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, ...

Comparative studies on the effect of diet on ruminal protein degrading activity were made with sheep fed either maize or timothy and with sheep fed either barley or lucerne. In both studies the levels of proteolytic activity and deaminase activity in the rumen, measured as non-protein nitrogen and ammonia production, respectively, during in-vitro incubation of rumen liquor and casein, were higher when cereal was given than when forage was given. The rate of casein degradation after intra-ruminal infusion was also higher with the cereal diets, whereas outflow rate from the rumen was lower. However, with all diets the fractional hydrolysis rate of casein was considerably greater than its fractional outflow rate and the calculated proportion of protein escaping ruminal breakdown was low. The effect of diet on the degradation of the protein of natural feedstuffs was examined by measuring N disappearance when samples of the four feeds and of three protein supplements (soya bean meal, groundnut meal and fish-meal) were incubated in dacron bags in the rumen. In contrast to the findings with casein, the rate of N disappearance for all feedstuffs, except maize which was similar on the two types of diet, was lower when sheep were given cereal than when given forage.

1. Estimates of degradability of nitrogen in the sheep rumen for a basal hay diet and for soya-bean meal (SBM), groundnut meal (GNM) and fish meal (FM), when given together with the hay, were determined from measurements of (1) duodenal N flow, (2) ammonia kinetics and (3) rumen N disappearance from polyester bags and rumen outflow rate. The ability of various in vitro procedures to predict in vivo N degradability was also examined.2. Four sheep were given a basal hay diet (800 g dry matter (DM) and 19 g N/d) either alone or supplemented with isonitrogenous amounts (15 g N/d) of SBM, GNM or FM. Duodenal non-ammonia-N flow (g/d) was increased more by FM (8.0) than by GNM (5.9) and SBM (5.8), whilst microbial N flow (g/d) was increased more by SBM (3.9) than by GNM (2.3) and FM (1.6). N degradability values calculated from these results were 0.88, 0.76 and 0.57 for the SBM, GNM and FM respectively. The corresponding value for hay was calculated to be 0.76.3. The irreversible loss of ammonia in the forestomachs (g N/d) was increased more by SBM (1 1.9) than by GNM (7.2) and FM (5.Q whilst ammonia outflow from the rumen (g N/d) was increased to a similar extent by all supplements ( I 'I, 0.9 and 0.8 respectively), as was the amount of microbial N (g/d) synthesized from sources other than rumen ammonia (1.8, 2.0 and 1.9 respectively). N degradability values calculated from these results were 0.84, 0.54 and 0.45 for the SBM, GNM and FM respectively.4. The fractional rate of N disappearance (/h) when the feedstuffs were incubated in polyester bags in the rumen of sheep receiving the basal hay diet (800 g DM/d) was the highest for SBM (0,145) and lowest for FM (0.037), with the hay (0.082) and GNM (0.071) intermediate, whilst the fractional outflow rates from the rumen (/h) of the three supplements were similar (0.034, 0.038 and 0,030 for SBM, GNM and FM respectively). N degradability values calculated from these results were 0.82, 0.67 and 0.60 for the SBM, GNM and FM respectively; the value for the hay was 0.73.5. Of a number of in vitro procedures tested, only N solubility in sodium hydroxide and ammonia or total non-protein-N (NPN) production during incubation with rumen fluid in the absence of hydrazine sulphate ranked the supplements, although not the hay, in the same order as the in vivo degradability procedures. In terms of absolute values, N solubility in NaOH, at room temperature, gave estimates similar to those derived from the duodenal flow measurements; estimates derived from ammonia and total NPN production were lower.

Sheep and steers were given, at a maintenance level of feeding, four diets consisting of either poor quality dried grass, good quality dried grass or separate mixtures (63 : 35) of each of the dried grasses and barley. Ammonia and total N concentrations in rumen liquor were significantly higher in sheep than in steers whereas total volatile fatty acid concentration was significantly lower and protein N concentration, pH and rumen fluid dilution rate did not differ significantly between species. For all rumen measurements, except total volatile fatty acid concentration, there were significant differences between diets with the dietary responses being similar in both species. Protein degrading activity in the rumen was measured in vitro with casein as the substrate and in situ by measuring N disappearance when soyabean meal, cotton seed meal, groundnut meal. meat and bone meal, fish meal and dried grass were incubated in polyester bags in the rumen. Casein degrading activity of rumen liquor did not differ significantly between species, whereas rumen in-situ degradation of all feedstuffs, except fish meal, was significantly higher in sheep. In both species, ruminal casein degrading activity was higher when the good quality forage was given than when the poor quality forage was given and also increased when part of each forage was replaced by barley. In contrast, rumen in-situ degradability of feedstuffs did not differ when the two all-forage diets were given and the inclusion of barley in the diet reduced the rate of degradation.In both species and with all diets the rumen in-situ degradability ranking of the feedstuffs was similar.

The N fraction of fresh grasses, silages and hays is generally considered to be extensively degraded in the rumen. However published N degradability values for each type of material vary over quite a wide range. This may, at least in part, be due to an effect of the stage of growth of the harvested material since with increasing maturity there are marked changes in chemical composition and botanical features. Accordingly the ruminal N degradability characteristics of perennial ryegrass, harvested at different stages of growth and conserved as hay or silage, were determined using the polyester bag procedure.Crops of young primary growth (C1), first regrowth (C2) second regrowth (C3) and mature primary growth (C4) were harvested from a field of perennial ryegrass c.v.S23. For each crop the fresh grass was stored frozen, conserved as field-cured hay and conserved by ensiling in laboratory silos either directly or after wilting and in both cases with or without formic acid addition (3l/tonne). The silos were opened after 90d and the silages subsequendy stored frozen.