Circular Biomanufacturing of Non-Grain Feeds: Synergizing Microbial Fermentation and Insect Bioreactors for Animal Health

The escalating global "food-feed competition" and the drive for agricultural carbon neutrality have intensified the search for sustainable animal feed alternatives. Non-grain feed resources (NGFRs) — encompassing crop residues, agro-industrial co-products, and food waste — represent a vast nutrient reservoir. However, their direct application in monogastric and analogous aquatic animal diets is severely hindered by inherent physicochemical barriers, such as high lignocellulosic content, anti-nutritional factors (ANFs), and biosafety risks (pathogens and mycotoxins). To unlock the nutritional value of NGFRs, biological transformation has emerged as a highly efficient paradigm. This review systematically evaluates current bioconversion strategies, transitioning from standalone microbial fermentation and enzymatic hydrolysis to advanced insect-based bioconversion using the Black Soldier Fly (Hermetia illucens, BSF). Specifically, we highlight the disruptive innovation of the "multi-stage bioconversion system" multi-stage bioconversion networks. This synergistic approach not only accelerates fiber depolymerization and ensures profound detoxification but also maximizes nitrogen recovery and biomass accumulation. Furthermore, by integrating recent multi-omics data — spanning 16S rRNA sequencing, transcriptomics, and metabolomics — this paper elucidates the underlying mechanisms by which these upcycled resources modulate the "feed-gut-muscle" axis. Bioconverted NGFRs actively remodel the host's intestinal microecology, leverage AMPs for pathogen competitive exclusion, and significantly enhance terminal meat quality through improved antioxidant capacity and optimized lipid deposition. Despite scale-up challenges, current techno-economic and life cycle assessments confirm the long-term profitability of these integrated systems. Ultimately, the multi-stage bioconversion of NGFRs stands as a vital engine for achieving sustainable bio-manufacturing, ensuring global protein security, and fostering a circular bioeconomy.