In studing in vitro rumen fermentation, cow and calf feces were investigated in place of rumen liquor as medium. Incubations were carried out in all glass artificial rumen for 24 hours, bubbling carbon dioxide continuously and volatile fatty acids (VFA for short) were determined by gas chromatography. It was observed that the composition of VFA produced by feces bacteria quite resembled that of rumen bacteria.
In order to compare soil bacteria with rumen bacteria, successive incubations of soil bacteria were conducted for 30 days in which half of the previous medium served as inoculum for the next 24 hours' fermentation.
Lactic and formic acids were produced first, and they then disappeared in a few days and propionic and higher acids increased in longer incubation instead of former two acids. The composition of VFA produced by soil bacteria became to be similar to that of rumen bacteria in 15 days.
When bacteria population in inoculum are decreased by diluting with salt solution, VFA production in the artificial rumen decreased and formic and lactic acids were produced simultaneously. From these results of the experiments it may be concluded that the rumen bacteria which are responsible for VFA production originate from soil and or feces source.