Effect of Hot Conditions on Adrenal Cortical, Thyroidal, and Other Metabolic Responses of Dairy Heifers

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Abstract
Physiological responses of ten Holstein heifers were measured under cool (38-65[degree]F) and controlled-hot (75-95[degree]F) conditions. Thyroxine secretion and turnover rates and PBI [protein bound iodine] values decreased (P< .01) under hot conditions. Blood plasma levels of 17-hydroxycorticosterone increased (P<.01) from 1.7 to 4.5 [mu]g per cent Average daily body weight gain and feed consumption decreased (P< .01) from 1.8 and 15.7 to 1.1 and 14.7 lb. respectively. Heat production, respiration rate, and rectal temperature rose following initial exposure to heat, then declined on continued exposure. These changes appeared to be the result of compensatory adjustments brought about by altered thyroid and adrenal cortical function. Within-animal correlation coefficients between thyroid and adrenal cortical function were not significant. Results indicated the animals did not undergo severe physiological stress.