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Repeatability of an Experimental Heat Tolerance Test and the Influence of Season¹

U. S. Department of Agriculture

Abstract

Jersey and Sindhi-Jersey crossbred cows were subjected for six hours to a standard hot atmosphere (105° F., and vapor pressure 34 mm. Hg) at repeated intervals of two months or longer, and observations made on rectal temperature and respiratory rate. Between tests the animals were kept in the barn or on pasture under climatic conditions prevailing at Beltsville, Maryland.

The rectal temperature responses are found to vary markedly with the time of year. The responses show a maximum in February, and a smaller maximum in August, with a minimum in May–June.

Repeatability coefficients for the rectal temperature responses in heifers (8–22 months of age) remain fairly low until the responses are sorted into three seasonal groups: April–July (low); August–September and February–March combined (high); and October–January (intermediate). They then reach high and satisfactory levels.

The results are explained in terms of two sets of response to stress—yield or slip responses, the compensatory responses—which are differentially affected by the seasonal conditions (hot, temperate, cold) under which the animals live between tests.

Repeatability coefficients for respiratory rate responses to the test hot atmosphere remain low, even after seasonal sorting.

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

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