The effect of essential oils (EO) of medical plant seeds and spices on rumen microbial fermentation of al-falfa hay, sugar beet pulp and barley grain (as substrate) were evaluated under in vitro conditions. In vitro incubations were carried out using the gas production method with glass syringes. Treatments were as fol-lows; a control (no additive), monensin, EO of cinnamon, black pepper seed, cumin seed, fennel seed and garlic oil (200 and 400 μL/g DM). Monensin was used as a positive control in the medium at 5 μmol. Data on gas production were fitted using an exponential equation. Results showed that compared to control treatments, monensin had a significant increase on gas production (P<0.05), and cumin seed EO decreased gas production of the feed samples (200 and 400 μL). The effects of treatments on in vitro ruminal fermen-tation characteristics were tested using an in vitro culture inoculated by mixed rumen microbes. The test treatments were as follows; control (no additive), EO of cinnamon, black pepper seed, cumin seed and fen-nel seed. Evaluations were made for medium pH, ammonia nitrogen concentration and dry matter disap-pearance after a 48 h incubation period. To evaluate the effect of EO on in vitro ruminal fungi populations, a sample was taken from the medium after a 120 h incubation period and fungal population was determina-tion by real-time polymerase chain reaction. Compared to the control treatment, cumin and cinnamon addi-tions resulted in a significant decrease (P<0.05) on disappearance of dry matter in the feed samples. In the present study, additions of all tested EO to alfalfa hay treatment showed a significant increase in the final pH of the culture (P<0.05). However, cinnamon addition resulted in a significant decrease in medium am-monia nitrogen concentration for each of the feed samples (P<0.05). Results of the present study also dem-onstrate that addition all of the tested EO to alfalfa hay had a significantly decrease on in vitro ruminal fun-gal population (P<0.05).

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