Abstract: We evaluated the effects of oregano essential oil on the nutrient degradability, rumen fermentation, and CH₄ production of sheep in an in vitro fermentation system. The experiment had a single-factor design. Oregano essential oil (OEO) was added to the medium at four different levels: 0 (CON), 100 (EO₁), 400 (EO₂), 700 (EO₃), and 1000 mg·L^-1 (EO₄). Each group had four replicates. After fermentation for 48 h, nutrient degradability and fermentation parameters were measured. The total gas and CH₄ production were measured at 1, 3, 6, 9, 12, 18, 24, 36, and 48 h during fermentation. The main results were as follows: 1) Dry matter degradability (DMD), crude protein degradability (CPD), and neutral detergent fiber degradability (NDFD) were significantly increased in the EO₁ group (P<0.05), significantly decreased in the EO₄ group (P<0.05), and unaffected in the EO₂ group (P>0.05) compared with the CON group. The DMD was significantly higher in the EO₃ group than in the CON group (P<0.05). Addition of OEO did not affect the acid detergent fiber degradability (ADFD) (P>0.05); 2) Among all groups, the EO₁ group had the lowest pH but the highest concentrations of total volatile fatty acids (TVFA) and propionate. The acetate concentration was higher in the EO₁ group than in the EO₂, EO₃, and EO₄ groups (P=0.011), however, the percentage of acetate/TVFA was significantly lower in the EO₁ group than in the EO₃ and EO₄ groups (P<0.001), and the ratio of acetate/propionate was significantly lower in the EO₁ group than in the CON group (P<0.05). Compared with the CON group, all the EO groups showed significantly decreased concentrations of butyrate and NH₃-N, and lower butyrate/TVFA ratios (P<0.05); 3) The total gas production and CH₄ production after 36 h of fermentation decreased as the OEO dose increased. Compared with the CON group, the EO₁ group showed significantly decreased total gas production from 18 to 48 h of fermentation (P<0.05), and the EO₂, EO₃, and EO₄ groups showed significantly decreased total gas production and CH₄ production after 18 h of fermentation (P<0.05). Compared with the CON group, all EO groups showed significantly lower CH₄ production from 12 to 36 h of fermentation (P<0.05). In summary, supplementation with oregano essential oil can improve nutrient degradability, regulate rumen fermentation, and decrease total gas and CH₄ production; the optimal supplementation level is less than 100 mg·L^-1 in in vitro fermentation conditions.

Key words: oregano essential oil, in vitro fermentation, nutrient degradability, fermentation characteristics, CH₄ production