Methane yields from Brahman cattle fed tropical grasses and legumes

In the national greenhouse inventory, methane emissions from the Australian tropical beef herd are derived from cattle fed two diets. In the experiments reported here, methane production was measured by open-circuit gas exchange from 13 Brahman cattle offered 22 diets from combinations of five tropical grass species and five legumes, with a minimum of three steers per diet. All diets were offered daily ad libitum, with the exception of three legume diets fed without grass and leucaena (Leucaena leucocephala) mixed with grass, which were offered at 15 g dry matter per kg liveweight. Diets were fed as long-chopped dried hay, with the exception of leucaena, which was harvested and fed within 2 days. For the data from cattle fed diets of grass and grass mixed with legumes, methane production could be predicted as 19.6 g/kg forage dry matter intake (residual standard deviation 12.3). Observed methane yields were not predictable from a stoichiometry, which used volatile fatty acid proportions in rumen fluid. Mean methane emission rates across all diets were equivalent to 8.6–13.4% of digestible energy intake, and 5.0–7.2% of gross energy intake. The latter values are comparable to IPCC (2006) recommendations (5.5–7.5%) for large ruminants fed low-quality crop residues and by-products. Methane yields per unit of ingested dry matter or digested organic matter were variable across diets but were related to digestibility and contents of fibre and protein. These results constitute a significant downward revision of the methane emissions attributable to the northern Australian beef herd grazing tropical pastures.