Dry olive cake, an olive by-product, as a functional fiber source in diets for laying pullets: effects on growth and gut health.

The European Union is the world's largest producer of olives (Olea europaea), generating substantial amounts of by-products that require revalorization. Among these, dry olive cake is rich in dietary fiber and phenolic compounds, making it a promising ingredient for poultry nutrition. This study evaluated the effects of including 2% dry olive cake in laying pullet diets compared with a control diet, assessing performance, apparent total tract retention (ATTR), intestinal morphology, gene expression, cecal microbiota, and volatile fatty acids (VFA) after 28 days of trial. Two diets were formulated: a control diet with 2% sepiolite and a diet with 2% dry olive cake (DOC), administered to 6-weeks-old pullets housed in 12 cages (6 replicate cages per dietary treatment, with three birds per cage). DOC improved ATTR (P<0.05), while performance remained unaffected (P>0.05). Gut morphology improved in the duodenum, where crypt depth decreased and the villus length-to-crypt depth ratio increased (P<0.05). Gene expression analysis revealed downregulation of inflammatory genes (IL-2, TNF-α, IFN-γ) and RARRES1, alongside upregulation of genes related to barrier function (MUC2), antimicrobial defense (AvBD4), and fatty acid transport (FAT-CD36) (P<0.05). Cecal microbiota showed trends toward distinct community structures (Jaccard and Bray-Curtis indices; P<0.10), with higher richness (Chao1 and Fisher; P<0.05), greater Butyricicoccaceae, and reduced Proteobacteria (P<0.05). These changes were accompanied by greater total VFA production, particularly acetic, propionic, butyric, and valeric acids, with a higher proportion of butyrate (P<0.05). Overall, 2% dry olive cake improved gut health, microbiota, and nutrient utilization without affecting performance, supporting its potential as a sustainable, functional fiber source in the feed of laying pullets.