The objective of the present study was to investigate the in vitro effect of some semi-arid medicinal plant essential oils on ruminal methane emission and feed fermentation efficiency (FFE). A mixed diet of alfalfa hay: concentrate (50:50, based on DM) was provided. It was then ground to pass through a 1- mm screen. Approximately, 500 mg of the diet alone (as control) or plus essential oil of cinnamon, dill, oregano or peppermint (100 µL/ g DM) were placed into a 125 mL serum bottle (n = 6) containing 50 mL of buffered-rumen fluid (2:1). Rumen fluid was obtained from 3 ruminally fistulated sheep (49.5 ± 2.5 kg, body weight), before the morning feeding. Bottles were placed in shaking water bath for 24 h at 38.5°C. Gas produced of each bottle was recorded using a pressure transducer and then sampled. Gas pres- sure was converted into volume using an experimentally calibrated curve. Then, bottle content was filtered (42 μm) and residual was dried (60°C, 48 h) to determine dry matter disappearance (DMD). Data were statistically analyzed using SAS (V. 9/1) and Dunnett’s test was used to compare the means (P < 0.05). Feed fermentation efficiency was estimated as FFE = DMD (g/kg)/cumulative gas (ml) produced at 24 h. Methane content of the produced gas was determined using gas chromatography procedure. Results indicated that these essential oils caused a significant (P < 0.05) decrease in methane and total gas produced over 24 incubation compared with those of the control (Table 1). The essential oil of Dill enhanced FFE (P < 0.05) compared with that of the control. Present results demonstrated a positive effect of the essential oils on ruminal fermentation pattern.

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