Optimizing feeding space and group size ameliorates high stocking density-induced impairments in broilers

This study investigated the behavioral and physiological mechanisms through which feeding space and group size influence the growth performance and health status of broilers reared under high stocking density (HSD) conditions. A total of 390 21-day-old Arbor Acres broilers were randomly assigned to four groups corresponding to different rearing conditions: normal stocking density (NSD; 15 broilers/m², 6.67 cm feeder space per broiler), HSD (20 broilers/m², 5 cm feeder space per broiler), HSD + increased feeding space (HSD + FS; 20 broilers/m², 10 cm feeder space per broiler), and HSD + reduced group size (HSD + GS; 20 broilers/m², 5 cm feeder space per broiler). The groups were compared in terms of growth performance, behavioral changes, serum biochemical, antioxidant, immunity, and cecal microbiota abundance. Compared with the NSD group, the HSD group exhibited significantly reduced feed intake (P < 0.05) and increased frequency of aggressive behavior (P < 0.01). Moreover, the HSD group exhibited reduced antioxidant capacity and downregulated mRNA expression of claudin-1 (P < 0.05). Additionally, the HSD group exhibited elevated stress and inflammation levels as well as differential enrichment of bacterial taxa such as Turicibacter and Helicobacter. In contrast, the negative effects of HSD on growth performance, aggression display, and stress and immune responses were alleviated in the HSD + FS and HSD + GS groups. The HSD + FS and HSD + GS groups exhibited upregulated mRNA expression of occludin (P < 0.01) and enrichment of bacterial taxa, such as Allobaculum, Bifidobacterium, Acinetobacter, Vogesella, and Azohydromonas. Increasing feeding space and reducing group size can improve the health status of broilers under HSD by mitigating oxidative stress, regulating immune function, upregulating the expression of tight junction-related genes, and improving gut microbiota.