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

• Greater feed intakes do not affect main digestive functions in sheep.
• Changes in organ size and fill compensate for greater intakes.
• However, changes in organ size and fill do not compensate for greater water losses in feces.

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