Objective. Determine the population density of cellulolytic bacteria, Butyrivibrio fibrisolvens and the concentration of vaccenic acid, by supplementing diets consisting of kikuyu grass (Pennisetum clandestinum Hoechst. Ex Chiov.) as base ingredient, together with cassava flour and biomass (effluent from ethanol production) in rumen simulator-Rusitec. Materials and methods. Four treatments (T) were evaluated, these were composed as: T1/Control 1: 100% kikuyu grass with a total protein intake of 23.9%, T2: a mixture of 70% kikuyu grass, 20% biomass and 10% cassava flour with a total protein intake of 19.4%; T3/Control 2: 100% kikuyu grass, with a 17.8% protein intake and T4: 70% kikuyu grass, 20% biomass and 10% cassava flour with a 15.3% protein intake. One and two-way variance analysis was made and the Pearson correlation coefficient was determined.Results. An increase was observed in the population density of viable cellulolytic bacteria (CFU/ml) and B. fibrisolvens statistically significant (p<0.005) with treatment T2, in contrast to T1, T3 and T4 treatments. In addition, there was a significant increase in the concentration of vaccenic acid (mg/L) in the ruminal content in Rusitec with the same treatment (T2). Conclusions. Results obtained in this ruminal simulation study are evidence to the benefits of kikuyu grass together with cassava flour and biomass diet implementation on the growth of ruminal cellulolytic and B. fibrisolvens bacteria, as well as on the production of vaccenic acid. The study also suggests the nutritional potential that such supplements could provide to grazing bovine feeding. RESUMENObjetivo. Determinar la densidad poblacional de bacterias celulolíticas, Butyrivibrio fibrisolvens y la concentración de ácido vaccénico, al suplementar dietas forrajeras de pasto kikuyo con harina de yuca y biomasa (resultante de la producción de etanol), en el simulador de rumen-Rusitec. Materiales 4948 REVISTA MVZ CÓRDOBA • Volumen 20(Supl) Diciembre 2015 y métodos. Se evaluaron cuatro tratamientos (T): T1/Control 1: 100% pasto kikuyo con un aporte total de 23.9% de proteína, T2: 70% pasto kikuyo, 20% biomasa y 10% harina de yuca con un aporte total de 19.4% proteína, T3/Control 2: 100% pasto kikuyo con un aporte total de 17.8% de proteína y T4: 70% pasto kikuyo, 20% biomasa y 10% harina de yuca con un aporte total de 15.3% de proteína. Se realizó un análisis de varianza (ANOVA) de una y dos vías y se determinó el coeficiente de correlación de Pearson. Resultados. Se observó un incremento en la densidad poblacional de bacterias celulolíticas viables (UFC/ml) y de B. fibrisolvens estadísticamente significativas (p<0.005) con el tratamiento T2, en comparación con los tratamientos T1, T3 y T4. Adicionalmente, se detectó un aumento significativo en la concentración de ácido vaccénico (mg/L) en el contenido ruminal del Rusitec con el mismo tratamiento (T2). Conclusiones. Los resultados obtenidos en este estudio de simulación ruminal indican que la suplementación del pasto kikuyo con harina de yuca y ...