A simple technique for the determination in vitro of the dry-or organic-matter digestibility ol small (0-5 g) samples of dried forages is described. It involves incubation first with rumen liquor and then with acid pepsin. Using 146 samples of grass, clover and lucerne of known //i v/vo digestibility (7), the regression equation X = 099 Jf-101 fS.E. ±2-31) has been calculated, where X ---in vitro dry-matter digestibility, This technique has been used for the .study of the digestibilities of plant breeder's material, of the leaf and stem fractions of herbage and of herbage consumed by animals.

Determinations of the in vitro digestibilities of leaf-blade, leaf-sheath and stem fractions of the grasses cocksfoot, perennial ryegrass, timothy and tall fescue and of the leaves and stems of lucerne and sainfoin have shown that all parts of the plant have a high digestibility at early stages of growth. With increasing maturity, however, the digestibility of the stem falls olT at a much faster rate than that of the leaf; in grasses, leaf-sheath digestibility declines at an intermediate rate. The decline of digestibility is associated with a reduction in the content of water-soluble and protein constituents in the plant, and with a reduction in the digestibility of fibre. This may provide a biochemical basis for the selection of forage plants of high digestibility in plant-breeding studies.

The extent of cellulose digestion by rumen micro‐organisms in vitro is dependent on the pH of the medium. Digestion is greatly reduced at pH values similar to those found within the rumen of sheep fed with diets rich in readily digestible carbohydrates. It is suggested that this may be one of the reasons for reduced digestion of the fibre component of forages when carbohydrate supplements are fed.

The amino-acid composition of lucerne herbage cut a t six stages of maturity has been determined by an ion-exchange chromatographic procedure and. compared with that of protein preparations isolated from lucerne and grasses.The protein preparations from grass and lucerne were of closely similar amino-acid composition ; no appreciable difference in composition could be detected between protein preparations from young or mature lucerne.The amino-acid composition of lucerne-herbage N differed from that of the protein preparations ; this difference arose because these herbages contained about 30% of their nitrogen in a water-soluble form, only part of which was present as free amino-acids. The amino-acid composition of lucerne herbage changed little as maturity increased.

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