In vitro rumen gas and methane production of grass silages differing in plant maturity and nitrogen fertilisation, compared to in vivo enteric methane production

The potential of an in vitro gas production (GP) system to predict the in vivo enteric methane (CH4) production for various ryegrass-based silages was evaluated, using adapted rumen fluid from cows. Rumen fluid from 12 lactating rumen-cannulated Holstein-Friesian cows were used for in vitro incubations and compared with in vivo CH4 production data derived from the same cows fed the same grass silages. The cows consumed a total mixed ration consisting of six different grass silages and concentrate at an 80:20 ratio on a dry matter (DM) basis. The grass silages differed in plant maturity at harvest (28, 41 and 62 days of regrowth) and N fertilisation (65 and 150 kg of N/ha). Rumen fluid from cows consuming each of the six grass silages was used to determine the in vitro organic matter (OM) fermentation and in vitro CH4 synthesis, using an automated GP technique. In vitro GP decreased with increasing maturity of the grass. In vitro CH4 production, expressed either in ml/g of OM, in ml/g of degraded OM (DOM) or as a% of the total GP, increased with increased N fertilisation (P < 0.05). Maturity of grass at harvest did not affect the CH4 synthesis expressed in ml/g of DOM and CH4 expressed as% of the total gas, whereas N fertilisation increased the in vitro CH4 synthesis, expressed in any unit. The in vitro data correlated poorly with the in vivo data. Across the six grass silages tested, the in vitro CH4 production, expressed in ml/g of OM after 8, 12, 24, and 72 h of incubation did not correlate with the in vivo enteric CH4 production, expressed in g/kg of DM intake (R2 = 0.01-0.08). Stepwise multiple regression showed a weak, but positive correlation between the observed in vivo CH4 synthesis, expressed in g/kg FPCM and the predicted CH4 per kg FPCM, using the amount of in vitro organic matter degraded (R2 = 0.40; P = 0.036). In vitro gas and CH4 parameters did not improve the accuracy of the prediction of the in vivo CH4 data.