Rumen Osmolality as a Factor in Feed Intake Control of Sheep

Five experiments were conducted to study the role of ruminal osmolality as a factor in feed intake control of sheep. A survey of post-prandial changes of rumen osmolalities showed that ruminal osmolalities seldom reached 400 mOsm/kg with high roughage or alfalfa silage rations and that such hypertonic levels only appeared in the rumen for a short period of time. When, in an experimental model system, ruminal osmolality was elevated to above 400 mOsm/kg feed intake decreased markedly in sheep. When carbocaine (a local anesthetic) was administered directly into the rumen simultaneously with an osmotic load (NaAc or NaCl), the osmolality induced VFC depression was reversed. VFC was not influenced by carbocaine administration to sheep fed a semi-practical roughage ration. An osmotic pressure greater than 400 mOsm/kg inhibited in vitro cellulose digestion. Disappearance rate of PEG was only slightly influenced by the intraruminal administration of an osmotic load. These data suggested that while rumen osmotic pressure may affect VFC in an experimental model system, the overall results would suggest that rumen osmolality is not an important factor in the control of feed intake in sheep.