The influence of body weight (fatness) on the energic efficiency of adult sheep

Abstract

Energy, carbon, and nitrogen balances were determined in adult wether sheep given a diet of lucerne hay and whole oats at several planes of nutrition between fasting and ad libitum. Four sheep were studied when their weight was c. 30 kg (10% fat) and later when they weighed c. 75 kg (33% fat); another four were studied at c. 70 kg (30% fat) and later at c. 45 kg (16% fat). The most obvious effect of fatness was loss of appetite. Voluntary food intake began to decline when body weight approached 60 kg and was half of the maximal amount when the sheep weighed 70 kg or more; one very fat sheep ate only 100–200 g food/day for several weeks. 1n addition, environmental changes such as transfer from a pen to a cage or respiration chamber often caused temporary inappetence when the sheep were in fat condition but seldom when they were lean. Fasting metabolic rate increased with body weight in accordance with the relationship generally applicable to adult sheep except when the sheep were in the anorectic phase of obesity, at which time their metabolic rate was 30–40% above normal. The digestibility of the diet was not dependent on the fatness of the sheep, nor was the relationship between metabolizable and digestible energy. At each level of feeding, the heaviest sheep produced most heat, but differences were less than at fasting; oxidation of fat, rather than protein, was responsible. When daily heat production and metabolizable energy were both expressed as multiples of the fasting energy loss, all sheep conformed to one relationship; the same held for the relation between energy balance and metabolizable energy. Net efficiency (change of energy balance divided by change of metabolizable energy intake) was 78% for maintenance and 55% for production, irrespective of body condition. Gross efficiency (energy storage divided by gross energy intake) was strongly influenced by body condition. When fed ad libitum, thin sheep achieved an efficiency of c. 26 % and fat sheep reached 21 % whereas anorectic very fat sheep never exceeded 10%. Thin, fat, and very fat sheep required 400, 500 and 650 g dry matter/day respectively for maintenance (zero gross efficiency).