Oyster mushroom (Pleurotus ostreatus) spent substrate replaced 10–20% of corn silage and improved dry matter digestibility from 41% to 46–48%. The 10% mushroom-waste diet cut methane from 65 to 14 mg/g dry matter, about a 78% reduction, while 20% inclusion pushed fiber digestibility highest at 65%. This turns mushroom farm leftovers into a practical livestock-feed tool for better digestion, lower emissions, and less agricultural waste.