The effect of feed intake on digesta passage, digestive organ fill and mass, and digesta dry matter content in sheep (Ovis aries): Flexibility in digestion but not in water reabsorption

The ruminant gastrointestinal tract (GIT) adapts to changes in diet quality or feed intake level, but studiesthat investigate changes in organ fill, tissue mass, and function simultaneously are rare. We used 3 groupsof 7 mature sheep each, fed at different DM intake levels (range 25–64 g kg−0.85d−1) for 3 weeks precedingslaughter. We determined the mean retention times (MRT) of a solute and two different-sized particlemarkers (and their ratios indicating particle sorting and digesta washing) in the reticulorumen (RR) andthe GIT, total tract digestibility, as well as digesta wet mass, wet organ tissue mass, and the dry matter(DM) concentration of digesta in the indvidual GIT sections. As DM intake increased, digesta wet mass inthe RR and spiral colon increased by organ distension. Simultaneous increases in digesta wet mass in theomasum and small intestine were parallel to increases in organ tissue mass. DM digestiblity, MRT in theRR, measures of the RR sorting mechanism (MRTlargeparticleRR/MRTsmallparticleRR) and RR digesta washing(MRTparticleRR/MRTsoluteRR) all remained constant across intake levels. Whereas the DM concentrationincreased in the rumen with intake, it remained significantly lower in the reticulum than in rumen. DMconcentration in the omasum and abomasum remained constant, but both MRT in the distal GIT andDM concentration in the spiral and distal colon digesta decreased with increasing intake, translating intohigher fecal water losses. These results indicate that the flexibility of the mature sheep’s GIT ensuresconstant digestive functions (such as digestibility, particle sorting, digesta washing) at different intakelevels but does not compensate for greater fecal water losses at increasing intakes