Consequences of underfeeding on digestion and absorption in sheep

The negative relationship between intake and digestibility of an unchanging diet has been widely described. The reviewof the different processes of ruminal digestion shows that the main cause of the variation in digestibility is the retention time of particles in the rumen. However, most experiments have been carried out at levels of intake higher than maintenance. For this reason, a special attention is paid to experiments carried out at lowlevels of intake. In these conditions, the response of digestibility to a decrease in intake is variable: it can increase, be stable or decrease.Uptill now, it has not been possible to determine the animal or nutritional factors which influence theway of variation, and especially the unexpected decrease in digestibility.However, it has been clearly shown that these decreases are not due to an insufficient retention time of particles in the rumen, or to an insufficient reduction of particle size. Differences in microbial activity have not been exhibited by in situ measurements. It is hypothesised that a reduction of bacterial growth, or of the expression of microbial degradation potential occurs at very lowintakes, butmechanisms are still not elucidated. The decrease in weight and oxygen uptake by the digestive tract contributes to the adaptation of animals maintenance requirements. At short term, the proportion of metabolisable energy intake which is recovered in the portal vein remains unchanged when animals are underfed but the supply of absorbable amino acids (AAs) cannot keep up with the gut requirements. The consequences of underfeeding on nutrition and absorption do not differ between small ruminants and cattle. © 2003 Elsevier Science B.V. All rights reserved.