Improving fibrous feeds utilization ensures low consumption of high-starch grains and may reduce various pathologies. Fibrolytic enzymes (FE) like cellulase and xylanase, and yeast addition can improve fibrous feeds utilization. Therefore, the current study aimed to study the effect of FE (Exp. 1) and yeast (Exp. 2) on the utilization of diet with 11.2% CP and 51.1% NDF. Thirty-two mares of Quarter Horse (450-500 kg BW) were used in a complete randomize design for 15 d, with 10 d for adaptation and 5 d for samples collection. For Exp. 1, mares (n=16) were distributed onto 4 treatments to be fed on the basal diet without FE (control), or plus cellulase at 10 mL/mare/d (CELL), plus xylanase at 10 mL/mare/d (XYL), or plus mixture cellulase and xylanase at 5 ml of each enzyme/mare/d (CX). For Exp. 2, mares (n=16) were distributed onto 4 treatments to be fed on the basal diet without yeast (control), or fed the control diets plus Procreatin 7 (1.5 × 10¹⁰ cfu/g S. cerevisiae) at 15 g/mare/d (P7), plus Biocell F53 (2 × 10¹⁰ cfu/g S. cerevisiae) at 11 g/mare/d (F53), or plus Biosaf SC47 (1.5 × 10¹⁰ cfu/g S. cerevisiae) at 15 g/mare/d (SC47); yeast products were in powder form. Both of enzyme and yeast doses were mix with the 1 kg of concentrate diet at 0400 h. Mares were fed the concentrates twice daily at 0400 and 1600 h, while the forage of oat straw was offered ad libitum at 0500 and 1700 h. Acid insoluble ash concentrations in feed and fecal samples were used for digestibility determination. No effects for FE and yeast were obtained in blood alanine transaminase, aspartate aminotransferase, urea, creatinine, total protein and glucose. Addition of CELL, XYL and CX increased (P = 0.001) nutrient intakes from oat straw versus control. Moreover, CELL, XYL and CX increased (P < 0.05) digestibilities of DM, OM, NDF and ADF. Enzymes decreased (P < 0.05) concentration of fecal coliform. In the contrary, F53 increased nutrients intake of oat and nutrients digestibility (P < 0.05) without difference compared with other treatments. Yeast had no effect on fecal coliform concentration. It could be concluded that addition of FE at 10 mL/mare/d or addition of Biocell F53 at 11 g/mare/d improved feed intake and nutrients digestibility without affecting mare’s blood parameters.