THE EFFECT OF CONDENSED TANNINS IN LOTUS-PEDUNCULATUS ON THE DIGESTION AND METABOLISM OF METHIONINE, CYSTINE AND INORGANIC SULFUR IN SHEEP

Two experiments were conducted with sheep fed on fresh Lotus peduncula tus 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 wa s determined using a continuous intraruminal infusion of indigestible markers, whilst plasma irreversible loss (IRL) of methionine, cystine and inorganic sulphate was determined using S-35 labelling. The propor tion of microbial non-NH3-N (NAN) in whole rumen digesta NAN and the I RL of reducible S from the rumen were determined using a continuous in traruminal infusion of (NH4)(2)(SO4)-S-35. The proportion of microbial NAN in whole rumen digesta NAN (0.44 v. 0.71) and the IRL of reducibl e S from the rumen (0.84 v. 2.49 p S/d) were lower in control than PEG sheep. PEG sheep lost 30% of ingested methionine and cystine across t he rumen, whereas the control sheep lost no methionine and cystine acr oss the rumen. Apparent absorption of methionine from the small intest ine was 27% higher in control than PEG sheep, but both groups had a si milar apparent absorption of cystine. The apparent digestibility of cy stine in the small intestine was lower in control (0.42) than PEG (0.5 3) sheep, whereas the apparent digestibility of methionine was similar (0.78) for both groups. CT had no effect on plasma methionine IRL, bu t markedly increased the IRL of cystine (39.8 v. 22.4 mu mol/min) and reduced the IRL of plasma inorganic sulphate (35.9 v. 50.2 mu mol/min) . A three-pool model comparing interconversions between the three plas ma metabolites showed that CT increased the. flow of cystine to body s ynthetic reactions (36.5 v. 17.3 mu mol/min). This was due to trans-su lphuration of methionine to cystine being greater in control than in P EG sheep, whilst the oxidation of both methionine and cystine were red uced in control sheep. It was concluded that CT reduced the proteolysi s of forage protein and the degradation of S amino acids to inorganic sulphide in the rumen, resulting in increased net absorption of methio nine and increased utilization of cystine for body synthetic reactions in sheep with a high capacity for wool growth (and, hence, high cysti ne requirement).