Exogenous enzyme pretreatment improves utilization of high-carbohydrate diets in rainbow trout (Onchorhynchus mykiss): effects on growth, digestibility, fillet quality, and gene expression.

Exogenous enzymes are used to enhance ingredient digestibility, overall fish growth, and feed utilization; however, different administration methods have not been extensively studied. In this study, a mixture of carbohydrases (α-amylase, cellulase, xylanase, and β-glucanase) was applied before (at B3, B6, B12, and B24 hours), during (D), or after (A) the production of high-carbohydrate diets (33% carbohydrates) to assess their effects on the growth and feed utilization of rainbow trout. The enzymes were added to the diets at the minimum required levels, determined by a reducing sugar test with dinitrosalicylic acid (DNS) based on the available wheat flour and soybean meal in the diet. Seven isoenergetic (21 kJ/g) and isonitrogenous (40%) diets were formulated and fed to fish (n = 210, 17.41 ± 1.76 g) for 8 weeks. Overall growth parameters improved in the B6 and B3 groups, which also showed the lowest FCR values (0.84 and 0.85, respectively) and significantly higher weight gain, PER, and SGR compared to the other treatments. The apparent digestibility of protein, lipid, ash, and dry matter was significantly enhanced in the B3 group. Additionally, DHA and omega-3 fatty acid content increased in B3 (p < 0.05), although no significant differences were observed in fillet contents of EPA and ARA (arachidonic acid) between treatments (p > 0.05). The pretreatment of ingredients (B3 and B6) likely induced a protein-sparing effect, as indicated by higher PER and increased amino acid retention in the fillets (p < 0.05). Moreover, higher metabolizable energy and glucose levels led to significant upregulation of the ghrelin hormone (GHRL), growth hormone (GH), and insulin-like growth factor (IGF-I) genes in the B3 and B6 treatments (p < 0.05). In conclusion, pretreatment of ingredients (soybean meal and wheat flour) with optimized multienzyme doses improved digestibility and activated regulatory genes, resulting in enhanced growth, feed utilization, and fillet quality in high-carbohydrate diets for rainbow trout.