FERMENTACIÓN RUMINAL Y PRODUCCIÓN DE METANO IN VITRO DE DIETAS PARA OVINOS CON INCLUSIÓN DE TANINOS CONDENSADOS Y NIVELES CRECIENTES DE ACEITE DE ORÉGANO (Lippia graveolens)

Background. Ruminants have been related to greenhouse gas production. Therefore, it has been necessary to find alternatives by using natural additives to reduce their production by modulation of rumen fermentation. Objective. It was to evaluate the effect of the addition of condensed tannins (CT) and different levels of essential oregano oil (Lippia graveolens) (EOO) in sheep's diets on in vitro fermentation products such as total gas production, methane (CH₄), carbon dioxide (CO₂), pH, ammoniacal nitrogen (N-NH₃), and volatile fatty acids (VFA). Methodology. There were evaluated six experimental diets: the control diet with 4% of CT kg/DM, and 0.02, 0.04, 0.06, 0.08 or 1% de EOO kg/DM, respectively. Total gas production, CH₄, CO₂, pH, N-NH₃, and AGV were measured at different in vitro incubation times. Results. Total gas production was not affected (P > 0.05) due to treatment or the interaction treatment*time. An exponential increment was observed (P < 0.05) according to incubation time. CH₄ and CO₂ production increased at 48 h. The lowest emissions of these gases were recorded when 0.04, 0.06, 0.08, and 1% of EOO were added to the diet. Nevertheless, no differences were found (P > 0.05) due to treatment or the interaction treatment*time. Acetic and butyric acid concentrations were affected due to incubation time and the interaction treatment*time; however, there was no effect due to treatment (P > 0.05). Propionic acid production and pH were not affected (P > 0.05) due to treatment, time, or interaction treatment*time. N-NH₃ production increased through incubation time (P < 0.05), with the highest concentration at 48 h for all treatments, without differences (P > 0.05) due to treatment or interaction treatment*time. Implications. These results suggest that the inclusion levels of TC and EOO in the diet were not enough to affect rumen fermentation or methane production. Therefore, it is necessary to evaluate higher levels of these compounds in sheep's diets. Conclusion. The addition of CT or the different levels of EOO included in sheep's diets evaluated in this study did not affect total gas production, CH₄, CO₂, pH, concentration of N-NH₃, AGV, and pH.