Abstract
This study evaluated the effects of dietary supplementation with soybean isoflavones (SIF) at 30, 50, and 100 mg/kg on reproductive performance, egg quality, ovarian function, tibia quality, serum hormone profile, antioxidant status, and immune function in late-phase laying hens. A total of 432 Hy-Line Brown hens with the average body weight of 2.18 ± 0.12 kg, aged 66 weeks, were randomly assigned to 4 dietary treatments: a basal diet (control) and basal diets supplemented with 30, 50, or 100 mg/kg SIF. The trial lasted for 8 weeks, with each group comprising 6 replicates of 18 birds. Results indicated that SIF supplementation at 30 and 50 mg/kg significantly improved egg production and albumen quality (P < 0.05). However, no significant differences were observed in eggshell strength and thickness, egg weight, mass, shape index, yolk color, average daily feed intake, and feed-to-egg ratio (P > 0.05). Dietary SIF linearly increased serum total cholesterol (P = 0.030) without affecting other biochemical indices. Serum superoxide dismutase activity was significantly upregulated by SIF (P = 0.038), while malondialdehyde levels declined (P = 0.037). Moreover, hens fed 30 mg/kg SIF showed elevated estrogen levels and an increased number of large white follicles compare with the control (P < 0.05). Supplementation with 50 mg/kg SIF increased tibial calcium and phosphorus contents (P < 0.05). These findings suggest that SIF may serve as a nutritional strategy to improve egg production, egg quality, hormone secretion, bone mineralization, ovarian function, and antioxidant capacity in aged hens. Notably, both 30 and 50 mg/kg inclusion levels produced similar improvements in performance and physiological indices. Therefore, aged laying hens may benefit from dietary SIF at a dose of 30 mg/kg to enhance their reproductive efficiency, owing that such positive effects can be achieved without the need for higher dosage supplementation.