Circular Economy in Animal Nutrition: Feed Valorisation, Precision Livestock Farming, and Sustainability

Livestock systems are expected to convert human-inedible biomass into animal-source food while reducing their environmental footprint, and the circular economy (CE) provides a framework for this transition. This review argues that the circular value of a feed resource depends less on its availability than on three conditions: the processing required before it can be fed safely, the controllability of its variability and hazards, and the comparability of the life cycle assessment (LCA) evidence for its environmental benefit. Accordingly, resources are classified as suitable for direct feed use (e.g., oilseed meals, distillery co-products, former foodstuffs), requiring biological or technological upgrading (e.g., solid-state fermented residues, insect meal, single-cell protein), or better suited to non-feed valorisation. Nutritional evidence, LCA findings, One Health hazards and regulatory constraints are compared across these classes, distinguishing LCA values reported by original studies from qualitative estimates. A Mediterranean case study on olive mill by-products and phytogenic additives in swine, largely conducted by the authors, is presented separately, with its limits of generalisation. Linkages and trade-offs with the Sustainable Development Goals are examined, and precision livestock farming is evaluated as a conditional enabler rather than a prerequisite. Research priorities are proposed.