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Exogenous fibrolytic enzymes (EFE) can increase ruminal digestion of neutral detergent fiber (NDF) and improve its fermentation for cattle. Twenty-four multiparous Holstein cows (33.72 ± 7.63 kg milk/d and 176 ± 82.27 days in milk-DIM, at the start of the experiment) were used in a replicated 4 × 4 Latin square experimental design. Increasing doses of a commercial product was used to evaluate the effect of xylanase activity (100 units per gram of product) on intake and total-tract digestion of nutrients, sorting index, chewing time, milk yield and composition, N use, and ruminal fermentation. Treatments consisted of 0, 8, 16 or 24 g/d per cow of EFE product mixed into the concentrate. Corn silage was used as forage source. The basal diet had a forage-to-concentrate ratio of 50:50 (dry matter basis). Even though EFE supplementation had a positive linear effect on dry matter and NDF intake, it had no impact on total-tract digestion of nutrients. Moreover, this enzyme promoted a linear increase of the intake of feed with larger particle size (>19 mm) and quadratically affected rumination and chewing activity (hours), reaching the highest values at intermediate doses (8 g/d and 16 g/d). Treatments had no effect on milk yield and composition; however, the N use efficiency was linearly decreased by EFE, reducing thus the ability to synthesize milk and quadratically decreasing N retention. No treatment effect was found on ruminal pH, whereas a negative quadratic effect on ruminal NH 3-N concentration was significant. Regarding ruminal parameters, this enzyme supplementation provided linear increased in acetate, propionate, butyrate, and total short-chain fatty acids synthesis. As result, EFE supplementation improved DM and NDF intake, increasing the time spent chewing and ruminating, leading to a greater total short-chain fatty acids production in rumen. Nonetheless, EFE did not improve milk yield and composition of mid-lactating dairy cows.

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