Fermentation in the rumen of the sheep. I. The production of volatile fatty acids and methane during the fermentation of wheaten hay and lucerne hay in vitro by micro-organisms from the rumen.

It is now generally accepted that the fermentation of carbohydrates in the rumen gives rise to the formation of large amounts of the lower volatile fatty acids; but the fact that these acids are absorbed in considerable amounts and at different rates from the rumen into the blood stream has made it clear that in vitro studies of the fermentation processes are necessary in order to ascertain the actual quantities in which they are made available to the ruminant. The course of carbohydrate dissimilation by the mixed flora and fauna of the rumen is determined primarily by an extraordinarily stable milieu (cf. Marston, 1939,1948), andso it might be expected thataclosely similar environment, andinocu-lation of the substrate with the natural mixed culture of organisms from the rumen, would be essential prerequisites for the fermentation to proceed in vitro as it does in the rumen. The study of pure or of' purified' cultures of organisms derived from the rumen may eventually illuminate the intermediate stages in the course of the dissimilations which occur there and the nature of the organisms responsible, but it cannot give precise information concerning products of the fermentation of natural fodders in the rumen. The same may also be said of studies made with related organisms derived from other sources—soil, faeces, etc.—or with fermentations carried out at temperatures or reactions which are outside the limits that obtain within the rumen. Most of the experiments described in the earlier literature on the in vitro fermentation of refractory carbohydrates—cellulose, etc.—must be included in the above categories. If information cogent to the nutritional physiology of the ruminant is to be obtained, it is essential that some major characteristics of the fermentation as it proceeds in the rumen should be used as criteria by which to gauge the similarity of the courses taken by in vitro and in vivo fermentations of natural foodstuffs. The extent to which certain insoluble carbohydrate fractions of natural fodders are broken down in the rumen, and the amount of methane produced from them, can be measured. If these criteria were met in vitro, under the appropriate conditions of temperature, reaction and anaerobiosis, then the fermentation might reasonably be considered to have taken a course closely similar to that occurring in the rumen, and the fatty acids produced could be taken to represent those actually formed in the rumen.