The methanogenic potential and C-isotope fractionation of different diet types represented by either C3 or C4 plants as evaluated in vitro and in dairy cows

The methanogenic potential of two major feed production systems, grassland and arable land, was tested in vitro and in vivo. Diets were differentiated into C-3 (grass, cereals) and C-4 plants (maize) to determine whether C-isotope fractionation during methane formation differed between the C-3 and C-4 plant based diets. In the in vitro experiment, four diets consisting of hay, maize, wheat (the latter two straw and grain) and a 1 : 1 mixture of the maize and wheat diet were investigated using the Rusitec system (n = 4 per diet). In the in vivo experiment with 12 lactating cows, diets were basically similar, but barley was used instead of wheat and no maize-cereal mixture was tested (n = 4 per diet). Diets were always iso-energetic and iso-nitrogenous. Methane was either measured by gas chromatography from complete daily collections (in vitro) or by putting cows into respiratory chambers. In vitro, the methanogenic potential of the diets was similar. Methane related to apparently degraded neutral detergent fibre (NDF) was 50% lower in the hay than in the wheat diet, while the other diets ranged in between. The largest C-isotope fractionation between fermentation gases (alpha CO2 - CH4) was found in the wheat diet. In vivo, methane formation was highest with the barley diet followed by the hay and the maize diet (580, 461 and 453 L/day). Relating methane emission to dry matter intake reduced differences among diets (average 34 L/kg). On average, 118 L of methane were produced per kg digested NDF. Methane conversion rate was higher than the IPCC default value of 6.5% given for diets with less than 90% concentrate. No differences in the C-isotope fractionation were found in vivo. It was shown that forage-only diets, containing higher levels of fermentable fibre, do not necessarily have a higher methanogenic potential than mixed forage-concentrate diets.