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Effect of Hot Conditions on Adrenal Cortical, Thyroidal, and Other Metabolic Responses of Dairy Heifers¹

Department of Dairy Science, Louisiana Agricultural Experiment Station

W. T. Burnett

Nuclear Science Center, Louisiana State University, Baton Rouge

ABSTRACT

Physiological responses of ten Holstein heifers were measured under cool (38–65 F) and controlled-hot (75–95 F) conditions. Thyroxine secretion and turnover rates and PBI values decreased (P < .01) under hot conditions. Blood plasma levels of 17-hydroxyeorticosterone increased (P < .01) from 1.7 to 4.5 µg per cent. The 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.

¹ The data are from a dissertation submitted by the senior author to the graduate faculty of Louisiana State University in partial fulfillment of the requirements for the degree of Doctor of Philosophy, 1961.

² Present address: U. S. Department of the Interior, Fish and Wildlife Service, Denver 25, Colorado.