In ovo delivery of mannan oligosaccharides and Limosilactobacillus reuteri DSM 17938 enhance early growth in layer pullets via differential modulation of intestinal morphology

Mannan based oligosaccharides and Lactobacillus strains has emerged as a promising bioactive substance to enhance intestinal development and improve early growth in poultry. However, their specific microarchitectural targets and differential microarchitectural role in post-hatch growth of layer pullet have not yet adequately characterized. This study explored the differential effects of in ovo delivery of two variants of mannan based oligosaccharides: D-mannan oligosaccharide (Dman) and Poly-D-mannan oligosaccharide (PolyD) and Limosilactobacillus reuteri DSM 17938 (LBR) on intestinal microarchitectures and growth in layer pullets. A total of 1,260 fertile eggs were divided into five groups: (1) not-injected control (NC); (2) saline-injected control (Sal; 100 μL of sterile saline); (3) LBR (100 μL of suspension containing 10⁸ CFU per egg); (4) PolyD (3 mg per egg); and (5) Dman (3 mg per egg). The solution was injected into amniotic on 17.5 day of incubation. Both mannan oligosaccharides (MOS) variants significantly enhanced early post-hatch growth, with Dman sustaining the effects up to 42 days (P < 0.05), whereas LBR had no significant effect (P > 0.05). In the ileum, both MOS variants increased villus width (VW) and villus surface area (VSA) on day 1, whereas LBR enhanced VW (P < 0.05). These effects persisted to day 7, with PolyD and LBR maintaining higher VW. In the jejunum, Dman increased VW on day 1 and crypt depth (CD) on day 7, while PolyD improved the villus height (VH):CD ratio on both days. By day 21, both VH and CD were elevated in the jejunum and ileum of both MOS variants treated chicks, with a sustained increase jejunal VH observed through day 42 (P < 0.05). Inclusively, these findings suggest that in ovo bioactive substance delivery exerts segment- and age-specific effects on intestinal development, supporting the concept of precision nutritional programming in layer chickens.