Comparisons of manual and automated incubation systems: Effects of venting procedures on in vitro ruminal fermentation

• Higher headspace pressure may not alter Methane production.
• Higher headspace pressure alters total and hydrogen gas production.
• Low pressure venting permits a reliable total gas, CH4 and H2 accumulation.
• Higher headspace pressure may alter pathway of VFA production.

Gas produced was measured based on headspace pressures in both manual and automated in vitro incubation systems, which were always vented at pre-set fixed times (FT) and at a pre-set threshold of fixed pressure (FP) respectively. This study was to investigate these two venting procedures (VP) on kinetics of total gas, methane (CH4) and hydrogen gas (gH2), substrate degradation and volatile fatty acid (VFA) production. Corn grain, rice straw, rice grain and tall fescue were selected for in vitro ruminal batch incubation. Headspace pressure of FT procedure was manually released at incubation times being 2, 4, 8, 12, 20, 28, 36, 48 and 72 h in manual incubation system, while headspace pressure of FP procedure was automatically released at threshold pressure being 9 kPa/bottle in automated incubation system. The FT procedure had particular high venting headspace pressure (> 40 kPa) at some incubation times for corn grain and rice grain. The FT procedure had a lower gas volume (P<0.001) and fractional rate of gas production (P<0.001) than FP procedure, which was caused by the decreased (P=0.01) substrate degradation and increased partial dissolution of CO2 under higher venting headspace pressure. The FP procedure had higher (P=0.016) fractional rate of CH4 production than FT procedure. The FT procedure had lower final asymptotic gH2 volume (P=0.01) and fractional rate of gH2 formation (P=0.002), and higher propionate production (P=0.05), in comparison with FP procedure. The higher propionate production under FT procedure indicated that more H2 was entering to VFA production, leading to less gH2 released into the headspace of bottle. In summary, the rate of gas, CH4 and gH2 generation could be different between VP and VT procedure, and higher venting headspace pressure caused by FT procedure altered kinetics of gas, CH4 and gH2 volume, substrate degradation and VFA production.