Effects of Temperature and Feed Intake on Thyroxine I131 Disappearance Rates of Cattle

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Abstract
Six lactating Holstein dairy cattle were used to determine the effects of environmental temperature and controlled feeding on the blood thyroxine I131 disappearance rates. Environmental temperature of 88[degree]F significantly depressed blood thyroxine I131 disappearance rates in ad libitum and control-fed animals. Temperature differences in the thyroxine I131 disappearance rate were greater (P< .10) for the control-fed animals than the ad libitum group. There was a highly significant difference (P< .10) in the thyroxine I131 disappearance rate due to environmental temperature for all animals. These results suggest that thyroxine I131 disappearance rates are depressed directly by high environmental and body temperature rather than by depressed feed intake.