The effect of condensed tannins in Lotus corniculatus on plasma metabolism of methionine, cystine and inorganic sulphate by sheep

Abstract

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.