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Abstract Although plant derivatives have shown promise in reducing enteric methane (CH4) emissions from ruminants in short-term studies, investigation on possible rumen microbial adaptation to these compounds is still limited. The objective of this study was to assess the possibility of mixed rumen microbial adaptation to antimethanogenic plant derivatives over relatively long-term in vitro incubation. Treatments were: garlic oil, carvacrol and thymol, each at a final concentration of 300 mg/l. A control, containing no additive, was included. The fermentations were done using the consecutive batch culture system with two serial subculture transfers at three-day intervals. The substrate was a mixture of forage and concentrate in a ratio of 60:40. Incubations were conducted in triplicate with rumen fermentation parameters being determined at days 3, 6 and 9. Indications of rumen microbial adaptation were observed in carvacrol and thymol as CH4 significantly increased from almost complete inhibition at day 3 to almost 5 % of total gas at day 9. This was closely associated with a rise in methanogenic archaea populations from being significantly lower (p = 0.0026) at day 3 to being numerically higher (p = 0.0632) than the control by day 9. Garlic oil consistently suppressed (p < 0.0001) CH4. Carvacrol and thymol suppressed overall fermentation, resulting in lower (p < 0.0001) VFA concentrations and hence higher culture pH (p < 0.0001). Garlic oil did not differ from the control on VFA concentrations (p > 0.05) except for butyrate (p = 0.04). Treatments had no overall effect on total bacteria (p = 0.235) and protozoa numbers (p = 0.835). The effects of carvacrol and thymol on fermentation parameters suggested that these supplements lower CH4 indirectly by suppressing overall fermentation whereas garlic oil selectively inhibits methanogens directly. We concluded that rumen microbes adapted to carvacrol and thymol. These supplements need to be tested in vivo to see if similar effects will be observed.

Assessment of Rumen Microbial Adaptation to Garlic Oil, Carvacrol and Thymol Using the Consecutive Batch Culture System