A study of heat tolerance of grade Australian Illawarra Shorthorn cows during early lactation.

Abstract

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.