Effects of dietary zinc fulvate supplementation on egg quality, serum biochemical indices, and intestinal health of aged laying hens.

Zinc fulvate (ZFA), a chelate of fulvic acid and zinc, exhibits superior bioavailability and multifunctional properties that benefit animal health. This study aimed to evaluate the effects of dietary ZFA supplementation at different levels on egg quality, serum biochemical indices, intestinal morphology, and cecal microbiota composition in aged laying hens. A total of 480 Hy-Line Gray laying hens (63 weeks old) were randomly assigned to four groups with eight replicates each (15 hens per replicate): a control group (CON) fed a basal diet, and three experimental groups supplemented with 600 mg/kg (ZFA 600), 1200 mg/kg (ZFA 1200), and 1800 mg/kg (ZFA 1800) of ZFA, respectively. The experiment lasted for 8 weeks after a 2 weeks adaptation period. The results revealed that: (1) Compared with the CON group, ZFA 600 and ZFA 1200 groups significantly increased eggshell thickness and strength (p < 0.05). (2) ZFA 1200 group had significantly higher serum copper-zinc superoxide dismutase (CuZn-SOD) activity and lower malondialdehyde (MDA) content (p < 0.05), while ZFA 600, ZFA 1200, and ZFA 1800 groups all showed elevated serum albumin (ALB) levels (p < 0.05). (3) ZFA 600 group significantly increased villus height (VH) in the duodenum, jejunum, and ileum, as well as the villus height-to-crypt depth ratio (VH/CD) in the duodenum and jejunum (p < 0.05), while ZFA 1200 group reduced crypt depth (CD) in the duodenum and jejunum (p < 0.05). (4) At the microbial level, ZFA 600 group significantly improved the Simpson and Shannon indices of cecal microbiota (p < 0.05), and increased the relative abundances of beneficial taxa including Firmicutes (Firmicutes_A, Firmicutes_B, Firmicutes_D), Actinobacteriota, Verrucomicrobiota at the phylum level, and Mediterraneibacter_A and Prevotella at the genus level (p < 0.05). Collectively, dietary ZFA supplementation improves eggshell quality, enhances antioxidant capacity, optimizes intestinal morphology, and modulates gut microbiota homeostasis in aged laying hens. Different ZFA doses have distinct advantage targets, and the suitable dose can be selected according to actual breeding needs, e.g., 600 mg/kg ZFA can be used as a recommended addition dose for balancing the overall cecal health of laying hens, while 1200 mg/kg ZFA is suitable for scenarios requiring focused alleviation of oxidative stress. Under the conditions of this study, 600 mg/kg ZFA is recommended as the optimal supplemental level based on overall beneficial effects and physiological rationality. However, the potential risk of excessive zinc intake at high ZFA doses was not fully considered in the present study, and lower doses of ZFA should be further evaluated in future research to meet nutritional standards more precisely.