Energetic efficiency of fattening sheep. I. Utilization of low-fibre and high-fibre food mixtures

Abstract

The energy, carbon, and nitrogen exchanges of nine castrate male sheep in moderately fat condition were determined with the aid of closed-circuit indirect calorimetry. Five of the sheep were kept on a diet containing equal parts of chopped lucerne hay and chopped wheaten hay (mixture A). The other four were given a pelleted 5:4:1 mixture of lucerne hay, maize meal, and peanut meal (mixture B). Each mixture was given at five different rates and each sheep was fasted on two occasions. Digestible energy averaged 62% for mixture A and 76% for mixture B, irrespective of feeding level. Of this, 10% was lost as methane and 5 to 13%, depending on level of feeding, in the urine, leaving on the average 81% metabolizable. Thus metabolizable energy amounted to 51 and 62% of the gross energy intake with mixtures A and B respectively, while net energy was 89 and 97% of the metabolizable energy intake at the lowest level of feeding and 61 and 69% at the highest. At any given level of metabolizable energy, mixture B provided 30% more digestible nitrogen than mixture A, but, allowing for differences between sheep in nitrogen economy, any additional energy obtained from mixture B was stored in fat. Consideration of the present results, along with data from earlier experiments with fattening sheep and cattle, showed that the net availability of metabolizable energy, both for maintenance and fattening, decreases regularly as the quantity of digestible fibre increases. Net energy could be estimated more accurately from this relation than by use of the commonly used factors of Kellner.