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Respiratory Activity as an Index of Heat Tolerance in Jersey and Sindhi x Jersey (F₁) Crossbred Cows¹

U. S. Department of Agriculture³

Abstract

Twenty purebred Jersey cows and 20 Red Sindhi x Jersey (F₁) cows were examined (a) at a "normal" temperature of 57°–84°F., and (b) after 3 hours exposure to 105° F. with a vapor pressure of 34 mmHg. Observations were made on rectal temperature, respiratory rate, and respiratory volume. The method of measuring respiratory volume is described.

The crosses showed a smaller rise in rectal temperature (1.87± 0.152° F.) than did the Jerseys (3.06±0.23° F.). Both the final respiratory volume and the rise in respiratory volume, however, were also slightly smaller in the crosses. Furthermore, the correlation between respiratory volume and rectal temperature in cows exposed to comparable heat stress was a positive one, often of high magnitude.

It is concluded that the superior heat tolerance of the crosses cannot be attributed to a more sensitive respiratory response, but that the higher respiratory activities are compensatory responses displayed by the individuals with the lower heat tolerance. The crosses, however, did show a more marked reduction of tidal volume (–0.48±0.131 l./resp.) than the Jerseys (–0.29±0.113 l./resp.), which should further reduce the risk of an induced alkalosis.

Respiratory rate proved to be a less sensitive index of heat tolerance than respiratory volume. Under these conditions there was no significant correlation between rectal temperature and respiratory rate.

¹ These studies were conducted by the Bureau of Dairy Industry at the Agriculture Research Center, Beltsville, Maryland.

² Professor of Physiological Climatology, Johns Hopkins University, Baltimore.

³ Bureau of Dairy Industry.