Methane conversion rate of bulls fattened on grass or maize silage as compared with the IPCC default values, and the long-term methane mitigation efficiency of adding acacia tannin, garlic, maca and lupine

Abating enteric methane in ruminants by feeding is considered promising, but long-term assessments are lacking. Thirty-six growing bulls were allocated to six treatments and received either grass or maize silage and concentrate. The maize silage diet was also supplemented with acacia tannin extract, garlic bulbs, maca hypocotyls, or lupine seeds. At ages of 5, 9 and 11 months, enteric methane was measured in open circuit respiratory chambers along with feed intake and excretion. Ruminal fluid was collected at slaughter. Slurry-derived methane was determined while stored at 14 °C and 27 °C for 15 weeks. Exchanging maize for grass silage increased enteric methane only on one occasion. Total system methane emission was similar between grass- and maize-silage fed bulls. Tannin decreased methane by up to 36% and, most importantly, this suppression persisted throughout the entire experiment. Tannins did not alter slurry-derived methane. Enteric methane conversion rate and methane conversion factors in slurry were always below the IPCC default values. Grass silage increased fibre digestibility and numbers of total and cellulolytic Bacteria. The study demonstrated a long-term efficacy of acacia tannin in mitigating methane. Further, the results suggest that IPCC appears to overestimate methane from growing cattle fattened on typical European diets.

► We measured methane emissions from bulls fattened on grass- and maize-based diets, and from their slurry.
► For all diets and for the entire fattening period, Ym was lower than the IPCC default value of 6.5%.
► Methane emissions were substantially and permanently suppressed by supplementation with acacia tannins.
► Enteric methane emissions from grass-silage fed bulls were only slightly higher than those from maize silage.
► Enteric and slurry-derived methane emissions were compensatory when opposing grass- and maize-silage fed bulls.