Energetic efficiency of fattening sheep. II. Effects of under-nutrition

Balances of energy, carbon, and nitrogen were determined with five castrate sheep at several levels of feeding and at fasting, when the sheep were in lean condition and when they were very thin; measurements had been made previously when they were fat. While the animals were on a constant low ration to cause loss of condition, the observations were continued under both thermoneutral and cold conditions, and glucose and ketone levels in the blood were also measured. The digestibility and metabolizability of the gross energy of the food were not affected by body condition. However, as the sheep lost weight, the net availability of metabolizable energy at each level of food intake declined. The change of efficiency was not apparent when comparisons were made at equivalent levels of energy storage. In the weight range 21–59 kg, the fasting heat production varied directly with body weight at a rate of 10 kcall24 hr/kg. Prediction of fasting metabolism from body weight is discussed. Changes of energy utilization were largely accounted for by changes of fat metabolism. When the sheep were emaciated, oxidation of protein during fasting was somewhat greater than when they were fat, but nitrogen supplied by the food was used more efficiently. Under cold stress, the heat production of the sheep increased to a high level, which did not vary with body weight until homeostasis failed when the animals were extremely emaciated. The apparent digestibility of the food (at the lowest intake) decreased by 0.47 unit/°C fall of ambient temperature. While the sheep were on the submaintenance ration, the levels of blood glucose and ketones remained at the higher end of the "normal" range under thermoneutral conditions and were somewhat elevated under cold conditions. Hypoglycaemia occurred only when homeostasis failed.