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.

S. 24 perennial ryegrass harvested in June, and subsequently in September from the same sward was conserved by rapid freezing. The digestion of the carbohydrate and protein components of the two diets was investigated, using sheep fitted with re-entrant cannulae at the proximal duodenum and terminal ileum and consuming daily 950 g of herbage dry matter.The higher content of soluble carbohydrate and the lower content of protein in the spring-cut herbage compared with the autumn cut, led to a more efficient fermentation in the rumen with a higher yield of volatile fatty acids per mole of substrate degraded, a proportionately higher yield of propionic acid and a substantially greater net gain of protein between mouth and duodenum on the spring cut herbage. Consequently, the quantity and composition of the amino acids entering the small intestine, or being absorbed therein, were not significantly different, despite the higher protein content noted on the autumn herbage.The total energy absorbed was markedly higher for the spring herbage (11-2 MJ/kg D.M.) compared with the autumn herbage (9-5) and the energy absorbed as protein from the two diets comprised 20 and 26 % of total energy absorbed respectively.The possible causal mechanisms within the rumen, and the subsequent effect of these on nutrient supply are discussed in relation to the well-established higher efficiency of use of metabolizable energy (ME) of spring-cut herbage compared with autumn cut herbage of similar apparent energy digestibility.TTtfTP fYnTTPTTfYNr autumn herbage of similar digestible organic-matter content. It has been consistently shown by research Attempts to identify causal factors have been only workers and confirmed in practice, that herbage partially successful. Both Corbett et al. (1966) and harvested in the autumn does not sustain the Blaxter et al. (1971) referred to the higher wateranimal performance observed with the same soluble carbohydrate content in the dry matter of herbage species harvested in the spring, although spring herbage, possible differences in protein quality the total apparently-digested nutrients in the two or the presence of oestrogenic compounds. The work materials is generally similar. Corbett et al. (1966) reported in this paper was undertaken to quantify showed with a mixed grass and clover pasture the end products of digestion of spring and autumn harvested in June and August, that whilst the harvested herbage, and determine if differences in apparent digestibility of energy and nitrogen were the end products of digestion could account for similar, and that of cellulose higher for the later-differences in performance and utilization, cut material, the net availabilities of metabolizable energy above maintenance were 43-5 and 32-5%MATERIALS AND METHODS respectively. Similarly, Blaxter et al. (1971) observed the net energy for fattening to be 47 % Preparation of diets higher on a mixed grass-clover sward harvested in A newly established sward of S. 24 perennial early June compared with a mid-August harvest,...

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.

SummaryThe mixed buffer salts of artificial saliva were incorporated at three rates: 0 (diet 1), 5·7 (diet 2) and 11·4% (diet 3) (w/w) in pelleted diets prepared from60% ground flaked maize and 40% dried ground grass. The control and mineral-supplemented diets were fed to sheep hourly (Expt 1) or twice daily (Expt 2).When the sheep were fed hourly or twice daily the pattern of fermentation in the rumen was altered from a predominantly propionate to mainly acetate fermentation. Dilution rate in the rumen increased from 0·036 to 0·053 to 0·064 when diets 1, 2 and 3 were fed hourly, and dilution rate was negatively related to the molar proportion of propionic acid (P< 0·05) and positively related (P< 0·05) to the molar proportion of acetic acid in rumen fluid, but these relationships were not apparent in Expt 2.Nitrogen retention was higher for lambs fed diet 2 than for those fed diets 1 and 3 (P< 0·05) in Expt 1.In sheep fed hourly, selenomonads and bacteroides were the predominant rumen bacteria associated with the control diet but the inclusion of mineral salts resulted in the bacteroides being replaced by large number of chain-forming cocci. Numbers of protozoa were variable. The relationship of these changes to the fermentation pattern is discussed. In sheep fed twice daily there was much variation between animals.