Evaluating effects of tannins on extent and rate of in vitro gas and CH4, production using an automated pressure evaluation system (APES)

An in vitro study was conducted to investigate effects of tannins on extent and rate of gas and CH4 production using an automated pressure evaluation system (APES). In this study three condensed tannins (CT; quebracho, grape seed, green tea tannins) and four hydrolysable tannins (HT; tara, valonea, myrabolan, chestnut tannins) were evaluated with lucerne as a control substrate. CT and HT were characterised by matrix assisted laser desorption ionisation–time of flight mass spectrometry (MALDI–TOF MS). Tannins were added to the substrate at an effective concentration of 100 g/kg, either with or without polyethylene glycol (PEG 6000), and incubated for 72 h in pooled buffered rumen liquid from four lactating dairy cows. After inoculation, fermentation bottles were immediately connected to the APES to measure total cumulative gas production (GP). During the incubation, 11 gas samples were collected from each bottle at 0, 1, 4, 7, 11, 15, 23, 30, 46, 52 and 72 h of incubation and analysed for CH4. A modified Michaelis–Menten model was fitted to the CH4 concentration patterns, and model estimates were used to calculate total cumulative CH4 production (GPCH4). GP and GPCH4 curves were fitted using a modified monophasic Michaelis–Menten model. Addition of quebracho reduced (P=0.002) GP, whilst the other tannins did not affect GP. Addition of PEG increased GP for quebracho (P=0.003), valonea (P=0.058) and grape seed tannins (P=0.071), suggesting that these tannins either inhibited, or tended to inhibit, fermentation. Addition of quebracho and grape seed tannins reduced (P=0.012) the maximum rate of gas production, indicating that microbial activity was affected. Quebracho, valonea, myrabolan and grape seed decreased (P=0.003) GPCH4 and the maximum rate (0.001=P=0.102) of CH4 production. Addition of chestnut, green tea and tara tannins did not affect total gas nor CH4 production. Vvalonea and myrabolan tannins have the most promise at reducing CH4 production as they had only a minor impact on gas production.