Effects of Mulberry Leaf Extract on the Growth Performance, Organ Indices, Flesh Quality, Collagen Metabolism, and Myofiber Development of Grass Carp (Ctenopharyngodon idella)

To investigate the effects of mulberry leaf extract (MLE) on the growth performance, organ indices, and flesh quality of grass carp (Ctenopharyngodon idella), a basal diet (Control, Crude protein 30.25%, Crude lipid 7.23%) and three MLE-supplemented diets containing 1%, 2%, and 4% MLE (MLE1%, MLE2%, MLE4%) were formulated. A total of 160 grass carp (259.7 ± 1.1 g) were randomly allocated into 16 cages (1 m × 1 m × 1.2 m), with 10 fish per cage and 4 replicate cages per treatment group. Throughout the 8-week feeding trial, juvenile grass carp were fed three times daily (08:00, 12:00, and 17:00) at a daily feeding rate of 1.5–2.0% body weight. At the end of the trial, growth performance was assessed. Additionally, four fish were randomly sampled from each cage to determine the organ indices, flesh quality parameters, and gene expression levels. In the results, the final average body weight of all groups ranged from 909.00 to 942.97 g, and all the groups exhibited no significant difference in growth performance (p > 0.05). MLE-supplemented groups had significantly decreased visceral somatic index and hepatic somatic index (p < 0.05). In terms of flesh quality, all MLE-supplemented groups increased the flesh shear force, springiness, whiteness, and lightness (p < 0.05). Compared with the Control group, the MLE4% group had increased flesh hardness, gumminess, and chewiness, from 2899.52 g, 1310.62, and 516.59 to 4122.99 g, 1958.48, and 887.76, respectively (p < 0.05). MLE significantly decreased the 48 h thawing loss (MLE2%), 96 h thawing loss (MLE1%, MLE2%, MLE4%), and flesh crude lipid content (MLE2%, MLE4%) (p < 0.05). In terms of collagen metabolism, compared with the Control group, MLE significantly increased the flesh collagen content (MLE2% and MLE4%) (p < 0.05), upregulated the expression level of La ribonucleoprotein 6 (MLE4%) and tissue inhibitor of metalloproteinase 1 (MLE4%). Matrix metallopeptidase 2 was significantly downregulated in all MLE-supplemented groups (p < 0.05). In terms of myofiber development, compared with the Control group, the myofiber density was higher, and the myofiber diameter was lower in all MLE-supplemented groups (p < 0.05). MLE significantly upregulated the expression level of mammalian target of rapamycin (MLE4%), musculoskeletal embryonic nuclear protein 1 (MLE4%), myogenic differentiation 1 (MLE2%, MLE4%), and myogenic factor 5 (MLE1%, MLE2%, MLE4%) (p < 0.05). Myotubularin-related protein 8 was significantly downregulated in all MLE–supplemented groups (p < 0.05). In conclusion, dietary supplementation with 2–4% MLE improves the flesh textural properties, chroma and water-holding capacity of grass carp. MLE improves the flesh quality of grass carp by enhancing collagen deposition and myofiber development.