielding space to an aggressor than did dominant bulls (P<0.05). Dominant bulls had lower cortisol levels (P<0.05) post-ACTH than pasture bulls for the intermediate bleeding period. Pasture bulls had higher (P<0.05) post-ACTH cortisol levels than penned bulls for the intermediate bleeding period. There were no differences (P>0.05) in pre-ACTH or post-ACTH cortisol levels among bulls across all treatments for the final bleeding period. Bulls raised in pens of this size were not under stress when compared to pasture raised bulls.