A theoretical consideration of the effect of preventing rumen fermentation on the efficiency of utilization of dietary energy and protein in lambs

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Abstract
1. The effect of preventing rumen fermentation of feed on the utilization of energy and protein by growing lambs has been studied by theoretical means.2. Initially, the differences in feed utilization resulting from microbial fermentation compared with digestion by host enzymes were estimated. It was assumed that a diet containing 85% of unspecified carbohydrate and 15% casein was placed directly into the rumen (ruminant lamb) or abomasum (non-ruminant lamb) of animals weighing 20 kg, and that in both instances the diet was completely digested. In the non-ruminant lamb from 39 to 45% more net energy was available for maintenance, and from 22 to 61% more net energywas available for production, than in the ruminant lamb. The smaller differences applied to concentrate diets and the larger differences to highly fibrous diets. When dietary protein was completely degraded by microbes approximately 50% less protein was absorbed from the small intestine in the ruminant lamb than in the non-ruminant lamb. It is suggested that this may significantly reduce tissue synthesis and growth in early-weaned lambs weighing less than 30 kg, but it may not limit growth in heavier animals.3. Because part of the food of ruminants generally escapes fermentation and is digested by enzymes in the small intestine, and because part of the food of non-ruminants is fermented in the hind-gut, the estimates were adjusted accordingly. In this situation the utilization of digested energy in the non-ruminant lamb was from 30 to 45 % more efficient for maintenance and from 10 to 60 % more efficient for production than in the ruminant lamb. Some dietary proteins are so resistant to microbial degradation that a lower efficiency of utilization in the ruminant lamb when these proteins are given could only result from a limit in availability of energy.4. Since ruminants digest crude fibre more efficiently than non-ruminants, it was calculated that the level of dietary crude fibre must exceed at least 22%, and in some instances 35%, of the dry matter before the ruminant digestive system results in a better utilization of dietary energy in lambs. Thus, many common feeding-stuffs would be more efficiently used by lambs if they by-passed the rumen. The possible limitations to the adoption of this procedure are discussed.5. The present techniques available for reducing the degradation of food within the rumen are outlined. It is suggested that an improvement in food utilization in ruminants could be achieved by feeding fats or formaldehyde-treated fat-casein complexes in solid diets, or alternatively, by feeding liquid diets which activate the reticular groove.