Adding oyster mushroom (Pleurotus spp.) spent substrate to feed increased digestibility by 29–37% while keeping fermentation stable, meaning animals extracted more usable energy from the same feed. Methane emissions dropped sharply by 35–60%, alongside 17–49% lower CO₂, driven by shifts toward more efficient fermentation pathways so agricultural waste became a high-impact feed upgrade that improved efficiency and meaningfully reduced livestock emissions.