Abstract
In four experiments male and female calves, heifers and bulls were housed in a climate chamber and exposed to two weeks of hot environmental conditions (30-35 degrees C and 60% relative humidity (RH) during the day; 25 degrees C and 60-70% RH during the night). The animals were subjected to milder conditions (15-18 degrees C and 70% during the day; 12-15 degrees C and 70% during the night) during the two week period preceding and the two weeks following heat exposure. Serum prolactin increased significantly (P < 0.01) when ambient temperature was increased and decreased after reduction of temperature. No changes could be observed for GH, TSH, LH and FSH. There was a significant (P < 0.01) sex difference in GH levels (males < females) in prepubertal calves but this difference was not apparent in older animals.
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