Effect of dietary levels of folic acid on growth performance, blood biochemistry, antioxidant capacity, and immunity of juvenile largemouth bass, Micropterus salmoides

Abstract To obtain the optimal requirement of dietary folic acid (FA) in feed of juvenile largemouth bass, Micropterus salmoides , five experimental diets were prepared with 0, 0.5, 1.5, 4.5, and 13.5 mg/kg of supplemental FA in the basal practical diet (measured FA value <0.18 mg/kg in the basal diet) and fed to juvenile largemouth bass with an initial body weight of (14.49 ± 0.06) g for 7 weeks. The results showed that the weight gain rate and specific growth rate of the experimental fish in each FA supplementation group were higher than those in the control group to varying degrees and reached maximum values at 1.5 mg/kg in the experimental group, which were significantly higher than those in the FA‐free group; the increase in the addition of FA to the diet significantly reduced the visceral to body ratio (visceral somatic index), liver to body ratio, and relative intestine weight. The addition of ≥0.5 mg/kg FA to the feed significantly increased the whole fish protein content. The hemoglobin content and hematocrit of whole blood cells were significantly increased in the fish group supplemented with 0.5–1.5 mg/kg FA, and the addition of 0.5 mg/kg FA significantly increased the number of red blood cells. Plasma total protein and albumin levels increased and then decreased with the addition of FA, reaching a maximum at 1.5 mg/kg, and differed significantly from the FA‐free group. The addition of 0.5 mg/kg and more of FA significantly decreased plasma alanine transaminase (GPT) activity and improved liver GPT activity, the addition of 0.5 mg/kg and more of FA significantly decreased serum malonaldehyde activity, 0.5 mg/kg of FA significantly increased total hepatic antioxidant capacity, 1.5 mg/kg of FA significantly increased hepatic catalase and head kidney lysozyme activity. The addition of 0.5 mg/kg and above of FA significantly reduced liver glycogen content. In conclusion, the addition of FA to the diet significantly improved the growth performance, hematopoiesis, metabolism of protein and carbohydrate, antioxidant capacity of the liver, and protected liver health of juvenile largemouth bass. Regression analysis based on growth performance showed that the optimal amount of FA in the feed of juvenile largemouth bass was 1.42–1.46 mg/kg.