Calorigenesis of Dairy Cattle as Influenced by Thyroxine and Environmental Temperature

Four Holstein cows were used to determine the influence of thyroxine on calorigenesis. The objective of this study was to determine the calorigenic action, latent period, biological effective time and peak metabolic response at different environmental temperatures. Injection of L-thyroxine at 18° and at 32° C. increased “resting or standing” metabolism, pulse rate and lactation. The latent period was 2 to 3 days and 1 to 2 days at 18° and 32° C, respectively. The biological effective time was found to be 4 to 5 days and 5 to 6 days at the comfort zone and under exposure to heat, respectively. The rate of thyroxine metabolism is slow in cattle and decreases progressively under hot conditions. The results suggest that estimates of thyroxine secretion rate are too low when calculated by blood methods and too high when the in vivothyroid measurement technique is used. High temperature (32° C.) decreased pulse rate, oxygen consumption, milk production and feed intake and increased rectal temperature and respiration rate, indicating a serious disturbance in the cow's thermal equilibrium.