Effects of solids and liquid flows on fermentation in continuous cultures. IV. pH and dilution rate [Rumen metabolism]

The purpose of his study was to examine the effect of various liquid dilution (D) rates and pH levels, at a constant solids retention time, on metabolism and fermentation patterns of rumen microbes in continuous culture. The D rates were 4, 8, 12 and 16%/h. At each D rate, fermentations were conducted at pH 4.5, 5.5, 6.5 and 7.5. Under these conditions, pH 6.5 was found to be optimum for fiber and organic matter digestion, with only small D effects. Total nonstructural carbohydrate digestion was high (89 to 95%) at all pH and D rate combinations. Protein digestion and microbial growth and efficiency were affected by specific sets of pH and D conditions. At pH 4.5 and 7.5, at the 4%/h D rate, values for these characteristics were high, but were decreased by increases in D. The data also suggest that energetic uncoupling resulted from high D rates at these pH levels. At pH 6.5 protein digestion, microbial growth and efficiency were high at all D rates, with increases as D rate increased. At pH 5.5, protein digestion and microbial growth were low at all D rates. Osmotic pressure changes were found to have little effect on fermentation or metabolism.