Se evaluó el efecto de un preparado de enzimas fibrolíticas exógenas (celulasas y xilanasas) en la degradabilidad in situ de la materia seca (DisMS), fibra detergente neutro (DFDNr) y fibra detergente ácido residual (DFDAr), en dietas altas o bajas en harina de nopal deshidratado. Se aplicaron concentraciones de 0, 1, 2 y 3 g de enzima por kilogramo de materia seca al inicio y 24 horas antes de la degradación in situ. Se determinó la concentración de ácidos grasos volátiles totales y de nitrógeno amoniacal a las 0, 3, 6, 9, 12 y 24 horas después de aplicarse la enzima. No se observaron efectos en DisMS, DFDNr y DFDAr; la aplicación al inicio de la degradación in situ mostró valores más altos que a 24 horas para DisMS y DFDNr, pero fue menor para DFDAr. No se observaron diferencias en las interacciones entre niveles de enzima, tipo de dieta y tiempo de pretratamiento. La aplicación de 1 y 3 g de enzima, en la dieta con bajo contenido de harina de nopal, tuvo efectos en el incremento de los ácidos grasos volátiles totales; para el nitrógeno amoniacal, los mejores resultados ocurrieron con 0 y 1 g de enzima.It was evaluated the effect of a fybrolitic enzyme product (cellulases and xylanases) on in situ digestibility of dry matter (DisMS), residual neutral detergent fiber (DFDNr) and acid detergent fiber (DFDAr), indehydrated ground prickly pear diets with a low or high level. Enzyme concentrations of 0, 1, 2, and 3 g kg-1 of dry matter applied at the beginning (0 hour) and 24 hours before starting in situ digestibility were used. Total volatile fatty acids and ammonia nitrogen were determined at: 0, 3, 6, 9, 12, and 24 hours after the enzyme application.There were no effects on DisMS, DFDNr, and DFDAr. Initial application of enzyme concentrations (0 hour) was higher than 24 hours for DisMS and DFDNr but lower for DFDAr. No differences were observed in interactionsamong enzyme level, diet and application time. Application of 1 and 3 g of the enzyme to the diet with the low level of prickly pear forage had effects on the increasing of volatile fatty acids; for ammonia nitrogen, the best results were obtained using 0 and 1 g of the enzyme.It was evaluated the effect of a fybrolitic enzyme product (cellulases and xylanases) on in situ digestibility of dry matter (DisMS), residual neutral detergent fiber (DFDNr) and acid detergent fiber (DFDAr), in dehydrated ground prickly pear diets with a low or high level. Enzyme concentrations of 0, 1, 2, and 3 g kg-1 of dry matter applied at the beginning (0 hour) and 24 hours before starting in situ digestibility were used. Total volatile fatty acids and ammonia nitrogen were determined at: 0, 3, 6, 9, 12, and 24 hours after the enzyme application. There were no effects on DisMS, DFDNr, and DFDAr. Initial application of enzyme concentrations (0 hour) was higher than 24 hours for DisMS and DFDNr but lower for DFDAr. No differences were observed in interactions among enzyme level, diet and application time. Application of 1 and 3 g of the enzyme to the diet with the low level of prickly pear forage had effects on the increasing of volatile fatty acids; for ammonia nitrogen, the best results were obtained using 0 and 1 g of the enzyme
