Systematic Review on Alternative Fiber/Protein Sources for Monogastric Livestock: SDG-Aligned Valorization of By-Products

Driven by escalating feed costs and Sustainable Development Goal (SDG) 12 targets for responsible resource use (Targets 12.2, 12.5) and waste reduction (Target 12.3), this systematic review synthesizes evidence on valorizing underutilized by-products—cattle rumen scrapings (CRES), epithelial scrapings, soybean shaft/hulls, and "aking rice"—as alternative protein and fiber sources in monogastric livestock. Through analysis of 85 peer-reviewed studies (2000–2023) sourced from Scopus, Web of Science, PubMed, AGRICOLA, and CAB Abstracts, we evaluated impacts on growth performance, nutrient digestibility, gut health, immune response, and economic viability in swine, poultry, and rabbits. Results demonstrate that CRES and epithelial scrapings can effectively replace 60–80% of fishmeal without compromising growth or health when subjected to appropriate processing protocols such as sterilization and defatting, thereby enhancing protein circularity. Soybean shaft/hulls at inclusion levels ≤25% improve fiber fermentation and short-chain fatty acid production, while "aking rice" (≤40%) delivers resistant starch to nourish beneficial hindgut microbiota. Collectively, these by-products reduced feed costs by 10–25% and lowered production costs per kilogram of gain. Critically, optimized inclusion levels varied by species and growth phase but consistently supported intestinal barrier function and nutrient utilization. Rigorous processing protocols, including heat treatment of animal by-products, effectively mitigated disease transmission risks, with serum biomarker profiles confirming physiological safety. Thematic analysis revealed these resources directly advance SDG Target 12.5 by diverting slaughterhouse and agro-processing waste from landfills while simultaneously supporting SDG 9.4 through scalable, low-carbon feed innovations. Future research should prioritize standardizing by-product processing methodologies and validating large-scale economic-environmental synergies to accelerate adoption within circular livestock production systems.