Influence of ambient temperatures on the heat production of pregnant ewes

Abstract

Two Merino ewes were kept on constant diets throughout pregnancy and, after shearing, were exposed to a series of ambient temperatures between 10 and 35°C on several occasions before lambing and once afterwards. Their heat production and urinary nitrogen output were determined at each temperature, and some blood constituents were also measured. Metabolizable energy intakes were 1300 and 1900 kcal/day. The estimated critical temperatures (minimal heat production) of the sheep ranged from 28 to 35°C, being lower at the higher level of feeding and also during pregnancy. A general relationship between thermoneutral heat production per sq. metre and the critical temperature of shorn sheep was derived; the heat increments of gestation and feeding merely increase thermoneutral heat production and so decrease the critical temperature. The heat increment of gestation was 80 and 90 kcal/24 hr/kg lamb at the high and low feeding levels respectively. Fat oxidation (as indicated by the respiratory quotient) and heat production increased up to twofold at subcritical temperatures. At 10°C there was no heat increment due to pregnancy or feeding; blood glucose was increased by 6–9 mg/100 ml in one sheep, and plasma free fatty acid content was highest at 10° in both sheep (0.9 m-equiv./l.). Heat production also tended to be elevated at the highest temperature. The change was greatest (8%) during pregnancy and at the higher level of feeding. Urinary nitrogen increased at the same time only when the sheep were not pregnant. Improved nitrogen economy during gestation was also apparent at other temperatures.