This study was performed to evaluate the effects of pre-treating barley silage-based diet with a fermentation extract derived from Trichoderma reesei (FETR, mixture of xylanase and cellulase; AB Vista, Marlborough, UK) on lactation performance, omasal nutrient flow and digestibility, rumen fermentation characteristics, and rumen pH profile in Holstein dairy cows during early lactation. The dairy trial was conducted using 9 Holstein dairy cows (averaging 46 ± 24 DIM and 697 ± 69 kg BW, 6 cows were fitted with a rumen cannula and 3 were non-cannulated). Two groups of cows were randomly assigned to each of the dietary treatments in a crossover design: control (without FETR supplementation) and supplemented (with 0.75 mL of FETR/kg DM of diet based on our previous study). The pre-treatment was applied to barley silage-based diet one hour before feeding by mixing FETR with the diet. The experiment consisted of 2 consecutive experimental periods of 27 d each. Within each period, the first 18 d were used for adaptation to the treatments, followed by 3 d of milk sampling, 3 d for the collection of the ruminal, omasal, and fecal samples, and the last 3 d for measuring the pH profile using indwelling pH probes. The application of FETR tended to decrease the DM intake compared with control (32.8 vs. 33.7; P = 0.08). There was a significant decrease (P = 0.05) in milk urea nitrogen by 7% and a numerical decrease (P = 0.16) in ruminal ammonia concentration by 14% as a consequence of adding FETR to the diet. In conclusion, dairy cows fed FETR pre-treated barley silage-based diet display enhanced milk yield with less feed. The positive effect of adding FETR could benefit the dairy industry in Western Canada where barley silage-based diets are common