Almond shell as an insoluble fibre source in laying pullets: influence of inclusion level and particle size on performance and intestinal health.

Dietary fibre (DF) is increasingly recognised for its role in poultry nutrition, particularly in supporting gut health. Almond shells (AS), rich in insoluble fibre (IF), represent a potential DF source. This study assessed the effects of fine (FAS) and coarse (CAS) AS included at 1% and 2% (FAS1, FAS2, CAS1, CAS2) compared with a control diet (NC) without supplemental IF on growth performance, dry matter digestibility, organ development, intestinal morphology, gene expression, caecal microbiota, and volatile fatty acids (VFA) in 6-week-old pullets over 28 days. AS inclusion did not influence productive parameters (P > 0.05). However, diets with 2% FAS, 1% CAS, and 2% CAS improved dry matter digestibility versus NC (P < 0.05). Proventriculus weight was greater in CAS-fed birds than in NC or FAS groups (P < 0.05), indicating a particle-size effect. Gizzard development was enhanced in FAS2, CAS1, and CAS2 compared with NC and FAS1 (P < 0.05), suggesting effects of both inclusion level and particle size. In intestinal morphology, CAS-fed birds exhibited longer villi, greater villus surface area, and thicker mucosa in the duodenum and jejunum than FAS-fed birds (P < 0.05). No dietary effects were detected on barrier-related or inflammatory gene expression, caecal microbiota, or VFA profiles (P > 0.05). In conclusion, AS inclusion as an IF source was associated with improved dry matter digestibility, increased relative weights of the proventriculus and gizzard, and enhanced intestinal morphology, particularly at 2% inclusion and in coarse particle form. Although, no effects were detected on growth performance, microbiota, or VFA profiles.