Effect of Quantity of Rumen Dry Matter and other Factors on Determinations of Rumen Fluid Volume with Polyethylene Glycol

The accuracy and repeatability of an analytical method to determine the concentration of polyethylene glycol (PEG) in rumen fluid was tested. PEG concentration and refrigerated storage did not affect the accuracy of the method. Analyses of PEG solutions exposed to light 72 hr. indicated no measurable destruction of PEG. When cows were fed alfalfa hay, a portion of water in rumen digesta was unavailable to PEG. In preliminary trials 80 to 90% of the total rumen water was available to PEG. Results of further trials indicated that dry matter percentage of rumen digesta affected the portion of total rumen water available to PEG. The relationship between dry matter percentage and available water percentage was expressed by a regression equation. Rumen digesta were removed from the rumen of a fistulated cow and weighed. Total dry matter was determined. At the same time PEG concentration in rumen fluid was determined and total dry matter was calculated. Good agreement between the two methods indicated that total weight of rumen dry matter of a hay-fed cow can be determined from PEG concentration. When cows were fed a high-grain, low-roughage ration the relationship between rumen dry matter percentage and available water percentage did not correlate significantly. However, an average available water percentage was established to determine total rumen volume and quantity of dry matter.