IN VITRO AND IN VIVO RELATIONSHIPS OF RUMEN FLUID DILUTION RATE TO DIETARY CHARACTERISTICS AND RUMEN FERMENTATION

An experiment involving four continuous flow fermenters, two periods and two dilution rates (D1 = 4.1%h; D2 = 5.7%h) was conducted in a crossover design. A medium-concentrate diet was provided to each fermenter at 0700 h and 1900 h daily. After a five-day adjustment period, fermenter contents were withdrawn at 4 h postfeeding for a five-day collection period. Faster dilution rate resulted in increased pH (P < .01), elevated (P < .01) protozoal concentration, decreased (P < .05) bacterial numbers, higher (P < .01) NH(,3) concentration, increased (P < .01) acetate proportion, depressed (P < .05) propionate proportion, lower (P < .05) valerate proportion and depressed (P < .05) total volatile fatty acid concentration. A trend (P < .10) for depressed apparent organic matter digestibility was observed at the faster dilution rate. Generally, values for variables measured were consistent with reported values, and altered dilution rate produced fermentation responses similar to previous reports.;Examination of dietary relationships with FVOL indicated that metabolizable energy concentration (MECO) best described FVOL (R('2) = .20). A positive relationship of ash concentration with FVOL was observed in the best two-variable model (R('2) = .23). When ratios were included, CPI:ADFI best described FVOL (R('2) = .27). The second variable to enter was acid-detergent fiber concentration (R('2) = .45). Generally, equations describing FDR and FVOL did not account for much variation in these variables.;Data from seven in vivo studies covering a wide range of diets were compiled to examine relationships of fluid dilution rate (FDR), fluid volume (FVOL), animal weight (WT), fermentation characteristics and dietary variables, using stepwise regression to produce equations. Of the six fermentation variables, only propionate and pH were significantly related to FDR, bu R('2)'s were only 3 and 12%, respectively. About 16% of the variation in FDR was explained by ash intake. The best two-variable model for FDR included dry matter intake (DMI) and WT (R('2) = .36). When dietary ratios were included as potential independent variables, DMI:WT best described FDR (R('2) = 31). An R('2) of .50 was achieved when crude protein intake:acid-detergent fiber intake (CPI:ADFI) and CPI:WT were used to describe FDR.