Responses of Beef Cattle to High Ambient Temperatures

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Abstract
Many reports indicate that production is lowered in cattle during the summer in tropical and subtropical areas and that tropical breeds are more heat tolerant than exotic breeds. Little work has been reported on the physiological and anatomical differences responsible for varying degrees of adaptability. In this study a heat chamber was used to subject eight animals simultaneously to temperatures of 105° F. and relative humidity of 50 per cent for an eight-hour period. A total of 366 animal observations were made. Analyses of the data showed significant breed differences with Herefords high, Brahmans low and the F1 between these intermediate but nearer the Brahman for body temperature and respiration rate. Similar differences were found for respiration rate taken in the field and weight gain during the summer in shadeless pastures except that the order for gain (high to low) was F1, Brahman and Hereford. Correlations between the above characters and some others that appeared to be related to heat tolerance were mostly significant but of low magnitude. Summer gain, only, was found to be sufficiently high in heritability (19 percent) to indicate usefulness in selection. Summer gain and winter gain appeared to be negatively correlated on a breed basis but independent within breeds. It appears that summer gain will be more useful in selection for heat tolerance under practical conditions than any other measure studied. Copyright © . .