Environmental stimuli provoke an animal to respond, which in turn can influence performance processes in five ways. Responses:
1. Alter internal functions. As an unintentional consequence, certain stress hormones secreted as part of long-term adaptive or stress responses can reduce a foal's growth rate.
2. Divert nutrients from other maintenance processes and performance. A nursling piglet that increases metabolic rate simply to keep its body warm in a chilly environment will have fewer nutrients left for disease resistance and growth.
3. Directly reduce animal productivity. Thermoregulatory responses to hot environments sometimes include reducing internal heat production. Eggs laid by heat-stressed hens weigh less than normal, due partly to decreased feed intake, partly to a homeokinetic reduction in egg synthesis (which gives off heat).
4. Impair disease resistance. As a consequence, e.g., individual feedlot cattle under social stress due to aggressive group mates are more likely to become infected and diseased.
5. Increase variation in animal performance. Individual animals differ in responses to stimuli and therefore in performance even when residing in the same adverse environment. Stress increases individual variation in performance.
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