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Total Body Solids Loss as a Measure of a Short-Term Heat Stress in Cattle¹

University of Missouri,³ Columbia 39762

Abstract

Twelve Friesian calves 6 months old were exposed in the climatic chamber for 3 days at 18.3 C and 50% relative humidity followed by 3 days at 32.2 C and 50% relative humidity. Total body water was determined during each exposure using the tritiated water and extrapolation technique. Total body water was subtracted from the average body weight to obtain the total body solids. Whole body ⁴⁰K, nitrogen retention and rectal temperature were determined on the third day of each thermal exposure. Heat caused a 15% decrease (P<.01) in total body solids. The percentage decrease in the latter, as a measure of heat stress, varied among the animals from 24.5% to 0.2%. Nitrogen retention decreased (P<.01) but the animals were in a positive balance indicating no lean mass loss with short-term heat exposure. This was confirmed by a lack of significant loss in whole body ⁴⁰K. Thus the body solids loss was attributed to a loss in body fat. Total body water increased 10.1 1 (P<.01), which replaced the 9.9 kg loss in body solids thus resulting in no significant change in body weight. Body water turnover rate increased (P<.01) from 3.8 to 6.4 ml/kg/hr., thus preventing the increase in rectal temperature (P<.01) from reaching a lethal level.

¹ A contribution from the Missouri Agricultural Experiment Station Series No. 5954. Approved by the Director.

² Bioclimatology Unit, Atomic Energy Establishment, Cairo. U.A.R.

³ Dairy Department.

⁴ The authors gratefully acknowledge the help of Dr. Jack Clark, assistant professor and supervisor of the low-level radiation laboratory in counting the whole body ⁴⁰K.