In vitro rumen fermentation of the tropical grass Pennisetum purpureum and mixtures with browse legumes: effects of tannin contents.

The fermentation of Pennisetum purpureum, alone (PP) or substituted with 0.30 of the tanniferous legumes Acacia cornigera (AC), Albizia lebbekoides (AL), Leucaena leucocephala (LL) and the saponin-rich Enterolobium cyclocarpum (EC) was studied in vitro, in presence or absence of polyethylene glycol (PEG) as tannin binder. Inactivation of tannins with PEG increased (p < 0.05) gas production with AL and LL from 8 and 12 h to the end of the incubation period respectively. When PEG was added, LL and AC promoted a higher (p < 0.05) gas volume than PP in the first 12 h incubation, and there were not differences between PP and AL. Substrate mixtures reduced (p < 0.05) methane volume produced compared with PP, but this was not related to PEG inclusion (p > 0.10). There was a trend (p = 0.06) for a higher 48 h bacterial attachment to the substrate when incubated without PEG. The decrease in fermentation of EC from 12 h incubation onwards could be associated with a negative mid-term effect of saponins over cellulolytic bacterial activity. It is concluded that the effects of tannins on microbial fermentation of mixed forage substrates are variable, depending on their nature, but do not have a marked impact on bacterial adhesion or methane production.