Valorization of olive oil industry by-products in livestock feeding: Nutritional value, functional properties, processing strategies, and research priorities

The olive oil industry generates a significant amount of by-products during harvesting and oil extraction, creating environmental challenges while offering opportunities for valorization within livestock production systems. This review provides a structured synthesis of current knowledge on chemical composition, nutritional characteristics, bioactive properties, processing strategies, and feeding applications of major olive oil by-products, including olive pomace, olive cake, olive leaves, and olive mill wastewater. These by-products are generally distinguished by high levels of structural carbohydrates, moderate residual lipid content rich in unsaturated fatty acids, low to moderate crude protein levels, and substantial concentrations of phenolic compounds, resulting in considerable variability in their nutritional value across animal species. Available evidence suggests that olive oil by-products can be incorporated into animal diets without detrimental effects on performance when appropriate processing methods and species-specific inclusion levels are applied. Ruminants exhibit a greater capacity to utilize these materials due to microbial fermentation of fibrous fractions, whereas their use in aquaculture, poultry, and swine species is typically restricted to low inclusion levels and is mainly associated with functional effects, like improved oxidative stability, modulation of gut health, and enhancement of product quality. However, their broader application is considered by various factors including high moisture content, limited fiber digestibility, lipid oxidation, compositional variability, and the presence of anti-nutritional compounds. Overall, the integration of olive oil industry by-products into livestock feeding systems represents a promising strategy to support circular economy approaches and improve the sustainability of livestock production, although further research is required to optimize their practical utilization.