An experiment was conducted at Palmerston North, New Zealand, to determine the effect of condensed tannins (CT) on the true and apparent digestion of methionine and cysteine in the small intestine (SI) of sheep fed fresh Lotus comkulatus. The lotus contained c. 30 g total CT/kg dry matter (DM) and was fed hourly to sheep in metabolism crates. Four sheep were prepared with rumen and abomasal cannulae which enabled the indigestible liquid phase marker, chromium ethylene diamine tetra-acetic acid (Cr-EDTA), to be infused into the rumen to estimate digesta flow. True digestibility of plant methionine and cysteine in the SI and their site of absorption in the SI were determined from 35S-labelled L. corniculatus homogenate continuously infused into the abomasum. After 9 h infusion of the 35S-labelled lotus homogenate, the sheep were slaughtered and digesta samples were taken at intervals along the small and large intestines. The effect of CT was determined by comparing two control sheep (CT-acting) with two sheep given a continuous intraruminal infusion of polyethylene glycol (PEG, MW 3500) to bind and inactivate the CT.

The CT reduced the true digestibility of plant methionine (0·72 v. 0·88) and cysteine (0·65 v. 0·81) in the SI relative to sheep receiving PEG. Condensed tannins also appeared to alter the site of digestion of both [35S]methionine and [35S]cysteine in the SI, and increased the flux of both amino acids in the mid and latter thirds of the SI. CT did not affect the apparent digestibility of total methionine (0·82 v. 0·84) in the SI but reduced the apparent digestibility of total cysteine from 0·77 to 0·66. In control sheep CT increased the abomasal flux (as a proportion of eaten) of total digesta methionine (0·88 v. 0·76) and total digesta cysteine (0·74 v. 0·62). The apparent absorption of total methionine (plant + microbial + endogenous) was increased by the action of CT (0·72 v. 0·63 g/g eaten) but was similar for total cysteine (0·49 v. 0·48 g/g eaten) in both groups. It was concluded that CT reduced the true digestibility of plant methionine and cysteine in the SI. However, it was calculated that the action of CT actually increased the total amounts (g/g eaten) of plant methionine and cysteine absorbed from the SI, due to its effect in increasing abomasal flux.

Fresh Lotus corniculatus containing 27 g extractable condensed tannin (CT)/kg dry matter (DM) and 8 g bound CT/kg DM was fed at hourly intervals to sheep held in metabolism cages to study the effects of CT on nutrient digestion and on metabolism of methionine, cystine and inorganic sulphate in plasma. Polyethylene glycol (PEG) was continuously infused into the rumen of half the sheep to remove the effects of CT. Principal measurements in the two groups were plasma irreversible loss (IRL) rate and interconversions of methionine, cystine and inorganic sulphate using 35S labelling. CT in Lotus corniculatus had no effects on the apparent digestion of cellulose and minerals, slightly depressed DM, organic matter and hemicellulose digestion and markedly reduced the apparent digestion of N (P 0·01). The concentration of NH3 and molar proportions of iso-butyric acid, iso-valeric acid and n−valeric acid in rumen fluid were markedly increased by the PEG infusion (P 0·01), whereas total volatile fatty acid concentration and molar proportions of acetic acid, propionic acid and n−butyric acid were not affected. PEG infusion temporarily increased rumen protozoa numbers. CT greatly increased the IRL of plasma cystine (131·1 v. 7·0 μmol/min; P 0·05) and reduced IRL of plasma inorganic sulphate (36·8 v. 48·1 μmol/min; P 0·01) but had no effect on methionine IRL. CT increased transulphuration of methionine to cystine (4·37 v. 1·24 μmol/min; P 0·05), increased cystine entering the plasma from whole-body protein turnover plus absorption from the small intestine (9·34 v. 5·75 μmol/min; P 0·05) and increased cystine flux to body synthetic reactions (11·89 v. 5·41 μmol/min; P 0·05). CT had no effect on the proportion of methionine total flux transferred to sulphate (0·05 v. 0·06; P 0·05), reduced the proportion of methionine flux transferred to body synthetic reactions (0·68 v. 0·86) and markedly reduced the proportion of cystine flux transferred to sulphate (0·09 v. 0·27; P 0·01). It was concluded that CT in Lotus corniculatus reduced rumen protein degradation and markedly increased utilization of plasma cystine for body synthetic reactions.

Two experiments were conducted with sheep fed on fresh Lotus pedunculatus containing 50–55 g condensed tannin (CT)/kg dry matter. Effects of CT were assessed by comparing control sheep (CT operating) with sheep receiving a continuous intraruminal infusion of polyethylene glycol (PEG) to bind and inactivate CT. Digestion of methionine and cystine was determined using a continuous intraruminal infusion of indigestible markers, whilst plasma irreversible loss (IRL) of methionine, cystine and inorganic sulphate was determined using 35S labelling. The proportion of microbial non-NH3-N (NAN) in whole rumen digesta NAN and the IRL of reducible S from the rumen were determined using a continuous intraruminal infusion of (NH4)235SO4. The proportion of microbial NAN in whole rumen digesta NAN (0·44 v. 0·71) and the IRL of reducible S from the rumen (0·84 v. 2·49 g S/d) were lower in control than PEG sheep. PEG sheep lost 30% of ingested methionine and cystine across the rumen, whereas the control sheep lost no methionine and cystine across the rumen. Apparent absorption of methionine from the small intestine was 27% higher in control than PEG sheep, but both groups had a similar apparent absorption of cystine. The apparent digestibility of cystine in the small intestine was lower in control (0·42) than PEG (0·53) sheep, whereas the apparent digestibility of methionine was similar (0·78) for both groups. CT had no effect on plasma methionine IRL, but markedly increased the IRL of cystine (39·8 v. 22·4 μmol/min) and reduced the IRL of plasma inorganic sulphate (35·9 v. 50·2 μmol/min). A three-pool model comparing interconversions between the three plasma metabolites showed that CT increased the flow of cystine to body synthetic reactions (36·5 v. 17·3 μmol/min). This was due to trans-sulphuration of methionine to cystine being greater in control than in PEG sheep, whilst the oxidation of both methionine and cystine were reduced in control sheep. It was concluded that CT reduced the proteolysis of forage protein and the degradation of S amino acids to inorganic sulphide in the rumen, resulting in increased net absorption of methionine and increased utilization of cystine for body synthetic reactions in sheep with a high capacity for wool growth (and, hence, high cystine requirement).