Net utilization of roughage and concentrate diets by sheep.

1. Two diets, an all-roughage diet and a high-concentrate diet, were fed at two levels, a low level of estimated 1-5 times maintenance energy requirement and a higher level of estimated two times maintenance energy requirement, to South African Mutton Merino castrated male sheep, aged 13 months and in fairly lean condition at the start of the 93 d experimental period. 2. Body composition and energy retention were determined using the comparative slaughter technique and two series of digestibility and balance studies were done during the course of the experiment. Metabolizability of each diet was estimated and corrected for fermentation heat using the fermentation balance approach. 3. Although there were significantly different rates of energy gain on different diets and feeding levels, fat energy gained (% total energy gained) was similar for the four groups, i.e. 78-80. 4. Regression of energy gain v. corrected metabolizable energy (ME) intake indicated that the maintenance energy requirements of sheep used in this experiment were 310-2 and 302-3 kJ ME/kg body-weight0-75 per d and the values for net utilization of ME for body energy gain were 0-411 and 0-479 with the roughage and concentrate diets respectively. 5. It was concluded that the estimated maintenance energy requirements of sheep obtained in this study are realistic values and that the efficiency of utilization of surplus ME for the two diets did not differ significantly.