| Summary: | 1. The fermentation of pasture grass at different stages of maturity has been studied during two seasons by the use of an in vitro technique. A variation in both total volatile fatty acid production and in the distribution of individual acids has been noted. Acid formation is highest at the stage of maximum leafy growth of the grass and declines with advancing maturity of the sward. Acetic acid tends to be the main end product of fermentation from grass during its early growth period, but propionic acid is formed in increasing amount as the percentage of fibre in the grass rises. The use of pure cellulose as an index of the relative activities of rumen inocula has been discussed. 2. Samples of dried and stored grasses prepared from the corresponding fresh material have, on fermentation, been shown to give rise to an altered distribution of acids. Acetic acid is formed in greatest quantity by all samples of the dried grass studied. This difference in fermentation behaviour has been shown to be related to changes, possibly in chemical composition of the soluble carbohydrate fraction, occurring during the drying process. Little further change is observed on storage. 3. The chemical composition of the dried material derived from fresh grass cut at different stages of growth has been determined, and the relationship of levels of soluble carbohydrate, fibre and protein to volatile fatty acid production discussed. Acid formation is considered to be a function, not of any major single component, but of the relative proportions and availabilities of all the constituent fractions. 4. An attempt has been made to examine the mode of formation of the individual volatile fatty acids by fractionating the dried grass. Partition on a water solubility basis indicates that the grass extract and the extracted residue each produce characteristic proportions of individual acids, and an approximate relationship between the fermentation products of whole grass and those of its simple separates has been shown to exist. It has not, however, proved possible to establish such a connection between the fermentation of isolated "pure" compounds and the fermentation of the whole grass. Cellulose has been found to react in a similar manner to the fibre fraction of grass, but fermentation of a grass extract containing water soluble carbohydrate gives rise to an acid distribution very different to that obtained from hexose sugars or from fructosan. 5. Factors determining the nature of breakdown of pasture grass and its constituents by rumen microorganisms have been discussed, and possible applications of in vitro techniques to problems of ruminant nutrition suggested.
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