1. To elucidate the causal relation between leucine and the lowering of hepatic NAD content of rats fed on a leucine-excessive diet (Yamada et al. 1979), the effect of leucine on intestinal absorption of tryptophan was investigated.

2. Co-administration of [3H]tryptophan and leucine, with leucine at ten times the level of tryptophan, delayed absorption of L-[side chain 2,3-3H]tryptophan from the digestive tract and incorporation of [3H]tryptophan into portal blood, the liver and a protein fraction of the liver. After 120 min, more than 95% of tryptophan was absorbed whether [3H]tryptophan was administered with or without leucine.

3. Co-administration of a mixture of ten essential amino acids, in proportions simulating casein, with [3H]tryptophan markedly delayed absorption of tryptophan from the digestive tract. The addition of supplementary leucine to the amino acid mixture, however, caused no further delay.

4. In rats prefed a leucine-excessive diet for 1 week [3H]tryptophan was absorbed at the same rate as in rats fed on a control diet.

5. The results indicate that competition between tryptophan and leucine for intestinal absorption did not cause lowering of hepatic NAD.

1. Studies on the quantitative significance of protozoa on carbon and nitrogen digestion and metabolism in the rumen were carried out in sheep given a diet of pelleted concentrate (500 g/d) and chopped hay (500 g/d).

2. Measurements were made of apparent digestibility; flows of organic matter and dietary and microbial non-ammonia N (NAN) (using 15NH+4) to the duodenum; and rates of production, interconversion and metabolism of the major C fermentation end-products (from mathematical modelling of 14C isotope values).

3. The population density of bacteria in the rumen increased as a result of defamation (28.6 compared with 8.2 x 10° organisms/ml). This high density was associated with greater utilization of volatile fatty acids (VFA) within the rumen.

4. The rate of irreversible loss (IL) of bicarbonatefcarbon dioxide from the rumen was greater in the defaunated animals (98.5 v. 57.2 g C/d) but the IL from the blood was greater in the faunated group (138.6 v. 106.1 g C/d). This is consistent with the hypothesis that the high population density of bacteria found in the rumen fluid of defaunated animals may result in increased fermentation of rumen VFA and digestible dietary carbohydrate, thereby increasing the output of CO2 from the rumen and reducing the quantity of VFA (hence energy) available to the host.

5. There was no difference in the flow of organic matter (OM) to the duodenum but there was a higher faecal excretion of OM in defaunated animals (apparent OM digestibility: 0.72 in faunated, 0.67 in defaunated).

6. Defamation did not significantly increase the flow of NAN to the duodenum, the percentage of duodenal NAN of bacterial origin or the quantity of microbial NAN synthesized/g organic matter fermented. Faecal excretion of NAN was higher in defaunated animals (5.3 v. 3.6 g N/d).

1. The conversion of methionine to cyst(e)ine was determined in sheep infused with different amounts of methionine (0-5 g/d) into the duodenum by assaying the incorporation of 36S from intravenously-infused L-[35S]methionine into cyst(e)ine in wool, plasma albumin and the free plasma pool.

2. The percentage of cystine-S in the plasma originating from methionine increased linearly from 4.5 to 18 with increasing supplemental methionine supply.

3. The percentage of cysteine-S in albumin increased from 15 to 50; methionine supply increased to 3 g/d but then remained constant, indicating that the transsulphuration pathway of the liver was exceeded.

4. The percentage of wool cysteine-S originating from methionine was high (∼ 70) at all methionine supplementation rates.

1. The effect of an exogenous supply of glucose, provided by the digestion of maize starch in the small intestine, on endogenous glucose metabolism and insulin action was studied in sheep using the euglycaemic insulin clamp procedure.

2. Insulin was infused intravenously at rates of 0.2, 0.5, 1.0 and 6.0 mU/min per kg live weight for four consecutive periods in each of four sheep fed on dried-grass and maize-based diets. Glucose was also infused intravenously at a variable rate, sufficient to maintain the plasma glucose concentration at basal levels. Whole-body rates of glucose metabolism were determined using a continuous infusion of [6-3H]glucose.

3. From the resultinginsulin dose-response curves, it was observed that, when the sheep were fed on the dried-grass diet, the responsiveness of glucose metabolism to insulin was less than that reported for non-ruminants.

4. When fed the maize-based diet, the glucose metabolic clearance rates (MCR) observed during insulin infusions were significantly greater (P < 0.05) than those observed for the dried-grass diet. However, after correcting for the non-insulin-mediated glucose disposal, differences between diets were not significant.

5. The sensitivity of glucose utilization to insulin was not affected by diet. The plasma insulin concentrations causing half-maximal insulin-mediated glucose MCR were 103 (SE 21) and 85 (SE 11) mU/l for the dried-grass and maize-based diets respectively.

6. The sensitivity of endogenous glucose production to insulin was also unaffected by diet. The plasma insulin concentrations resulting in the suppression of endogenous glucose production to half the basal level were 80 (SE 26) and 89 (SE 29) mU/l for the dried-grass and maize-based diets respectively.

7. It is concluded that the observed increase in glucose utilization on the maize-based diet was due partly to a slight change in responsiveness to insulin and also partly to a change in the rate of non-insulin-mediated glucose disposal.

1. Mixed-breed wethers (40–50 kg), 9 months old, were maintained on high-energy (HED) and low-energy (LED) diets for 2 weeks.

2. After a 15 h fast, a primed-dose constant infusion of L-[U-14C]leucine and α-[4,5-3H]ketoisocaproate (KIC) was given.

3. After 2 h, plasma samples were taken and plasma-specific radioactivities of I4C- and 3H-labelled leucine and KIC were measured and analysed by using an open two-pool model.

4. Less than 20% of the total leucine-C entering the circulation was converted to the KIC pool, and 42% of the KIC was converted back to the leucine pool; transamination of the leucine to KIC and reamination of KIC to leucine was much less than in other species.

5. Additional dietary energy resulted in a decrease in tissue protein synthesis, leucine oxidation and interconversion of leucine and KIC. Total leucine-C entry was also lower in sheep given HED, which was most likely due to a suppression of endogenous proteolysis.

6. Plasma glucagon concentration was significantly higher (P < 0.05) in sheep given LED compared with those given HED. The concentration of glucagon was closely correlated in all treatments with the leucine-C entry (proteolysis + absorbed leucine) and also with KIC-C exit (oxidation).

1. Rapeseed (Brussicu nupus), barley grain, ryegrass (Loliurnperenne) and barley straw were labelled with 15N as an internal marker. The dilution of 15N was used to estimate microbial N (RMN15N) in the feed residues in nylon bags incubated in the rumen for 5, 12 and 24 h. For comparative purposes, diaminopimelic acid (DAP) content of the residues was also determined and rumen microbial N (RMNDAP-N) in the feed residues calculated using DAP as a bacterial marker. The influence of two bag pore sizes (20 μm and 40 μm) with different sample sizes (1 g and 5 g respectively) was also studied.

2. For all feeds, the average disappearance of 15N was faster than that of total N, the difference between N and 15N disappearance being marked with barley, ryegrass and barley straw. The disappearance of microbially corrected dry matter (DM; correction calculated from the 15N values) was, accordingly, always faster than the uncorrected DM disappearance. Except for the bag pore/sample size effect for N disappearance, significant (P < 0.01–0.001) feed, pore/sample size and incubation-time effects were always found for the disappearance values.

3. Errors (%) resulting from the microbial contamination (calculated from the 15N values) in N-loss measurement with rapeseed, barley, ryegrass and barley straw, at 5, 12 and 24 h in 20 μm bags were respectively: –1.8, –3.9, –0.9; –3.8, –22.4, –3.8; –7.2, –4.1, –2.9; –164.5, –146.3, –204.6. In 40 μm bags the corresponding errors were respectively: –4.4, –1.2, –0.7; –26.1, –10.5, –3.9; –13.2, –6.4, –5.5; –221.2, –310.1, –1284.6.

4. The largest residual proportions of RMN15N, RMNDAP-N and DAP-N (% of total N) were found in barley straw, followed by barley, ryegrass and rapeseed, in that order. RMN15N (g/kg residual DM) followed the descending order: barley, ryegrass, straw, rapeseed. RMNDAP-N, (g/kg residual DM) and DAP (mg/kg residual DM) followed the descending order: barley, ryegrass, rapeseed and barley straw. Feed, pore/sample size and incubation-time effects were always significant (P < 0.001).

5. With 40 μm bags RMN15N values of barley, ryegrass and barley straw (expressed as % of total N or g/kg residual DM) were substantially higher than those of RMNDAP-N With 20 μm bags the RMN15N and RMNDAP-N values were generally quite close for these feeds. With rapeseed residues, RMN15N was clearly lower than RMNDAP-N with 20 μm bags, but only small differences were found with 40 μm bags.

6. A more diverse microbial colonization of feed samples was indicated in the 40 μm bags compared with the 20 μm bags. It was also noticed, consequently, that in most cases larger error resulted from the microbial contamination in 40 μm bags.

1. Concentrations of magnesium in plasma, the ionic concentration in rumen digesta supernatant fractions, Mg balance and electropotential differences were measured in three ewes which were fed on grass and suppleyented with potassium and Mg by intraruminal infusion.

2. Mean plasma Mg concentrations were unaltered by combined K and Mg treatments, but fell (P < 0.001) when K alone was infused.

3. The mean concentrations, in rumen digesta, of sodium and K varied reciprocally (P < 0.001) with each other when K was infused, but were unaffected by Mg infusion. The mean Mg concentrations in rumen digesta fell (P < 0.01) with K infusion but rose (P < 0.001) with Mg infusion.

4. Absorption and excretion of Mg rose (P < 0.001) when Mg intake was increased but was unaffected by K intake.

1. The effect of the gut microflora on protein turnover in pectoral muscle (M. pectoralis profundus) was studied by means of dietary infusion of L-[UJ-14C]phenylalanine and of massive dose injection of L-[4-3H]phenylalanine in chicks fed on a semi-purified casein—gelatin (SCG) diet until 19 d of age, and in those subsequently given either a nitrogen-free (NF) diet or NF supplemented with methionine and arginine (MA) for a further 9 d.

2. Time-course changes in radioactivity released in expired carbon dioxide during the 8 h infusion period showed that isotopic equilibrium was reached in 4 h with the SCG diet and in 5 h with the MA diet. However, with the protein-deprived chicks given the NF diet, isotopic equilibrium was not achieved since radioactivity in CO2 increased linearly throughout.

3. On feeding the NF diet, fractional protein synthesis rate and the absolute amount of protein synthesized in chick breast muscle were reduced. These reductions were partially alleviated by supplementing the NF diet with methionine and arginine.

4. The fractional degradation rate of breast muscle was increased in chicks given the NF diet, while the absolute amount of protein degraded was decreased. The addition of methionine and arginine counteracted these changes brought about by protein starvation.

5. Generally speaking, the presence of the gut microflora had little, if any, effect on protein turnover rate in chick-breast muscle.

1. Three experiments were conducted to examine the effect of monensin on growth performance, feed utilization and volatile fatty acids (VFA) in the forestomach and caecum of hamsters.

2. In Expt 1, monensin was fed at levels of 0, 5, 10 and 20 mg/kg to the growing male and female hamsters given a commercial diet (major component: lucerne (Medicago sativa) meal). In Expt 2, monensin was fed at levels of 0, 5, 15, 45 and 135 mg/kg to the growing male hamsters given a semi-purified diet containing 10 g urea/kg (main components: maize starch, sucrose, casein and cellulose). In Expt 3, monensin was fed at levels of 0 and 40 mg/kg to the growing male hamsters given the commercial diet containing lucerne meal or a semi-purified diet.

3. In Expt 1, monensin improved feed conversion efficiency and growth performances in the young growing hamsters, but monensin did not affect the hamsters at a later growing stage.

4. In response to monensin the proportion of acetic acid increased and that of propionic acid decreased in the forestomach, whereas the proportion of acetic acid decreased and that of propionic acid increased in the caecum in Expt 2. The hamsters given 135 mg monensin/kg ate less, developed diarrhoea and died.

5. The apparent digestibility of crude protein (nitrogen x 6.25) was improved by monensin but those of dry matter and neutral-detergent fibre (NDF) were decreased in hamsters given the semi-purified diet in Expt 3. Monensin did not appear to have a significant effect on the apparent digestibility of the diet containing lucerne meal.

6. The responses to monensin in hamsters are compared with those in ruminants.

1. The use of a nylon-bag technique for pig feed digestibility determination was studied. Bags, measuring 25 × 40 mm and containing feed samples, were introduced into the pig gastrointestinal tract through a duodenal cannula, and recovered in the faeces between 23 and 69 h later. The disappearance of organic matter and crude protein (nitrogen × 6.25) from the bags was compared with in vivo apparent digestibility, determined by conventional faecal-collection methods, and neutral-detergent-fibre content for eleven feeds. The residues left in the bags after passage through the intestine from whole-crop-pea (Pisum sativum) and barley-grain samples were analysed for starch, non-starch polysaccharide residues, Klason lignin, crude protein and ash.

2. Dry matter disappearance of barley or whole-crop peas was not influenced by increasing bag pore size from 10 to 36 μm or sample weight from 250 to 1000 mg. Pepsin (EC 3.4.2.1) pretreatment had no effect on the degradation in the bags of the feeds investigated.

3. Organic matter and crude protein disappearance from the bags exceeded in vivo apparent digestibility by up to 0.10 and 0.42 units respectively. In vivo apparent organic matter digestibility could be predicted (P < 0.001) by the organic matter disappearance from the bags and the neutral-detergent-fibre content of the feed, while in vivo apparent crude protein digestibility was highly correlated (P < 0.001) to all these indices but poorly to crude protein disappearance from the bags.

4. Klason lignin was the least degraded component measured in the whole-crop-pea and barley residues from the bags, while starch was completely digested. Of the non-starch polysaccharide residues, xylose was the most resistant to degradation in both samples whereas other sugars vaned in susceptibility to solubilization between samples.

5. Results are discussed in relation to the potential uses of the nylon-bag technique described in the present paper for studies in simple-stomached animals.

1. In a 4 x 4 Latin square experiment four cows were given, twice daily, diets consisting of (g/kg dry matter (DM)) 500 barley, 400 grass silage and 100 soya-bean meal. The diets were given at either 1.15 (L) or 2.3 (H) times maintenance energy requirements and the soya-bean meal was either untreated (U) or formaldehyde (HCH0)-treated (T).

2. The passage of digesta to the duodenum was estimated using chromic oxide as a flow marker; 35S was used to estimate the amount of microbial protein entering the small intestine. A microbial fraction was prepared by differential centrifugation from duodenal digesta. Samples of bacteria and of protozoa from rumen digesta were also prepared.

3. The total amino acid contents of feedingstuffs, duodenal digesta, duodenal microbial material, rumen bacteria and rumen protozoa were determined by ion-exchange chromatography. The D-alanine and D-glutamic acid contents of the samples were determined by gas–liquid chromatography.

4. The quantity of each amino acid entering the small intestine was significantly (P < 0,001) increased by increasing DM intake and tended to be increased by formaldehyde-treatment of the soya-bean meal. There were net losses of all amino acids across the forestomachs except for lysine, methione, o-alanine and D-glutamic acid for which there were net gains.

5. There were significant (P < 0.05) differences in amino acid composition between rumen bacteria and duodenal microbial material; differences in amino acid composition between rumen bacteria and rumen protozoa were also observed.

6. D-Alanine and D-glutamic acid were present in the silage but not in the barley or either of the soya-bean meals. All samples of microbes and digesta contained D-alanine and D-glutamic acid.

7. The use of D-ahine and D-glUtamiC acid as markers for microbial nitrogen entering the small intestine was assessed. Estimates of the quantities of microbial N entering the small intestine based on the D-alanine or D-glutamic acid contents of rumen bacteria or duodenal microbes were significantly higher than those determined using 35S as a marker.

1. After Nr-methylhistidine (Nr-MH) distribution among the various organs or the tissues was determined in male broiler chickens of 15 d of age, the rates of degradation of myofibrillar proteins in male layer and broiler chickens at different stages of growth were determined by means of Nr-MH.

2. About 75 and 8% of the total Nr-MH in the tissues occurred respectively in skeletal muscle and stomach, and most of the remainder in the intestine and the skin.

3. The rates of degradation of myofibrillar proteins in the male layer and broiler chickens of 21, 42 and 63 d of age were calculated to be 6.1, 4.5 and 2.4%/d (layer) and 5.0, 2.8 and 0.9%/d (broiler) respectively. These calculations involve the assumption that 80% of the total excreted Nr-MH was derived from skeletal muscle.

4. The results strongly indicate that the rapid growth of the broiler chicken is facilitated by the reduced rate of protein degradation.

1. The present work with growing rats was undertaken to study the effect of protein quality, gastrointestinal microbial activity and the level of nitrogen intake on protein utilization and energy digestibiiity. The experiment involved a total of thirty-six dietary treatments in a 9 x 4 factorial design, with five rats per treatment. The thirty-six diets resulted from nine protein sources. Each diet was composed of a basal N-free mixture plus minerals and vitamins, with N sources added at the expense of the N-free mixture to provide 15.0 gN/kg dry matter (DM) in the first three protein-addition treatments and 30.0 gN/kg DM in the fourth protein-addition treatment. The nine protein sources were soya-bean meal, casein, wheat gluten, skim-milk powder, meat-and-bone meal, wheat bran, barley, wheat and cooked brown beans (Phaseolus vulgaris). The four formulations for each protein source incorporated the protein unsupplemented at 15.0 gN/kg DM, unsupplemented at 30.0 gN/kg DM, or supplemented at 15.0 gN/kg DM with the estimated first-limiting amino acid or the antibiotic Nebacitin.

2. With all protein sources, the inclusion of the first-limiting amino acid had no effect on either protein or energy digestibility.

3. The microbial activity in the digestive tract affected protein utilization and energy digestibility to a different degree depending primarily on the level and type of dietary fibre. True protein digestibility (TD) of skim-milk powder and brown beans, both rich in easily-fermentable energy, increased from 0.959 to 1.000 and from 0.680 to 0.777 respectively by the addition of Nebacitin. TD of the other protein sources was only marginally affected by the antibiotic treatment. Only with brown beans was the biological value (BV) markedly affected by Nebacitin with an increase from 0.482 to 0.557 by the treatment. Energy digestibility was significantly lower in rats given antibiotic with soya-bean meal, wheat bran, barley, wheat and brown beans.

4. The effect of level of N intake on protein utilization was dependent on both protein quality and the fibre concentration of the diet. Protein sources with high BV were more affected than proteins of lower BV. It was concluded that TD is not always independent of dietary protein concentration.

1. The effect of peroral administration of xylitol on the absorption of iron and the activities of xanthine oxidase (EC 1. 2. 3. 2) and ferroxidase in rat duodenal wall was studied.

2. Adult male rats were given the basal diet containing 200 g xylitol/kg or the same diet containing no added carbohydrates for 8 weeks. Both feeding groups comprised twelve animals.

3. Xylitol significantly increased serum and liver Fe concentrations with a concomitant, significant increase in the duodenal xanthine oxidase activities, but caused a marginal increase in the duodenal ferroxidase activities.

4. In vitro, sugar alcohols reduced the binding rate of Fe to transferrin.

5. The xylitol-induced increase of Fe absorption may involve the following mechanism: the high intraluminal xylitol concentration of the xylitol-fed rats keeps Fe in the form of a soluble complex for a prolonged period of time, due to the slow absorption of xylitol. The polyol-Fe complex in turn induces xanthine oxidase and ferroxidase formation.

1. Nitrogen balance was studied in growing male SPF-rats fed on diets in which each essential amino acid was varied from zero to about 120% of optimum requirement. From the balance results, optimum and maintenance requirements were estimated as well as the efficiency of utilization of amino acids for growth and growth + maintenance.

2. N balance increased with increasing dietary level of the deficient amino acid; the response gradually diminished as the content of the amino acid approached optimum. At zero level of intake, negative N balance was found for all amino acids except histidine. The highest loss of body N was found in the sulphur-amino-acid-free diet and the lowest one in the lysine-free diet.

3. Maximal utilization of essential amino acids for growth was found at dietary levels corresponding to 30–60% of optimum requirement and ranged from about 0.65 to 0.85 except for S amino acids and histidine. The utilization of S amino acids was about 0.55 while that of histidine exceeded 1.0. The utilization of amino acids for growth-tmaintenance was maximal at the lowest levels of intake and gradually decreased as the dietary concentration of the limiting amino acid increased. At dietary levels near optimum the utilization was about 06–07, except for S amino acids where the utilization was less than 0.5.

I. The present work with growing rats was undertaken to compare the effect of wheat bran and barley husk on nutrient bioavailability. The experiment involved a total of nine dietary treatments consisting of a control group, without wheat bran or barley husk, and two series of four groups with increasing amounts of fibre from 50 to 117 g/kg dry matter (DM) from the two fibre sources. Dietary nitrogen concentration was kept constant at 15 gN/kg DM by adjusting the diets with an N-free mixture. Protein sources were casein, fortified with methionine and white wheat flour. True protein digestibility (TD), biological value (BV), net protein utilization (NPU) and digestible energy (DE) were estimated.

2. TD decreased when total dietary fibre (TDF) increased, the effect being greater in the case of wheat bran. The difference in response can be explained by the larger N contribution from bran than from barley husk. N from barley husk was actually digested less than N from wheat bran.

3. Changes in TD due to fibre were small, both for wheat and barley husk. It was concluded that decreased TD with fibre at moderate levels was due to poor digestibility of the N associated with the fibre source rather than decreased digestibility of N from other dietary components.

4. BV was only marginally affected by the fibre levels, indicating that the relatively high lysine content in both wheat bran and barley husk had a low availability.

5. Wheat bran and barley husk showed almost the same negative effect on DE and DM digestibility (DMD). DMD correlated significantly with DE, demonstrating that DMD is a simple and convenient means of monitoring DE.

1. Diets of fresh kale (Brassica oleracea) and ryegrass (Loliumperenne) – elover (Trifolium repens) herbage were fed to growing sheep in three experiments. In Expts 1 and 3 the sheep were confined indoors and fed at hourly intervals, and all were given supplementary iodine to counteract kale goitrogens. Lambs grazed the two forages for 24 weeks in Expt 2, with and without intramuscular injections of iodized oil. The kale and herbage contained respectively 11 and <0.1 g S-methyl-L-cysteine sulphoxide (SMCO)/kg dry matter (DM) and values for readily fermentable: structural carbohydrate (CHO) were 3.1 and 0.8, respectively.

2. Blood samples were withdrawn from indwelling catheters (Expts 1 and 3) or venipuncture (Expt 2) and the plasma analysed for a range of hormones using radioimmunoassay procedures. Glucose irreversible loss (GIL) was measured in Expt 1 using primed continuous infusions of D-[U-14C]glucose. Samples of adipose tissue were removed from the shoulder area in Expt 3, and rates of D-[U-14C]glucose and [U-14C]acetate incoporation and oxidation were measured in vitro, together with the rate of glycerol release.

3. In the presence of supplementary I2, kale feeding was associated with an elevation in plasma concentration of free thyroxine (T4) Regardless of I2 supplementation, sheep fed on kale had much higher plasma growth hormone concentrations than sheep fed on ryegrass-clover herbage, and this was accompanied by reduced plasma somatostatin concentrations.

4. Plasma insulin and glucagon concentrations were similar for sheep fed on the two diets; GIL tended to be slightly but not significantly greater (9.4%) for sheep fed on kale than for those fed on ryegrass-clover herbage.

5. Kale feeding was associated with increased uptakes of acetate and glucose into adipose tissue, reduced rates of oxidation of both substrates and no difference in rate of glycerol release. Each 1 nmol increase in glucose uptake was associated with 8.7 nmol acetate uptake ( P < 0.001).

6. It is proposed that ruminants counteract protein inactivation, caused by production of dimethyl disulphide from SMCO in the rumen, through increasing circulating concentrations of growth hormone and T4, which then stimulate synthesis of replacement body proteins.

1. 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.

1. Responses to twice-weekly oral supplementation with 4.0 g methionine were measured with lambs (27 kg) grazing kale (Brassica oleracea) for 10 weeks (Expt 1). In a second experiment with sheep fed on kale at hourly intervals, rumen fractional outflow rates of CrEDTA and ruthenium Tris(1, 10 phenanthroline) markers were measured from the rates of decline in their concentrations. Rumen turnover of S-methyl-L-cysteine sulphoxide (SMCO) and of carbohydrate (CHO) constituents were also measured. The kale fed contained 11.4 g SMCO/kg dry matter and the ratio, readily-fermentable: structural CHO was high at 2.9.

2. Severe haemolytic anaemia, associated with low live-weight gain (LWG), occurred in the lambs during the initial 5 weeks of grazing, due to rumen fermentation of SMCO. Oral methionine supplementation raised plasma concentrations of methionine and cysteine, increased wool growth rate, and increased LWG during the initial 5 weeks. Methionine supplementation also increased rumen pool and plasma SMCO concentrations, suggesting reduced rumen SMCO fermentation.

3. In Expt 2, rumen degradation rate of SMCO (1.2/h) was calculated to be twice as fast as that of the most rapidly fermented dietary CHO constituents and eight times faster than the rate of water outflow (0.16/h), thus explaining its virtually complete rumen degradation and toxicity in brassica diets. It was estimated that 1.2 g of each 4.0 g methionine administered would have escaped rumen degradation, due to the high rate of water outflow from the rumen.

4. Disappearance rates of CHO constituents from the rumen were as predicted for normal ruminant diets, showing that rumen metabolism of SMCO did not have a depressive effect. Hemicellulose disappeared at a slower rate than the other CHO, suggesting that hemicellulose digestion may be rate-limiting for cell-wall digestion.

5. It is suggested that the responses to oral methionine supplementation can be explained through methionine reducing rumen SMCO degradation, and through a significant portion of the methionine escaping rumen degradation.

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 eight0.75, 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 < 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 < 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 < 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-animal model of N flows between the digestive tract and the tissues.