Effect of Ruminal Heating on Thyroid Function and Heat Production of Cattle

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Abstract
The hypothesis that ruminal heating decreases thyroid function and metabolic rate comparable to the decreases obtained by exposure of the whole animal to heat was tested. During the investigation, four nonlactating cows with rumen fistulas were kept at a thermoneutral temperature. At this temperature, the rumen was heated with hot water circulating through stainless steel coils placed in the rumen via the fistula. The effect of ruminal heating on thyroid was studied by the thyroxine I131 disappearance rate (K) and PBI. Heat production was measured with an indirect calorimeter. Ruminal heating significantly decreased K and heat production. Respiratory rates and rectal temperatures were not affected using the exogenous heat load of 125 kcal./hr. but were increased by the 300 kcal./hr. heat load. Heating the rumen is suggested as exerting its effect by one or both of the following mechanisms: (a) by stimulating the “central warmth detectors” through impulses from the nerve endings in the rumen to depress the TRF; (b) by alteration of the requirement of the tissues for thyroid hormone. The latter may occur indirectly by a modification of some substrate level which in turn decreases thyroid function via the feed-back mechanism. Copyright © 1968. American Society of Animal Science . Copyright 1968 by American Society of Animal Science

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