Co-ensiling corn silage with oilseeds and spent oyster mushroom substrate (Pleurotus ostreatus) improved feed quality while lowering methane output in an in-vitro fermentation model. Across increasing inclusion levels, crude protein and beneficial lipids rose, structural fibers decreased, and methane declined in a clear linear pattern, while the combination ensiled for 42 days (the 70:15:15 mix) delivered the best balance of digestibility, microbial mass, and reduced emissions.