Complexes of the condensed tannins of sainfoin (<i>Onobrychis viciifolia</i> scop.) with fraction 1 leaf protein and with submaxillary mucoprotein, and their reversal by polyethylene glycol and pH

Abstract In vitro studies were made to determine the nature of complexes formed between the condensed tannins (i.e. pro‐anthocyanidins or flavolans) of sainfoin ( Onobrychis viciifolia ) and either the major soluble dietary protein, Fraction 1 protein, of green leaves, or bovine salivary mucoprotein. Fraction 1 protein, uniformly labelled with 14 C, formed only insoluble complexes with these tannins. Bovine salivary mucoprotein produced insoluble complexes only at temperatures below 25°C. Fraction 1 protein complexes were stable between pH 3.5 and pH 7.0. At pHs below 3.0 greater than 90% of the protein was solubilised in the presence of PEG but at pH 8.0 only 30% of the protein was released from the complex. In vivo experiments with sheep fed sainfoin, confirmed the stability of the tannin‐Fraction 1 complexes in the rumen (pH 6.5) and break‐up of the complexes in anterior duodenal samples (pH 2.5) as shown by the extractability of condensed tannin from the complex. The significance of these results with respect to (a) bloat in cattle, (b) nitrogen metabolism of ruminants, is discussed. Polyethylene‐glycol, mol.wt. 4000 (PEG) exchanged with protein in the complex releasing protein into solution. The effectiveness of PEG was dependent on the amount of tannin in the complex and the age of the complex at the time of addition of PEG. The results explain the observation that PEG is unsuitable as a marker for rumen volume determination when animals are fed herbage that contains condensed tannins. A method for determining any deleterious effects of tannins on nitrogen metabolism by ruminants, is suggested.