The artificial fibre bag technique has been widely used in studies of forage dry-matter digestibility (Van Keuren & Heinemann, 1962) and of the release of a range of plant constituents during fermentation in the rumen (Playne, McLeod & Dekker, 1972). More recently, the technique has been promoted as a simple means of estimating the rate and extent of food protein degradability in the rumen (θrskov & Mehrez, 1977; Mathers, Horton & Miller, 1977).

1. The characteristics of digestion, passage and rumen fill of three hays: early-and late-cut perennial ryegrass (Loliurn perenne cv. Endura) and white clover hay (Trifolium repens cv. Blanca and Pronitro) were studied using six rumen-cannulated sheep fed at a restricted level of intake (18 g dry matter (DM)/kg live weight (LW) per d), in a two 3 x 3 Latin square design. 2.Voluntary intake of the same diets was measured using a further six non-cannulated sheep in a similar design. 3.Rate of digestion of the three hays was measured using dacron bags and the rates of digestion of DM and neutral-detergent fibre (NDF) for clover hay were significantly ( P i 0.05 and P < 0.001 respectively) faster than those for the two grass hays whose rates did not differ. Rates of passage, determined using chromium-mordanted hay, did not differ between treatments.4. Rumen pool sizes of DM, organic matter and fibre were generally greatest for the late-cut grass hay and lowest for the clover hay, while voluntary intake was highest (P < 0.001) for the clover hay (36.6 g DM/kg LW per d j and lowest for the late-cut grass hay (24.7 g/kg LW per d). 5.The net rate of removal of indigestible fibre from the rumen appeared to vary within the day, with maximal disappearance occurring during eating, followed by a lag phase between 5 and 10 h after feeding, with a second increase in rate between 10 and 24 h post-feeding.The use of forages as diets for ruminants is associated with prolonged retention times within the digestive tract (Warner, 1981). The degree of distension and fill of the reticulo-rumen can therefore be an important factor limiting the voluntary intake of forage diets (Balch & Campling, 1969). However, the exact mechanism by which such diets can control intake through their effect on the extent of fill of the digestive organs is not clear.The extent of rumen fill is governed by factors which affect digestion in and passage from the rumen. A high content of structural carbohydrates which are fermented more slowly than other substrates (Van Soest, 1975) will lead to a higher degree of rumen fill, and Osbourn et ul. (1974) found that intake of forages by sheep was highly correlated with the cell-wall (CW) content of the diet. However, such a relation would not be expected to hold for diets with characteristically low CW contents, where metabolic satiety is likely to exert a dominant effect on intake (Conrad, 1966).The objectives of the experiments reported here were to study the removal of digesta from the rumen and the voluntary intake by sheep offered forages of contrasting digestibility and dietary fibre content. Grass hay, cut at two levels of maturity, provided diets of high and low digestibility with corresponding low and high CW contents, although both were considered to be within the range where physical limitation of intake was expected to dominate. In addition, the influence of forage species was investigated, with a white clover (Trifoliurn repens) hay intermediate in digestibility between the two grass hays, but with ...

1. Eight wether sheep were offered a diet of perennial ryeigrass (Loliumperenne cv. Endura) hay once daily at two levels of intake (1 1 and 16.5 g dry matter (DM)/kg live weight (LW)) with or without maize starch (175 g DM/kg hay DM). The experiment consisted of four periods. each lasting 32 d. Rates of digestion of the hay were measured by incubation in dacron bags and rates of passagc using chromium-mordanted hay. Rumen pool sizes of DM, organic matter and fibre were measured by emptying the rumen.2. The inclusion of starch in the diet appeared to increase significantly (P < 0.01) the lag phase before the start of fibre digestion, as observed both in the dacron bag studies and in the slower initial disappearance of digestible neutral-detergent fibre (NDF) from the rumen recorded by emptying of rumen contents. However, there was no significant effect of starch on apparent digestibility of fibre in the whole tract.3. The main effect of increasing the level of feeding wax, to increase the rate of passage with a consequent decrease in overall digestibility. The fractional rate of passage increased from 0.0318 to 0.0400 as the level of feeding increased, while apparent digestibility of NDF decreased from 0.755 to 0,724.4. On all treatments the weight of indigestible fibre in the rumen remained more or less constant between 5, 10 and 15 h after feeding, but was significantly lower at 24 h. These results suggest that a high proportion of the outflow of material from the rumen not associated with feeding appears to occur during the second half of the feeding cycle.The quantity of digesta present in the rumen can be an important factor in influencing the voluntary intake of roughage diets by ruminants (Balch & Campling, 1969). Roughages contain a high level of fibre and since the rate of digestion of fibre is slower than that of other fractions (Van Soest, 1975), factors affecting this rate may thus be implicated in intake control. The effect of soluble carbohydrates in depressing cellulolysis is well documented (El-Shady et al. 1961; Terry et al. 1969;Mertens & Loften, 1980) and often leads to a depression in voluntary intake. However, the relation between this depression and rumen volume has not been studied in detail.The relation between rumen pool sizes and intake level has been studied by a number of workers, mainly with diets of oat straw (Pearce, 1967) , 1984;Ulyatt et al. 1984). The increase in rumen pool size has generally been less than the increase in intake, presumably due to the increase in rate of passage as intake level increased. The effect of concentrate: forage on the rate of passage is not clear, since Uden (1984) observed no effect of an increased ratio while Colucci et al. (1982) reported higher rates of passage with high-forage diets compared with low-forage diets.or forages (Moseley & JonesThe experiment reported here was designed to study the effects of supplementation of a forage diet with starch and to see how the responses obtained were influenced by level of feeding. The responses measured w...

A mathematical model is described which was developed with the objective of analysing and interpreting data from rumen-emptying studies to estimate rates of digestion (kd) and passage (k,) in sheep offered hay once daily. It assumed that firstorder kinetics applied to digestion in and passage from the rumen within a 24 h feeding cycle. The model predictions of kd and k, were compared with rates of digestion estimated by incubation of the hays in dacron bags and rates of passage determined using chromium-mordanted hay, in the same experiments. The predictions for rates of passage determined over the period 5-15 h postfeeding were lower than those determined from the chromium-mordant data and the simulations indicated that the fractional rate of passage during feeding was higher than that recorded in the postfeeding period. This suggests either that more than one fractional rate constant applies within a 24 h period, or that first-order kinetics do not apply. The estimates for kd were considerably higher than those determined in dacron bags. Evaluation of the ability of the combined dacron bag and mordant techniques to estimate extent of digestion by comparison with in-vivo digestibility values indicated that the extent of digestion was underestimated. The predictions obtained from the rumen-emptying data were much closer to the in-vivo values.

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