Role of the omasum in regulation of digesta flow and feed intake in sheep
The aim of the work was to study whether the stimuli of osmotic pressure (OP), or volatile fatty acid (VFA), or pH to the omasum would affect feed intake (DMI) and the passage of digesta from the reticulo-rumen. The technique adapted was to infuse solutions of different OP, VFA concentrations or pH into the omasum or abomasum via permanent catheters. Rumen digesta outflow was measured by means of markers pulse dosed through and sampled from a rumen cannula. A total of eight experiments were carried out. In the first series (1-3) one sheep and in the second series (4-8) three or four sheep were used. Latin square or change over designs with one days measurement at three or four day intervals were used. The main measurements taken were of daily and two hourly feed intake, ruminal solid and liquid outflow rates and volumes (solid, Cr-mordanted hay; and liquid, Co-EDTA or PEG) and water intake. Diet digestibility, serum OP, urine and faeces output and rumen OP, VFA and pH were also measured. Ruminal samples were collected at 2h intervals for 24h. Feed intake, solid and liquid outflow rates from the reticulo-rumen were (not always significantly) reduced by increasing the OP of the infusate in the range 400 to 2000 mOsmol/kg which gave calculated omasal OPs up to 480 mOsmol/kg (perfect mixing of digesta assumed). Feed intake and fractional ruminal liquid outflow rate were significantly increased with VFA concentration of infusate in the range 50 to 250 mMol/l (calculated omasal VFA up to 150 mMol/l) with suggested decreases above this range. pH in the range 5.0 to 7.0 was without effect. Serum OP was not affected by any infusate. Faeces output on the day of infusion decreased in some cases. There was no or only a weak response to abomasal infusion compared with omasal infusion when the same infusate was used. The study demonstrated that the omasum responds to changes in the composition of digesta. Increasing OP reduced DMI and reduced digesta outflow from the reticulo-rumen. Increasing VFA concentration increased DMI and liquid outflow rate and decreased DMI and liquid outflow rate with high VFA concentration. It is concluded that the omasum has a role in the control of digesta outflow from the rumen.