Fermenting leftover oyster mushroom (Pleurotus ostreatus) substrates created a low-cost poultry supplement that boosted growth and feed efficiency. Broilers given 30 mL per liter of the fermented mix showed a 16 percent improvement in feed conversion, while the highest weight gain came from the 40 mL dose. This circular approach turns farm waste into a functional feed additive that supports small farmers and reduces reliance on expensive commercial feeds.