ABSTRACT The objective of this study was to evaluate in vitro effects on ruminal fermentation of by-product from the production of edible mushroom Pleurotus ostreatus. The following by-products were tested: (I) non-colonized sugarcane bagasse (control); (II) sugarcane bagasse from non-enriched mushrooms; and (III) sugarcane bagasse from selenium-enriched mushrooms. A semi-automatic in vitro system was used. The pH and neutral detergent fiber (NDF) residue were determined at 0, 3, 6, 9, 12, 18, 24, 48, 72, and 96 h of incubation. […]