The extent to which dietary components are fermented in the rumen is a function of both rate of fermentation and residence time in the rumen. The latter, usually expressed as the mean retention time (i.e. the reciprocal of the fractional outflow rate; MRT), can be determined from the decrease in the concentration of a non-absorbable marker in rumen digesta after an intraruminal dose of marker. This technique, whilst generally satisfactory for solute markers, is less reliable for particulate markers because of difficulties in obtaining representative samples of rumen digesta. The faecal marker excretion technique (Grovum & Williams, 1973) overcomes the problem of representative sampling and also has the added advantage that fistulated animals are not necessarily required. It is based on the fact that the pattern of marker excretion in the faeces after an intraruminal dose of marker reflects the cumulative effects of marker residence time in the various sections of the digestive tract. Provided a satisfactory mathematical description of the excretion curve can be achieved and the component parts identified, or at least that part relating to the rumen, the MRT in the rumen can be obtained. Blaxter et al. (1956) suggested that the ruminant gut is essentially composed of two mixing compartments and a tubular compartment, and that digesta flow can be described by a model consisting of two exponential terms and a time delay. Subsequently, Grovum & Williams (1973) used this model to describe the change in marker concentration in sheep faeces following an intraruminal dose of marker and showed that the longer MRT was associated with the rumen. However, other workers (e.g. In their paper on the theoretical considerations and computer simulation of digesta passage, Grovum & Phillips (1973) concluded that 'a poor fit between the observed concentration of marker and the predicted values for the two-pool model may indicate that a model with two compartments is not descriptive of passage of marker through the gut. Thus new models and new equations can be sought that are more appropriate.' In the present paper an alternative model to describe faecal marker excretion is proposed and was derived by considering digesta flow as a multicompartmental exponential process. It consists of a multiplicative equation containing an exponential term and a doubleexponential term :, available at https://www.cambridge.org/core/terms. https://doi

I . The ability of ytterbium acetabe (Yb acetate) to fulfil the requirements of a particulate-phase digesta-flow marker in a dual-phase marker system, and of the indigestible acid-detergent-fibre fraction of the feed (IADF) to act as a digesta flow marker, were examined using six mature wether sheep given a diet of dried grass (1 kg dry matter (DM)/d).2. CrEDTA was continuously infused (240 mg chromium/d) into the rumen of all sheep and Yb acetate was also continuously infused (100 mg Yb/d) into the rumen of three of the sheep. At this level of infusion the equilibrium concentration of Yb in rumen, duodenal and ileal digesta and in faeces could be reliably measured by atomic absorption spectrometry. 3.Estimates of faecal DM excretion based on either Yb or IADF did not differ (P > 0.05) from that determined by total collection, whereas estimates based on Cr were significantly (P < 0.05) lower. Urinary excretion accounted for 3.1 % of the infused Cr but no Yb was detected in urine. Estimates of ileal DM flow, assuming total marker recovery, were similar (P > 0.05) with all three markers, whereas the estimate of duodenal DM flow based on IADF was lower (P < 0.05) than the estimates based on either Cr or Yb.4. Compared with the infusion of Cr alone, the infusion of Cr and Yb had no effect (P > 0.05) on nutrient flows at the duodenum, ileum and in faeces nor on microbial degradative activity, volatile fatty acid production and N metabolism in the rumen. 5. Polyester bag and in vitro studies showed that pre-labelling the dried grass with up to 285 mg Yb/g DM did not affect its susceptibility to microbial degradation.6. The Yb in rumen, duodenal and ileal digesta was predominantly (> 90%) associated with the particulate matter but was not uniformly distributed and its concentration increased as particle size decreased.7. The use of CrEDTA and Yb acetate as a dual-phase marker system proved more reliable in estimating 'true' duodenal flow than the use of the individual markers when the digesta sample was unrepresentative.Net absorption from or secretion into the different parts of the gastrointestinal tract can be determined by measuring the quantity and composition of digesta flowing past cannulas inserted at various points along the tract (MacRae, 1975). To avoid the problems associated with total digesta collection, estimates of flow are usually obtained from the relation between the equilibrium concentration of a marker in a sample of the digesta and the amount of marker continuously infused or ingested. When re-entrant cannulas are used in conjunction with automatic sampling machines (Canaway & Thomson, 1977), which allow continuous sampling and so provide a representative sample of the total digesta flow, only one marker is required. However, because digesta flow from the rumen is a discontinuous process with the fluid and particulate phases flowing at different rates, the use of a single marker with spot sampling from either re-entrant cannulas or simple ' T'-shaped cannulas may give erroneous estimates of flow, ...

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1. Nitrogen kinetics were studied in six sheep (45-55 kg live weight) consuming either a high-N grass silage or a low-N dried grass made from swards of perennial ryegrass (Lolium perenne). The diets were fed hourly at a level of 600 g dry matter/d and supplied 19.5 and 11.0 g N/d respectively.2. The amounts of organic matter (OM) consumed and flowing at the duodenum and ileum and excreted in the faeces were similar (P > 0.05) with both diets. Each diet supplied 23 g digestible OM/d per kg live eight^''^, which was sufficient to maintain body-weight.3. There were no differences (P > 0.05) between diets in rumen fluid volume, fractional outflow rate of fluid from the rumen, total concentration of volatile fatty acids or molar proportion of acetate in the rumen. The pH and molar proportion of propionate in rumen fluid were higher (P i 0.01), and molar proportion of butyrate lower (P < 0,001) when the silage was given. 4.There was a net loss of N (4.0 g/d) between mouth and duodenum when the silage was consumed but a net gain (5.5 g/d) when the dried grass was consumed. As a result, total non-ammonia-N (NAN) flow at the duodenum did not differ (P > 0.05) between diets. Rumen microbial NAN flow at the duodenum, based on 15N as the marker, also did not differ (P > 0.05) between diets but the efficiency of microbial N synthesis in the rumen (g/kg OM apparently digested) was higher (P < 0.05) with the dried grass.5. When the sheep were consuming silage they had a higher concentration of ammonia in rumen fluid (P < 0.01), a higher rate of irreversible loss of ammonia from the rumen (P < 0.05) and a higher rate of absorption of ammonia across the rumen wall (P i 0.01). The rate of absorption was found to be more closely related to the unionized ammonia concentration in rumen fluid (r2 0.85) than to the total ammonia concentration (r2 0.36).6. Endogenous N entry into the forestomachs was calculated to be 5.5 g/d when the silage was given and 9.4 g/d when the dried grass was given, of which 1.7 and 3.5 g/d respectively were in the form of urea. Thus, approximately 4-6 g N/d were derived from non-urea materials.7. Within the small intestine the apparent absorption coefficient of rumen microbial NAN (0.72) did not differ (P > 0.05) between diets but the apparent absorption coefficient of total NAN was lower (P < 0.05) when the I silage was given, owing to a lower (P i 0.01) absorption coefficient of the non-microbial NAN fraction (undegraded feed and endogenous). 8.Within the large intestine, diet had no effect (P > 0 05) on the apparent absorption coefficients of total N (0.22) and rumen microbial NAN (0.63). 9. Plasma urea concentration, the rate of urea synthesis in the body and urinary urea excretion were higher (P < 0.001) when the silage was consumed. However, the transfer of urea to the whole digestive tract and to the post-ruminal part of the tract did not differ (P > 0.05) between diets; urea transfer to the rumen was higher (P < 0.01) when the dried grass was given.10. The results were used to construct a whole-anim...

A new model for describing forage degradation kinetics during incubation in the rumen using polyester bags is presented. Attention is given to dealing with the problem of deviations from exponential behaviour in the early stages of degradation by devising a function capable of representing exponential or sigmoidal trends. This is achieved by allowing part of the fractional degradation rate to vary with time of incubation, thus enabling responses other than those expected under simple first-order kinetics to be described. Seven sets of data consisting of 620 curves were analysed to study the performance of the new model compared with a commonly used exponential model. The proportion of significantly better fits varied from set to set. The new model deals successfully with sigmoidal behaviour and, thus, provides a means of analysing the degradation profiles of low-quality forage feeds. Forage: Rumen: Compartmental modelThe extent to which feed components, i.e. dry matter (DM), organic matter (OM), N, etc. are degraded within the rumen is a function of both their rate of degradation and residence time. Rate of degradation has been studied using polyester bags containing feed samples placed in the rumen (e.g. Van Keuren & Heinemann, 1962; Schoeman et al. 1972), and also by in uitro incubation of feed with either rumen microbes or enzymes (e.g. Broderick, 1978; Siddons et al. 1985 b). Typically, the disappearance curves of DM, OM, neutral-detergent fibre (NDF), crude protein (N x 6.25; CP) and other feed components obtained from these systems are then analysed using a model based on simple, first-order kinetics (Orskov & available at https://www.cambridge.org/core/terms. https://doi