Objective This study aimed to investigate the effects of short-term (30 d) and long-term (60 d) feeding with mulberry leaf extract (MLE) on growth performance, serum biochemical parameters, antioxidant capacity, histomorphology of liver and intestine, and muscle quality of seabass (Lateolabrax maculatus), providing references for the application of MLE in L. maculatus feed.

Method Based on the basal diet, two isonitrogenous and isolipidic diets containing 0% (CK) and 0.5% MLE were formulated. L. maculatus with an average initial weight of 150.0 g were fed in pond cages, with serum, intestine, liver, and muscle samples collected at 30 d and 60 d for physiological and biochemical analysis.

Result Both short-term (30 d) and long-term (60 d) feeding with MLE improved the growth performance and reduced the feed conversion ratio of L. maculatus, but the differences were not significant. Serum antioxidant and biochemical results showed that compared with the CK, after 30 days of feeding with MLE, the total cholesterol (T-CHOL), high-density lipoprotein cholesterol, triglyceride, and glucose contents in serum were significantly reduced, while superoxide dismutase and reduced glutathione activities increased significantly by 31.05% and 12.00%, respectively. After 60 days, the aspartate aminotransferase (AST), alanine aminotransferase (ALT) activities, and malondialdehyde content in serum were decreased significantly by 20.99%, 32.57%, and 23.10%, respectively. Compared with the CK, the lipase activity in intestine and alkaline phosphatase activity (ALP) in liver of L. maculatus fed with MLE for 30 d and 60 d were significantly increased by 24.10% and 38.55%, 69.40% and 32.28%, respectively; The lysozyme activity in liver of L. maculatus fed with MLE for 60 d was significantly increased by 24.90%. Histomorphological results of intestine and liver showed that the villus height and width of L. maculatus fed with MLE for 30 d were significantly higher than those of CK by 22.90% and 24.11%, respectively, while the symptoms of hepatocyte vacuolization and nuclear deviation were significantly reduced at 60 d. Analysis of muscle nutritional components revealed that compared with the CK, L. maculatus fed with MLE for 30 days showed a significant increase of 3.40% and 3.49% in umami amino acids and essential amino acids content in muscle respectively, a significant decrease of 20 percentage points in crude fat content, while no significant differences were observed in total amino acids and inosinic acid contents. Additionally, with prolonged feeding duration, both groups of fish showed significant increases in weight gain rate, serum T-CHOL content, AST and ALT activities, total antioxidant capacity, intestinal trypsin activity, villus height and muscle layer thickness, as well as crude fat content in muscle. Meanwhile, significant decreases were observed in hepatic ALP activity, intestinal villus width, threonine and umami amino acids contents in muscle.

Conclusion Short-term feeding of MLE can promote the development of intestine in seabass, reduce blood lipids and crude lipid content in muscle, significantly enhance the antioxidant capacity of serum, and increase the content of umami amino acids and essential amino acids in muscle, thereby improving muscle quality. On the other hand, long-term feeding of MLE is more beneficial for the health of liver in seabass.