Effect of glucose fermentation on fiber digestion by ruminal microorganisms in vitro.

Two in vitro digestion trials were performed to determine whether the negative effect on fiber digestion when pH was maintained at > 6.2 was attributable to glucose alone or to end products of glucose fermentation. In some treatments, glucose was depleted by a previous 6-h incubation; the supernatant from this incubation was used as the buffer source for treatments using the fermented glucose medium. In trial 1, mixed cultures were grown on cellulose, soybean hulls, and corn bran in fresh media with 0 (control) or 25 mM glucose, in media previously fermented for 6 h with 0 (control) or 25 mM glucose, or in fermented control medium plus 25 mM lactic acid. The rate of NDF digestion was decreased with fermented glucose medium but not with fresh glucose medium or lactic acid medium. Concentrations of lactate, propionate, and butyrate did not appear to affect NDF digestion directly. In trial 2, six treatment media were used: control and glucose media that were either fresh or previously fermented for 6 h and fermented control and glucose media treated with a protease. Rate of NDF digestion was slower in cultures with fermented glucose medium that was treated with protease than in fermented control medium without protease. When treated with protease, rate of NDF digestion was not different between the fermented control medium and the fermented glucose medium. Thus, the negative effect on fiber digestion appeared to be attributable partially to a proteinaceous inhibitor that was produced in culture media containing a rapidly fermented sugar.