Changes of bacterial numbers and carbohydrate fermenting groups during in vitro rumen incubations with feedstuff...

Short in vitro incubations of mixed ruminal bacteria with feedstuff materials were conducted. In each series of experiments, the rumen bacterial population was divided into equal portions. One portion was washed anaerobically by centrifugation in a mineral buffer prior to initiation of incubation, whereas the other portion was not washed. Changes of carbohydrate group composition of the populations, of bacterial cell composition, and of incubation fluid characteristics were monitored over 24 h. Washing the bacterial inoculum did not affect adversely the ensuing feed fermentation and resulted in lower background carbohydrates and fermentation acids. Unwashed bacterial incubations had a greater tendency to produce more lactate. When the feedstuff substrate contained more soluble nutrients, the microbial population exhibited batch culture type growth, and bacteria fermenting soluble carbohydrate predominated. When alfalfa hay was the sole substrate, a more balanced fermentation resulted with respect to both carbohydrate group composition and fermentation acid production. Implications of these findings in relation to use of in vitro methods to estimate digestibility of feedstuff are addressed.