Effect of easily fermentable carbohydrates on in vitro rumen methane emission at different pH

An automated batch system consisting of 45 bottles equipped with gas pressure sensors, was used to investigate the effects of addition of easily fermentable carbohydrates to grass hay on the kinetics of gas production (GP), feed degradability parameters, ammonia content and methane production, at three pH levels: 6.4, 6.6 and 6.8.
Four diets were compared: 100% grass hay (GH); 50% grass hay and 50% starch (GHS); 50% grass hay and 50% sucrose (GHSU); 50% grass hay, 25% starch and 25% sucrose (GHSSU). Each diet (0.50 g total) was incubated in triplicate for each pH level, with 15 ml of rumen fluid and 45 ml of medium.
A preliminary incubation for 96 h was performed in order to fit the GP kinetics. For other runs the fermentations were stopped at 10, 24 and 48 h. Data of GP at 96 h were fitted with a model the which parameters are the asymptotic GP (A), the time at which half of A is produced (t½) and the sharpness of the curve (c). NDF degradability at 48h, pH and ammonia content at the various times were measured on the residual fluids of fermentation. The true organic matter degradability (TOMd) was computed from OM content of feed and from NDF degradability.
Methane produced at various times was collected in a plastic bag connected to each bottle and measured by gas chromatography. Final pH was affected by initial pH (P<0.05) and by diets, being lower for GHS, GHSU and GHSSU compared to GH (P<0.05). With respect to the initial value at 0 h, the largest variations of ammonia concentration were observed at 24 h in all the diets except for GHSU. Ammonia concentrations were affected by the initial pH level both at 10 and 24 h (P<0.05). Gas production was always significantly lower for GH compared to the other 3 diets (P<0.05). Gas production of GHSU was significantly higher (P<0.05) compared to that of GHS and GHSSU after 10 h of fermentation only. GH showed the highest NDF degradability, whereas TOMd resulted higher for GHS, GHSU and GHSSU (P<0.05). Both at 10, 24 and 48 h, methane production (mmol/g OM) resulted higher for GHS, GHSU and GHSSU compared to GH (P<0.05), and increased at increasing pH level in all the diets. When expressed as proportion of total gas, methane production at 10 h resulted higher for GHS, GHSU and GHSSU compared to GH (P<0.05), whereas no differences were observed at 24 and 48 h (P>0.05). In conclusion, kinetics of GP, degradability, ammonia content and methane production were affected by diet and pH level, whereas any substantial difference was observed between the effect of starch and sucrose.