The ruminal degradability of fibre explains part of the low nutritional value and reduced methanogenesis in highly tanniniferous tropical legumes

BACKGROUND: Limited forage quality and low methanogenesis from fermentation of tropical shrub legumes is often attributed to high contents of condensed tannins (CTs), but characteristics of the fibre may be important, too. Using the rumen-simulation technique an attempt was made to separate the influence of tannins and fibre on ruminal digestion and methanogenesis. Legumes with CTs (Calliandra calothyrsus, Flemingia macrophylla) and plants free of CTs (Vigna unguiculata, Brachiaria humidicola) were used. To separate effects, either CTs were inactivated using polyethylene glycol or purified fibres free of CTs were obtained. In experiment 1, legumes replaced one third of grass in the diet and urea was supplemented. In experiment 2, only purified fibres and casein were incubated. RESULTS: Purified fibres had a different degradability (low in legumes with CTs). Lignin prevented mainly hemicellulose from being degraded while the extent of degradation of hemicellulose seemed to have determined methanogenesis. In the mixed diets these effects were small when the supply of N was sufficient. When CTs were inactivated, fibre degradation was still lower with Calliandra than with Vigna. CONCLUSION: Properties of fibre, especially lignification, of CT-rich plants help to explain the low feeding value of such forages, but CTs represent the over-riding factor. (C) 2008 Society of Chemical Industry.