Effect of plant extracts from several tanniferous browse legumes on in vitro microbial fermentation of the tropical grass Pennisetum purpureum

To what extent microbial fermentation of tropical grasses is affected by tannins from browse legumes was studied in vitro using acetone extracts from tanniferous plants in four 48 h incubation experiments. A proportion of plant extracts equivalent to 240 mg of Acacia cornigera (AC), Albizia lebbekoides (AL) and Leucaena leucocephala (LL) were added to each bottle with 800 mg Pennisetum purpureum as substrate, and contrasted with P. purpureum alone (PP) or with 64 mg quebracho tannins (QT). Gas production was lowest with QT and AL (P<0.05), and AC and LL reduced gas volume compared to PP from 10 to 24 h (P<0.001) and from 4 to 8 h (P<0.01), respectively. Compared with PP, total VFA concentration was reduced with AL and QT after 12 h (P<0.01), and the same trend (P=0.06) occurred with AC and AL after 24 h. Acetate molar proportion at 24 h was reduced, and that of propionate increased, with AC and AL compared with PP (P<0.05), whereas AL and QT reduced butyrate proportion (P<0.05). Estimated methane concentration per unit fermented organic matter after 24 h was lower with AC and AL than with PP (P<0.01). Proportion of R. albus was reduced with LL, AC and AL compared with PP at 12 h (P<0.01) and 24 h (P<0.001) of incubation, and the relative concentration of F. succinogenes tended (P=0.08) to be reduced with QT at 12 h, whereas there was no effect on R. flavefaciens. Plant extracts from A. cornigera, A. lebekkoides and L. leucocephala affected microbial fermentation of the tropical grass P. purpureum, reducing the rate of gas production from 4 to 16 h and increasing propionate at the expense of acetate, to the largest extent with A. lebbekoides. A reduction of methanogenesis, and therefore an increase in efficiency of energy utilisation, is consequently expected. No major effect on the bacterial population attached to the substrate occurred, but concentrations of R. albus seemed to be sensitive to tannins.