Effects of Protein Levels of Diets and Yeast Culture Supplementation on Performance, Antioxidant Capacity, and Intestinal Health in Laying Hens

Yeast culture (YC) can improve laying hen performance, but whether this effect depends on dietary protein level remains unclear. This experiment investigated the effects of adding YC to diets with different protein levels on the performance and intestinal health of laying hens. A total of 504 Lohmann pink laying hens aged 55 weeks were selected and randomly assigned to 4 treatments, with 7 replicates per treatment and 18 hens per replicate. A 2 × 2 factorial design was used, with two CP levels of 15.5% (NP) and 14.5% (LCP), and two YC supplementation levels of 0% and 1%. The feeding trial lasted 12 weeks. YC significantly increased the laying rate (p < 0.05). CP and YC interacted to affect the laying rate (p < 0.05). YC significantly increased serum CAT and GSH-Px activities, reduced PCO, and increased trypsin activity (p < 0.05). CP and YC interacted to affect GSH-Px and trypsin activities (p < 0.05). GSH-Px and trypsin activities were significantly higher in the LCP-YC group than in the LCP-0 group (p < 0.05). YC significantly increased jejunal villus height (VH) (p < 0.05). A significant CP × YC interaction was observed for jejunal VH; supplementing the LCP diet with YC significantly increased VH (p < 0.05). YC significantly increased the percentage of goblet cells in the jejunum, the contents of IgG and IgM in the jejunal mucosa, and decreased TNF-α content (p < 0.05). IFN-γ content was also significantly lower in the LCP-YC group than in the LCP-0 group (p < 0.05). YC affected cecal microbial α-diversity, while CP and YC interacted to affect β-diversity and the relative abundance of certain bacterial taxa (p < 0.05). In conclusion, under these experimental conditions, YC improves the laying rate and antioxidant capacity and supports intestinal health in laying hens. A CP × YC interaction was observed, but the apparently stronger response to YC in the LCP group should be interpreted cautiously because the LCP-0 and LCP-YC diets differed in analyzed calcium concentration.