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Journal of Dairy Science Vol. 53 No. 12 1734-1738
© 1970 by American Dairy Science Association ®
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Whole Body 40K Loss as a Predictor of Heat Tolerance in Cattle1

T. H. Kamal2 and H. D. Johnson

Dairy Husbandry Department, University of Missouri, Columbia 65201

ABSTRACT

Eleven nonlactating Holstein cows were exposed for 3 days at 18.3C and 50% relative humidity followed by 3 days at 32.2C and 50% relative humidity in the climatic laboratory. Feed and water were provided ad libitum. Liquid scintillation whole body counting of 40K was performed at the end of each climatic exposure. Heat caused an increase in rectal temperature (P < 0.01) and decreases in whole body 40K (P < 0.01), nitrogen retention (P < 0.01), and body weight (P < 0.05). Predicted heat tolerance was measured by subtracting the percentage decrease in body potassium, due to heat exposure, from 100. The coefficient ranged from 75 to 102 among the cows studied. It was possible to arrange the animals arbitrarily into 6 classes of predicted heat tolerance in the ascending order of: 70 to 75, 76 to 80, 81 to 85, 86 to 90, 91 to 95, and 96 to 100. The 40K of the rumen contents decreased insignificantly with heat exposure and such a decrease was too low compared to the whole body 40K loss; thus, it did not interfere with accuracy of the test. There was no significant correlation (P > 0.05) between the final rise in rectal temperature and the percentage decrease in 40K or in body weight caused by heat exposure. The percentage decreases in body weight and in 40K due to heat also were not correlated (P > 0.05).

FOOTNOTES

1 Contribution from the Missouri Agricultural Experimental Station Series no. 5952. Approved by the Director.

2 Bioclimatology Unit, Atomic Energy Establishment, Cairo, United Arab Republic.






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Copyright © 1970 by the American Dairy Science Association ®.