Role of Omasum in the Control of Feed Intake and Rumen Digesta Outflow

The objective of this study was to test whether the stimuli of osmotic pressure (OP), VFA or pH in the omasum would affect the passage of digesta from the rumen and feed intake in sheep. Five experements were carried out. Different solutions with specific, OP, VFA concentration and pH were infused into the omasal body. Rumen digesta volumes and outflow rates (liquid and solid) were measured by pulse dosing of liquid and solid marker into the rumen. Feed intake, solid and liquid outflow rates from the reticulo-rumen were (not always significant) reduced by increasing the (OP) of the infusate in the range of ۴۰۰ to ۲۰۰۰ mOsmol/kg which gave calculated omasal OP up to ۴۸۰ mOsmol/kg (perfect mixing with digesta was assumed). Feed intake and fractional ruminal liquid out-flow rate were significantly increased by increasing VFA concentration of infusate in the range of ۵۰ to ۲۵۰ mMol/l or calculated omasal VFA concentrations up to ۱۵۰ mOsmol/kg with the suggestion of a decrease above this range. pH in the range of ۵.۰ to ۷.۰ was with-out effect. Serum OP was not affected by any infusate. There was no or only a weak re-sponse to abomasal infusion compared with omasal infusion when the same infusate was used. The study demonstrated that omasum responds to changes in the composition of di-gesta. Increasing OP reduced DMI (dry matter intake) and reduced digesta outflow from the reticulo-rumen. Increasing VFA concentration increased DMI and liquid outflow, high VFA concentration decreased DMI and liquid outflow rate. It can be concluded that omasum has a role in the control of digesta outflow from the rumen.

Keywords:

Feed intake , Omasum , Outflow rate , Osmotic pressure , VFA