Manipulating rumen fermentation and methanogenesis using an essential oil and monensin in beef cattle fed a tropical grass hay

• CRINA does not significantly reduce enteric methanogenesis.
• Steers given monensin at a rate of 250 mg/d have DM intakes reduced by 18%.
• Methyl coenzyme-M reductase clone libraries were generated from rumen microbial DNA.
• Correlated decline in methanogens was out of phase with methane production.

The objective of this study was to determine if a specific blend of essential oils (CRINA® Ruminants) compared to monensin could reduce enteric methane production in beef cattle fed medium to low quality Rhodes grass (Chloris gayana) hay. Five Brahman steers [mean live weight (LW); 226 kg] were allocated to one of five groups: control (no additive), CRINA1 (CRINA 1 g/d), CRINA2 (CRINA 2 g/d), Mon1 (monensin 60 mg/d) and Mon2 (monensin 250 mg/d) as a 5 × 5 Latin square. Individual LW, dry matter (DM, kg/d) intake, rumen pH, fermentation patterns, and ruminal fungal colonisation was measured. Methyl coenzyme-M reductase (mcrA) clone libraries (methanogen diversity) were generated from microbial DNA extracted from the rumen. Total methane production (g/d) was measured over 24 h using open circuit respiration chambers. The DM intake for animals given CRINA at either dose rate was not different (P>0.05) to the control (5.4 kg/d). However, Mon2 (P<0.05) reduced DM intake by 18%, compared with the control with no effect on rumen pH or total VFA production. CRINA significantly increased butyrate and iso-valerate concentrations compared with the control. Mon2 also reduced acetate:propionate compared with CRINA and the control. Based on sporangia counts from rumen fluid collected throughout the experimental period a reduction in fungal colonisation was observed for both monensin and CRINA treatments.The use of Mon1 or CRINA did not affect methane production. Mean methane production was reduced to 10.2 g/kg DM intake for Mon2 treated animals, compared with the control group (14.6 g/d DM intake), but this was also associated with lower DM intakes. A shift in methanogen diversity for monensin treated animals was due to a decrease in Methanomicrobium genus and concurrent increase in Methanobrevibacter genus. The specific blend of essential oils used in this study had no direct effect on methane emissions; however the potential to manipulate rumen fungi with CRINA and/or monensin and the relationship with methanogens may be a novel strategy to indirectly reduce enteric methanogenesis.