The influence of head-space and inoculum dilution on in vitro ruminal methane measurements

The in vitro gas production technique (GPT) has demonstrated the ability to predict methane (CH4) emission from ruminants. However, modifications in substrate size (SS), inoculum dilution (ID) and head-space (HS) affect fermentation. This work aimed to evaluate the influence of HS and ID on in vitro CH4 measurement. GPT was carried out in 160-ml bottles with two different SS-to-ID ratios, named HS60 (60 ml HS; 20% ID) and HS85 (85 ml HS; 33% ID). Four plants, Myracrodruon urundeuva (Myr), Mimosa tenuiflora (Mim), Sorghum bicolor grains (Sorg) and Cynodon sp. (Cyn) were used, applying sheep rumen digesta as inoculum. HS pressure was measured over 72 h. Total GP and kinetics as well as CH4 were significantly higher for HS85 (p < 0.01). GP and CH4 were linearly correlated (r = 0.81; p < 0.01) in HS85, while HS60 only maintained the linearity after 24 h incubation. HS and ID in HS85 seemed to offer better environment for in vitro CH4 measurement, initially.