In vitro fermentation of browse species using goat rumen fluid in relation to browse polyphenol content and composition

Abstract

The effect of browse species tannins (using polyethylene glycol, PEG 6000) on in vitro gas production (GP), methane (CH4), ammonia (NH3), volatile fatty acids (VFA) and in vitro organic matter digestibility (IVOMD) were studied. Approximately 0.5g of air-dried A. etbaica, C. farinosa, C. tomentosa, D. angustifolia, D. cinerea, E. racemosa, M. angolensis, M. senegalensis, R. natalensis and S. singueana leaves were used as substrates in an automated in vitro system. Proanthocyanidin (PA) contents were quantified using the modified HCl-butanol method and the PA composition analysed by ultra performance liquid chromatography using a diode array detector coupled with electrospray ionization mass spectrometry (UPLC-DAD-ESI–MS/MS). Substrates were inoculated for 72h in buffered rumen fluid (2:1, v/v), pooled from three goats fed a grass silage and concentrate diet, on three separate occasions. Each substrate was incubated in triplicate with or without inclusion of PEG 6000 and GP measured. During incubation, head space gas samples were taken at 0, 3, 6, 9, 12, 24, 30, 48, 54, and 72h and analysed for CH4. Volatile fatty acids, NH3 and IVOMD were determined after 72h of incubation. Data from three runs were averaged for analysis. Addition of PEG increased (P<0.0001) GP, CH4, NH3 and total VFA indicating that PA were mainly involved in reducing methanogenesis but also digestibility. Prodelphinidins were found to be the major PA affecting fermentation. Also the contribution of quercetin, myricetin and kaempferol derivatives in CH4 reduction were evident. Changes in the molar proportions of VFA with PEG addition indicated that PA affected fermentation pathways. The absence of PEG effect on IVOMD was due to artefacts from the tannin-PEG complexes interfering with the incubation residue measurement. Overall, the effect of tannin-containing browse on in vitro fermentation characteristics was mainly due to PA with the possible minor effects of other phenolic and non-phenolic compounds.