Forage Integration for Sustainable Intensification in Mixed Crop–Livestock Systems: Mechanisms, Trade-Offs and Design Principles

Mixed crop–livestock systems remain central to sustainable intensification because they reconnect feed, biomass, soil cover, livestock productivity, and household livelihoods. However, forage integration benefits are neither automatic nor uniformly transferable across regions. This structured critical integrative review asks when forage integration generates net system-level gains. It also asks when integration merely shifts costs among production, labor, water, nutrients, emissions, household risk, or territorial nutrient balances. The review synthesizes evidence on forage legumes, tropical and temperate grasses, dual-purpose crops, grazed cover crops, pasture rotations, silvopastoral arrangements, and beyond-farm feed–manure exchanges. Examples from the Brazilian Cerrado, western São Paulo, and Minas Gerais illustrate tropical pathways involving Urochloa/Brachiaria integration, crop–pasture rotations, crop–livestock–forest systems, and habitat-mediated pest regulation. These examples are interpreted alongside evidence from African, Asian, European, and temperate systems to maintain regional balance. The review contributes a diagnostic framework for assessing system design, evidence strength, and scaling feasibility. The framework links forage portfolios to integration niches, management quality, resource constraints, gendered labor implications, external-input dependency, and adaptive scaling pathways. Forage integration can improve feed quality, animal performance, soil cover, nutrient cycling, biodiversity functions, and emission intensity. These gains require careful management of establishment, grazing, manure distribution, phosphorus and potassium balances, water demand, labor allocation, markets, and governance. Future research should move beyond short-term demonstrations toward causal inference, longitudinal whole-system accounting, transparent evidence-quality grading, and documentation of failed, partial, or discontinued interventions.