The effect of the dietary plane upon the reactions of two breeds of sheep during short exposures to hot environments

Abstract

Five Camden Park Merino and six Corriedale ewes, each breed divided into three groups and offered the food corresponding respectively to high, medium, and low dietary levels, were exposed for seven hours a day, one to three days a week, to each of several selected hot atmospheres over a range of temperatures 30 to 45°C, and a range of vapour pressures 15 to 40 mm.Hg. Observations were made on the rectal temperature, pulse rate, respiratory rate, respiratory volume, evaporative water loss, food and water consumption, body weight, and behaviour. These sheep were relatively tolerant of the hot conditions, only the highplane Corriedales having to be removed before the completion of the seven hours' exposure from the atmosphere 40°C./40 mm.Hg., with a rectal temperature over 107° F. A sharp rise in respiratory rate followed commencement of the exposure, the rate in the Merino being the higher under all hot conditions above the mildest (300 °C./30 mm.Hg.). The average maximum rates obtained for four sheep were 370/min. for the Merinos and 290/min. for the Corriedales. Pulse rates showed little change from normal rates of 60-65min. until air temperate or humidity reached high levels. The rise in vapour pressure at an air temperature of 40°C., which produced the same effect as a rise of 1°C. in air temperature at a vapour pressure of 30 mm.Hg., was 0.25 mm.Hg. for rectal temperature, 1.7 mm.Hg. for respiratory rate, and 2.3 mm.Hg. for respiratory volume. A high-plane diet reduced the heat tolerance of sheep, and heavy individuals actually lost weight during the experimental period. A very low diet, on the other hand, also impaired heat regulation, probably as part of a general depression of function. Sheep on a medium-plane diet fared best. It is difficult to account for observed water losses under hot conditions unless sweating is admitted.