5. Conclusions
Our data extend our understanding of the relationship between plasma and tissue concentrations of corticosterone in laying hens. It is apparent that plasma concentrations of corticosterone in laying hens may not always reflect the tissue concentrations of corticosterone, and this relationship is likely to change under acute or chronic conditions. This underscores the need to consider the nature of the stressor when determining physiological actions of corticosterone within particular tissues. It also suggests that measuring plasma corticosterone concentration alone may be of limited value when assessing hen welfare because it may not accurately reflect the impact of the stressor on hen physiology. Future research of this nature will make a substantial contribution to our understanding of the actions of corticosterone, the physiological consequences of these actions and, in turn, improved understanding of the impact of stress in laying hens.
Plasma corticosterone concentration is often measured to evaluate the welfare of laying hens. Our experiments strongly suggest prudence when evaluating stress (and consequently welfare) in laying hens if plasma corticosterone concentration is the only measure taken. There are a number of downstream mechanisms involved in the stress response that can alter the concentration of corticosterone in tissue and regulate its effect on tissue function. We have provided solid evidence that plasma corticosterone can only tell part of the story and should be interpreted in conjunction with other welfare indicators. When applied to the assessment of animal welfare, our approach is likely to be more informative and perhaps provide enhanced measures to assess the welfare of laying hens.