From Spent Mushroom Substrate to Biofeed: Source Heterogeneity, Dual Cell-Wall Barriers, and Targeted Bioprocessing

Background: Industrial mushroom cultivation generates large, concentrated quantities of spent mushroom substrate (SMS), but SMS is not a uniform feed ingredient. Mushroom species, the original substrate, the cultivation cycle, and storage history all influence its nutrient content, safety, digestibility, and sensory properties. Purpose: This review examines SMS as a biofeed resource and asks how different sources can be made safer, more digestible, and more acceptable to animals. Methods: This critical narrative review integrates source-characterisation studies, mechanistic evidence, and animal-feeding trials, giving greater weight to studies that report SMS source, treatment, and dietary inclusion. Results: SMS retains lignocellulose, fungal biomass, protein, minerals, and bioactive metabolites. Its feeding value is limited by two distinct structural materials: residual plant cell walls and newly formed fungal walls containing chitin, beta-glucans, and mannoproteins. Mushroom volatiles, bitter compounds, high water-holding capacity, ash, and storage spoilage may further reduce intake. Feeding studies support moderate roughage replacement in ruminants and rabbits and low-dose functional use in pigs, poultry, and tilapia, but the response depends strongly on source, treatment, dose, and diet formulation. Conclusions: A more practical approach is to identify what limits each batch and then decide whether it can be used directly, preserved, treated with enzymes, fermented with selected cultures, or kept out of feed. A provisional four-grade research hypothesis is proposed to organise feed suitability and treatment requirements; it is intended for future validation rather than as an established feed standard. The practical question is which material can be used, for which animal, at what inclusion level, and after what treatment.