The maximum metabolic response of sheep to cold: effects of rectal temperature, shearing, feed consumption, body posture, and body weight

Abstract

Shorn, adult sheep were exposed to an ambient temperature of about 1°C and a rate of fall in rectal temperature of 0.5–2.0°C/hr was obtained by varying wind velocity and wetness of the fleece. Respiratory gaseous exchange was monitored continuously, and the records were used to calculate metabolic rate. The maximum instantaneous metabolism was 2–36% higher than the maximum metabolism averaged over a 20 min period-termed summit metabolism. Since the larger differences were due to an additive effect of voluntary muscular activity, summit metabolism was adopted as the estimate of the maximum metabolic response to cold. There was no consistent relation between summit metabolism and rectal temperatures above 37°C, but there was a direct relationship at rectal temperatures between 30 and 37°. Shearing the sheep just prior to a determination, or fasting the animal for 19–21 hr, did not influence the magnitude of the summit metabolism. Fasting for 3 days reduced the maximum response by an amount equal to the heat increment of the ration. Sheep generally remained standing during the exposure to extreme cold. While the sheep were lying down, summit metabolism was 26–28% lower than the value during standing, and heat production was reduced to a greater extent than heat loss, since the rate of fall in rectal temperature was greater than that during standing. Summit metabolism was proportional to fleece-free body weight raised to the power 0.9, and summit metabolism per unit of surface area increased with increasing body weight. When sheep were housed in a warm or thermoneutral environment prior to the tests to prevent acclimatization to cold, their summit metabolism averaged 10 watts/kg or 25 watts/kg0.75 and day to day variation in individual sheep was about 6%.