Stress in farm animals: a need for reevaluation.

In animal husbandry, stress has usually been conceived as a reflex reaction that occurs ineluctably when animals are exposed to adverse environmental conditions, and which is the cause of many unfavorable consequences, ranging from discomfort to death. The inadequacy of this view is apparent from the new concepts that have been developed from research aimed at understanding the relationships between hormonal and behavioral reactions to stressful situations. Psychological aspects of environmental stimuli are powerful activators of endocrine responses. The amount of psychological stress that an animal experiences determines how much the pituitary-adrenal axis responds. Indeed, removing the variable of emotional arousal reduces or eliminates responses to some other stressors such as heat and cold. This means that one of the most important characteristics of the stress response, its nonspecificity, lies in the afferent part of the response, not the efferent. Hormonal and behavioral responses are intimately related in stressful situations. In particular, the perception and ensuing behavior of the subject are critical to the nature and intensity of hormonal response. Evidence from experiments in farm animals is presented to support these concepts. Because adjustment abilities are limited by genetics and previous experience, the respective role of each of these factors needs to be delineated more accurately. In addition, most experimental studies have been concerned with acute stress, while chronic multiple stress, which is more likely to be encountered in intensive husbandry, has received little attention. The approach in this field is hampered by the lack of suitable physiological criteria to assess long-term adaptive changes. Opportunities for further research are delineated and the need for a more integrated view of stress reactions in farm animals is emphasized.