1. An experiment was conducted to determine the effect of the abomasal parasite, Haernonchus contortus, on the pattern of digestion and nutrient utilization in Merino sheep. There were three groups of sheep: infected with H. contortus (300 larvae/kg live weight) (n 5), sham-infected by transferring blood from the jugular vein to the abomasum, and uninfected (control) sheep (n 9) which were fed daily rations equal to amounts consumed by 'paired' animals in the two other treatment groups. A diet containing (g/kg): lucerne (Medicago sativa) chaff 490, oat chaff 480, ground limestone 10, urea 10, and sodium chloride 10, was given in equal amounts at 3-h intervals.2. Continuous intrarumen infusions (8 d) of chromium and ytterbium were made in order to measure the flow of digesta through the rumen, duodenum and ileum with 16NH,CI included in the infusate for the final 3 d. The toss of blood into the gastrointestinal tract was measured using 51Cr-labelled erythrocytes and the rate of irreversible loss of plasma urea was measured with reference to a single intravenous injection of [Wlurea. Samples of rumen fluid were taken for analysis of volatile fatty acid (VFA) concentrations.3. The infected and sham-infected sheep developed severe anaemia during the period over which digestion and metabolism measurements were made (packed cell volume 0 118 (SE 00042) and 0.146 (SE 0.0073) respectively). The corresponding rates of blood loss into the gastrointestinal tracts were 253 (SE 23) and 145 (SE 17) ml/d. 4. The proportions of VFA in rumen fluid were altered (P < 005) in the infected group with a decrease in the ratio, acetate: propionate (control 3.28, infected 2.58, standard error of difference (SED) 0.21). There was also an increase in rumen fluid outflow rate (P < 0.01) from 405 litres/d in the control group to 5.53 litres/d in the infected group (SED 043). Water intake was higher (P c 005) in the infected than in the control animals (2.25 and 1.84 litres/d respectively; SED 0.14). 5.There was a decrease (P i 0.05) in apparent digestion of organic matter in the forestomachs of infected sheep (032 compared with 0.39 in the control, SED 0.02). There was also a decrease (P < 0.05) in the apparent digestion of organic matter across the whole digestive tract (0.65 control, 061 infected, SED 0.013).6. There was a loss of 2.6 and 1.8 g blood nitrogen/d into the gastrointestinal tract of the infected and shaminfected sheep respectively. In the infected sheep approximately 50 % of this N was accounted for as additional ammonia leaving the abomasum compared with 20% in the sham-infected group. The additional nonammonia-N (NAN) entering the duodenum of parasitized or sham-infected animals was reabsorbed before the ileum. There was no effect of infection or sham-infection on the synthesis or digestion of microbial NAN.7. There was a higher (P < 0.001) rate of plasma urea irreversible loss in the infected sheep (8.9 control, 12.2 infected, 109 sham-infected, SED (control v. treated) 087 g N/d). This was apparently due to increased abso...

I. Six sheep, each fitted with a rumen cannula and re-entrant cannulas in the proximal duodenum and distal ileum, were given two diets (600 g dry matter (DM)/d) consisting of either grass silage (32.1 g nitrogen/kg DM) or dried grass (18.3 g N/kg DM). A net loss of N occurred between mouth and duodenum with the silage diet, indicating extensive ruminal degradation of dietary N, compared with a net gain on the dried-grass diet. Consequently, despite higher N intakes when silage was given, N flow at the duodenum was similar for both diets.2. The proportion of microbial N in duodenal digesta N was estimated using diaminopimelic acid (DAPA),[35S]methionine (35S), 15N-enriched non-ammonia-N (I5NAN) and amino acid profiles (AAP) as microbial markers. Isotopic labelling of rumen micro-organisms was achieved by intraruminal infusions of Na,YSO, and (15NH4),S04.3. A comparison of all methods was made based on the marker concentrations in microbial fractions isolated by differential centrifuagation of strained rumen contents. With both diets, DAPA gave the highest estimates and AAP the lowest. Estimates based on 35S and 15NAN were intermediate and did not differ significantly (P > 0.05). 4.For the I5NAN, 35S and AAP methods, the effect of site of sampling of the microbial fraction, i.e. from rumen contents or duodenal digesta, was examined and in all instances mean estimates based on duodenally-derived microbes were higher. However, the differences were significant for only I3NAN with both diets ( P < 0.001), for 35S with the dried grass (P < 0.05), and for AAP with the silage (P c 0.05). Estimates based on duodenally-derived microbes were higher (P < 0.05) using 15NAN than those obtained using 35S with both diets. 5.Depending on the method used for estimating microbial N, estimates of the efficieny of microbial N synthesis in the rumen (g microbial N flow at duodenum/kg organic matter apparently digested in the rumen) ranged between 16 and 38 for the silage diet and 10 and 46 for the dried grass diet. Similarly, estimates of feed N degradability in the rumen ranged between 0.62 and 0.97 for the silage and 0.00 and 0.93 for the dried grass.Digesta entering the small intestine of ruminants contains protein originating from three different sources, i.e. microbial protein synthesized in the rumen, feed protein which has passed undegraded through the rumen, and endogenous protein in the form of abomasal secretions and desquamated epithelial cells. If a suitable marker exists which is present in only one of the three fractions, the proportion of that fraction can be estimated by marker dilution, by measuring the marker concentration in an isolated sample of the fraction and in unfractionated duodenal digesta. The proportion can then be used in conjunction with measurements of total protein flow to the duodenum to obtain the duodenal flow of that particular fraction.A number of different markers have been used in this respect to identify microbial protein in duodenal digesta. Two commonly used, naturally-occurring markers are d...

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