The heat tolerance of two breeds of calves from 1 to 12 months of age.

Abstract

Two grade Australian Illawarra Shorthorn bull calves and two Zebu- Hereford cross bull calves were exposed for 7 hr twice a week to each of several atmospheres having dry-bulb temperatures ranging from 86°F (30°C) to 108.5°F (42.5°C) and absolute humidities ranging from 6.5 to 16.6 gr moisture/cu.ft. dry air (vapour pressure 15-40 mm Hg). Such exposures were carried out a t different ages: 1-3 months, 6-8 months, and 12-13 months. The following reactions were studied: rectal temperature, pulse rate, respiratory rate, evaporative loss, skin temperature (Zebu-Hereford only), and behaviour. For air temperatures above 95°F the rise in rectal temperature was greater and equilibrium was reached more slowly in Illawarra Shorthorns than in the Zebu-Hereford. Increase in humidity at temperatures over 80°F produced more stress in the animals than increase of dry-bulb temperature. Pulse rates changed little during heat exposure, but there was a fall in rate with age under all circumstances. As the calves increased in age the panting rate decreased for a given hot environment (markedly so for the Zebu-Hereford crossbred). Evaporative loss increased as dry-bulb temperature was raised. The Zebu-Hereford showed a high transcutaneous water loss soon after birth with a continued increase up to 12 months. Skin evaporation was relatively small in the Illawarra Shorthorn at 1-2 months but increased a t 6-8 months, although it did not attain the high rate of the crossbred. In the Zebu-Hereford cross, sweat glands became active a t a skin temperature of approximately 96°F. With the onset of sweating a fall in skin temperature was observed. A comparison of the reactions of the Illawarra Shorthorn and the Zebu-Hereford crossbred with four Jersey calves aged 1-3 months, exposed to similar conditions by Riek and Lee, shows that the Zebu-Hereford has a better heat tolerance than either of the other breeds. Evidence is presented to indicate that this superiority of the Zebu crossbred is associated with at least two factors, ( a ) a better sweating mechanism and (b) decreased heat production. The Jersey and Illawarra Shorthorn calves have similar reaction values a t this age.