Researchers tested spent oyster mushroom (Pleurotus ostreatus) substrate as a partial feed replacement for goats and sheep, evaluating both digestion and fermentation results. When used at 20% of the total diet and fortified with urea and molasses, the substrate maintained nutrient use and fermentation similar to standard feed. Higher inclusion levels (35–50%) reduced protein digestibility and gas production, showing that moderate use of mushroom waste can sustainably supplement animal feed without harming performance.