The aim of the research was to evaluate the effect of three Eucalyptus globulus extracts rich in phenolic compounds, especially flavonoids, on rumen fermentation, methane (CH4) production, organic matter degradability and protozoa population using an in vitro gas production technique. Four concentrations (0, 50, 75 and 100 mg) of three Eucalyptus extracts (ethyl acetate, n-butanol and aqueous) were added to a diet of ruminants (forage: concentrate ratio 60:40) and incubated at 39°C under anaerobiosis with buffered rumen fluid. After 24 h, the fermentation fluid was analysed for ammonia-N and volatile fatty acids (VFA). Organic matter degradability (OMD) and protozoa were also determined; in vitro gas production was also recorded and CH4 concentration was measured. Compared to the control, CH4 production was significantly lower for ethyl acetate extract (P<0.05), but higher for n-butanol and aqueous extracts. Production of ammonia- N was lower in all Eucalyptus extracts (P<0.05). Propionate production (P<0.05) increased for ethyl acetate and n-butanol extracts, whereas no effect was registered for VFA, for all Eucalyptus extracts. Ethyl acetate extract decreased in vitro OMD (P<0.05), whereas n-butanol and aqueous extracts were comparable to the control. Protozoa population decreased (P<0.05) for all extracts in comparison with the control. Eucalyptus ethyl acetate extract might be promising to be used as a potent anti-methanogenic additive. Moreover, the assessment of the right dosage seems to be important to decrease methane production, without reducing feed nutritional value.

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