SUMMARY1. Three methods, based on treatment with neutral detergent or acid detergent, or involving ultrasonic disintegration, are described and compared for the direct estimation of undigested dietary nitrogen in individual samples of sheep faeces. Estimates of the true digestibility of the nitrogen in several sheep diets derived from analyses performed with these methods agreed well with each other, and were in accord with published estimates, derived by extrapolation techniques. Two other methods, based on treatment with phenol–acetic acid–water, and lysozyme–trypsin, respectively, were found to be unsuitable for such estimates.2. The quantitative distribution of nitrogen between undigested dietary residues, bacterial residues, endogenous debris residues and the water soluble fraction was determined chemically. It was concluded that 57–81% of the non-dietary faecal nitrogen was associated with bacterial material.3. Indirect evidence suggested that most of the bacterial nitrogen in faeces originated in the rumen.

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I . The concentrations of volatile fatty acids (VFA) in the rumen and caecum were compared when sheep were given barley or dried grass. The molar proportions of VFA in the caecum resembled those in the rumen except that the proportions of isobutyric and isovaleric acids were higher in caecal than in rumen fluid indicating an extensive breakdown of protein during fermentation in the caecum.2. The capacity for starch digestion in the large intestine was studied in two sheep receiving dried grass. Starch was infused into the caecum, the amount given increasing by daily increments of 20 g until 300 g daily were infused. Infused starch in excess of 138 g daily largely appeared in the faeces, indicating a limited but substantial capacity for starch fermentation in the large intestine. The infusion caused a marked increase in the molar proportion of butyric acid.3. Determinations made with a single sheep showed that the infusion of starch into the caecum caused large changes in the microflora, and species similar to Butyrivibrio became prevalent. The viable count on a starch medium was increased by 83 % by starch infusion.4. The infusions of starch increased the excretion of nitrogen in the faeces from 5.8 to 9.0 g daily. Using a fractionation procedure it was shown that when IOO g of starch were fermented in the large intestine the ' bacterial and endogenous debris' nitrogen increased by about 1.0 g.When starch was given orally to two sheep the increase in this fraction was about 0.4g nitrogen/Ioo g starch digested.5 . It is suggested that dietary factors that cause changes in the site of fermentation from the rumen to the caecum will render less microbial nitrogen available to the host animal per unit of carbohydrate fermented and decrease the apparent digestibility of nitrogen.

The extent and complexity of nitrogen metabolism in the mammalian large gut (hind-gut) varies considerably between species and is influenced by the physiological age of the animal. It is most highly developed in herbivores and omnivores whose diets, post-weaning, ensure a fairly constant supply of fermentable materials to the lower tract. The present paper reviews some of these topics, with particular emphasis on recent findings in the sheep and pig. N supply to the large gutIn both species of animal the amount (3-18 g/d) and composition of the ileal N passing to the hind-gut is related to body size, but the nature and quantity of the feed is also important. In the sheep, for example, this supply is influenced to an appreciable extent ( The wall of the large gut supplies N in the form of urea, mucins and sloughed epithelial cells. In one study (Nolan et al. 1976) urea, which crossed the wall close to or within the caecum of the sheep, provided about 27% ( I . 3 g/d) of the NH,-N produced in its lumen, but only 4170 (2.4 g/d) of the urea degraded in the digestive tract was metabolized in the rumen and caecum. Presumably much of the remainder (3.4 g/d) was degraded by bacteria living in close association with the hind-gut wall, in regions other than the caecum. In the goat, urea influx into the colon increased proportionally with increase in plasma urea concentrations for a particular diet (Engelhardt & Hinderer, 1976) and, in contrast to the situation in the rumen, the permeability of the colon wall to this compound appears to be only slightly modified by diet (Engelhardt et af. 1978). Little information is currently available at https://www.cambridge.org/core/terms. https://doi

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