The effect of heat exposure on blood serum levels of anterior pituitary hormones in calves, heifers and bulls

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°C and 60% relative humidity (RH) during the day; 25°C and 60-70% RH during the night). The animals were subjected to milder conditions (15-18°C and 70% during the day, 12-15°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.