In vitro fermentation of intact and fractionated tropical herbaceous and tree legumes containing tannins and alkaloids

Ruminal fermentation of tropical herbaceous and tree forage legumes with or without appreciable amounts of tannins and alkaloids was studied in vitro. The legumes studied included Calliandra calothyrsus Meissner, Acacia angustissima Kuntze, Desmodium uncinatum DC, Desmodium intortum DC (all containing tannins), Crotalaria ochroleuca Gidon and Lupinus albus L. (containing alkaloids). The plants with low levels of tannins and alkaloids were Neonotonia wightii Lackey and Lablab purpureus L. cv. Rongai (Sweet). Both unfractionated legumes and their isolated neutral detergent fiber (xNDF) were studied. In addition, polyethylene glycol (PEG, MW 4000) was added to the fermentations of tannin-containing forages and their isolated NDF. Unfractionated legumes varied widely in NDF digestibility. The NDF digestibility of intact C. calothyrsus, A. angustissima, C. ochroleuca and D. intortum was less than 40% and lower than that of the other legumes. C. calothyrsus and C. ochroleuca had the greatest increase (P<0.05) in NDF digestibility following extraction with ND solution when compared to their respective intact forages (0.32 vs. 0.23 and 0.46 vs. 0.22, respectively). There was no difference (P>0.05) in degradability of isolated and intact NDF in the other legumes. Cumulative gas production was nonlinearly related to NDF degradability (y = 15.5 ln(x) + 27.8; R2 = 0.94). Generally, the rate of gas production among the intact legumes was lower in tannin-containing legumes and C. ochroleuca compared to legumes with lower levels of anti-nutritional compounds. Addition of PEG to fermentations of tannin-containing legumes significantly (P<0.05) improved the amount and rate of gas production compared to the respective non-PEG controls. Microbial protein synthesis was significantly higher (P<0.05) in C. calothyrsus, L. purpureus and C. ochroleuca than in the other legumes. L. purpureus and C. calothyrsus contained more sugar (131 and 154 g/kg DM, respectively) than the other forages. Tannins suppressed production of volatile fatty acids (VFA) compared to VFA production from forages with low levels of anti-nutritional factors (ANF). Addition of PEG increased VFA production from fermentation of tannin-containing forages. This study suggests that tannins and alkaloids in forages can adversely affect their fermentation characteristics and thereby their nutritive value.