Experimental climatological work was carried out on a Jersey cow and a Corriedale ewe. These animals were exposed to a series of controlled environmental conditions in a psychrometric room for periods of 7 hr and their reactions observed. Both animals showed an increase in body temperature, respiratory rate and volume, and respiratory and transcutaneous water losses during heat exposure. Values and trends in these reactions were calculated for each exposure. Transcutaneous water loss by unit area of Jersey cow skin was of the same order as loss through human skin. Respiratory water loss formed only a small fraction — a fifth to a ninth — of the total water loss. In the sheep, however, respiratory water loss reached as much as one-third of the total water loss and the transcutaneous loss was one-third of that through human skin. It was concluded that the Jersey cow efficiently maintained a heat balance presumably by a good sweating mechanism. The Corriedale ewe, however, was a poorly sweating ruminant, relying mainly on the respiratory mechanism for approaching a heat balance.
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.
Four grade Australian Illawarra Shorthorn cows, aged 3 years, were exposed for 7 hr. twice a week to each of several atmospheres having dry-bulb temperatures ranging from 86° to 108.5°F. and absolute humidities ranging from 8 to 16 g./cu. ft. The following reactions were studied: rectal temperature, pulse rate, respiratory rate, evaporative loss, skin temperature, and behaviour. Average 9 a.m. ante-room rectal temperature reading was 101.5 ± 1.29°F. Dry-bulb temperatures of 95°F. and above caused a noticeable rise in rectal temperature and rise in body temperature paralleled rise in humidity. Humidity had a greater effect on the animal than dry-bulb temperature. Pulse rate was essentially unaffected by rise in temperature. Environmental conditions markedly influenced respiratory rate. On exposure to heat, the rate increased considerably before any rise in rectal temperature was apparent. When dry-bulb temperature increased, there was a parallel rise of evaporative loss with rise in respiratory rate. Humidity had little effect on water loss. Comparisons of the reactions of Australian Illawarra Shorthorn cows were made with four Jersey cows of similar age and lactation, exposed to atmospheric conditions similar to those observed by Riek and Lee two years previously. The following points were brought out: (i) The heat tolerance is greater in Jerseys with all combinations of dry- and wet-bulb temperatures. (ii) There is no significant change in pulse rate with breed. (iii) Respiratory rates are similar in the two breeds until the highest temperatures are reached, when that of the Illawarra Shorthorn increases above that of the Jersey. (iv) A greater increase in water loss occurs in the Jersey. (v) There is a larger body temperature-skin temperature gradient in the IIIawarra Shorthorn.
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