Dietary Mixture of Astragalus mongholicus and Codonopsis pilosula Improved the Growth, Immunity, and Lipid Metabolism in the Largemouth Bass

This study investigated the effects of dietary supplementation with a mixture of Astragalus mongholicus and Codonopsis pilosula extracts (QSY) on growth, immunity, antioxidant capacity, and lipid metabolism in juvenile largemouth bass ( Micropterus salmoides ). Fish (initial weight, 10.33 ± 0.07 g) were fed diets containing graded QSY levels (0%, 0.05%, 0.1%, and 0.2%) for 72 days. The results demonstrated that dietary supplementation with 0.1% QSY significantly improved growth performance, feed utilization, and nutrient apparent digestibility. Compared with the control diet, dietary 0.1% QSY enhanced intestinal function by increasing villus length (VL), villus width (VW), wall thickness (WT), and trypsin activity. Dietary 0.1% QSY could enhance immune‐related enzyme activity and upregulate the expression of anti‐inflammatory cytokines, thereby improving the survival rate of largemouth bass following a challenge with Aeromonas veronii . Meanwhile, dietary 0.1% QSY regulated the nuclear factor erythroid 2‐related factor 2/kelch‐like ECH‐associated protein 1 (Nrf2‐Keap1) signaling axis at the transcriptional level, hence enhancing antioxidant enzyme activities in the liver, intestine, and serum, while reducing liver damage and attenuating lipid peroxidation levels in various tissues. Furthermore, compared with the control group, dietary 0.1% QSY reduced hepatic lipid deposition through regulating lipid metabolism, specifically by promoting fatty acid β‐oxidation while inhibiting lipogenesis. However, 0.2% QSY exhibited adverse effects on the growth and survival rate against Aeromonas veronii challenge, suggesting excessive addition should be avoided. In conclusion, appropriate QSY supplementation (0.1%) effectively enhanced growth, immunity, and disease resistance in M . salmoides , supporting its potential as a functional feed additive in aquaculture.