The objective of the present study was to investigate the effect of different doses of some 7 natural semi-arid medicinal plants’ essential oils on in vitro ruminal digestion and fermentation patterns of a mid-forage (alfalfa hay: concentrates, 1:1) diet. Treatments consisted of either basal diet alone (control) or added with 35, 70, 140 or 280 μl L-1 of coriander, oregano, caraway, cumin, cinnamon, pistachio hull and thyme essential oils, incubated for 24 hours at 38.7ºC. The essential oils of cinnamon and pistachio applied as 280 μl L-1 and thyme applied at 140 and 280 μl L-1 caused a decrease in DM disappearance as compared with control. Thyme and pistachio essential oils (used at 280 μl L-1) resulted in a decrease of NDF disappearance, while caraway (70 μl L-1) and cumin (140 μl L-1) resulted in an increase in it (14.8% and +18.2%, respectively). Relative to control, the essential oils applied, did not significantly affect the medium N-NH3 concentration (except thyme at 140 and 280 μl L-1), pH (except thyme and cumin essential oils, 6.41 and 6.22 vs. 6.3, respectively), gas produced (except thyme at 280 μl L-1) and Feed Fermentation Efficiency (FFE). Relative to control, addition of all the essential oils resulted in a decrease of CP disappearance and CH4 (except for cumin) production as Mm-1 incubated. Findings revealed that these essential oils may allow manipulation of rumen microbial fermentation.

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