The in vitro rumen methane output of perennial grass species and white clover varieties, and associative effects for their binary mixtures, evaluated using a batch-culture technique

Grassland swards containing white clover varieties (WCV) may result in lower enteric methane (CH4) output from grazing ruminants than swards of only perennial grass species (PGS) due to differences in their chemical composition and rumen fermentation dynamics. The objectives of the present study were to compare the chemical composition, in vitro rumen fermentation variables and CH4 output per unit of feed for a range of common PGS and WCV harvested in May from simulated grazing regimes, and to determine the effects of binary mixtures of the PGS and WCV on in vitro rumen fermentation variables and CH4 output, using a batch-culture technique. Four PGS (perennial ryegrass, cocksfoot, meadow fescue and timothy) and three WCV (Aran, Chieftain and Crusader) were incubated as sole substrates or as part of binary mixtures (PGS : WCV ratios of 0 : 1, 0.25 : 0.75, 0.5 : 0.5, 0.75 : 0.25 and 1 : 0) for 24 h at 39°C with buffered rumen fluid. All WCV had lower (P < 0.001) CH4 output per unit of apparent DM disappeared (aDMD; mean value across WCV (s.e.m.) of 27.0 (1.35) mL/g aDMD v. 36.1 (0.90) mL/g aDMD across PGS) during the in vitro rumen incubation than did all PGS. The WCV also had lower CH4 output per unit of total volatile fatty acids (corresponding values of 0.147 (0.0090) v. 0.199 (0.0073) mmol/mmol total volatile fatty acid output) and per unit of total gas produced (0.118 (0.0022) v. 0.153 (0.0024) mmol/mmol total gas produced) than did the PGS. In addition, Aran and Crusader had lower (P < 0.001) CH4 output per unit of feed DM incubated than did all PGS. There were synergistic associative effects (i.e. where the response was greater for the mixtures than the arithmetic calculation using the responses for PGS and WCV alone; P < 0.05) of mixing the PGS and WCV in binary combinations on all CH4 output variables.