Dietary compound Chinese herbs improved the growth, immunity response and resistance against bacterial infection in largemouth bass ( Micropterus salmoides )

The present study was conducted to evaluate the effects of a Chinese herbal compound and its feeding strategy on growth performance, immunity, and resistance to bacterial infection in largemouth bass ( Micropterus salmoides ). A basal commercial diet was designated as the control (Con). Two experimental groups were established: a group fed the basal diet supplemented with the compound Chinese herbs (ChF), and an interval-feeding group (IF) that received the herbal-supplemented diet and the basal diet on alternating weeks. The herbal supplemented diet was formulated by adding 2% of the compound Chinese herbs (composed of Artemisia annua , Astragalus membranaceus , Isatis indigotica Fort, Polygonatum sibiricum Redouté, and Glycyrrhiza uralensis Fisch, and the ratio of the five herbs is 1:1:1:1:2) to the basal diet, with a corresponding reduction in wheat flour to maintain isonitrogenous and isolipidic conditions. Juvenile largemouth bass with an initial mean body weight of 27.96 ± 0.33 g were fed the three experimental diets for 8 weeks, with four replicate tanks per group. The results showed that the ChF group exhibited significantly higher final body weight, weight gain rate, survival rate, and intestinal protease and lipase activities, as well as a significantly lower feed conversion ratio, compared with the Con group. In terms of antioxidant and non-specific immune parameters, serum alkaline phosphatase and lysozyme activities in both the ChF and IF groups were significantly higher, while malondialdehyde content was significantly lower than that in the Con group. Additionally, serum superoxide dismutase and catalase activities in the ChF group were significantly higher than those in the Con group, and the cumulative mortality following bacterial challenge was significantly reduced. Regarding intestinal and hepatic health, intestinal villus height and intestinal wall thickness were significantly greater in the ChF group than in the Con group. Moreover, fish fed the herbal-supplemented diets exhibited a reduced number of vacuolated hepatocytes and a decrease in vacuolar area. In conclusion, dietary supplementation with the Chinese herbs effectively improved growth performance, immunity, antioxidant capacity, and intestinal and hepatic histomorphology in largemouth bass. Furthermore, continuous long-term feeding of the herbal-supplemented diet yielded significantly better effects than the alternate feeding regimen.