Effect of dietary β-mannanase supplementation on growth performance, digestibility, and gene expression levels of Cyprinus carpio (Linnaeus) fingerlings fed a plant protein-rich diet

The aim of this study was to assess possible beneficial effects of dietary β-mannanase supplementation on the nutrient digestibility, growth performance, digestive and metabolic enzyme activity, and immune response of common carp (Cyprinus carpio) fed plant protein-rich diets. An experiment was conducted in triplicate, and a total of 225 fingerlings of common carp with an average body weight of 13.17 ± 0.12 g were stocked in 15 fiberglass tanks (15 fish/tank). Five dietary treatments (control 35% crude protein, plant-rich basal diet without supplement and four diets supplemented with β-mannanase from two sources (commercially available and locally isolated), each at two dosage levels (500 and 1,000 U/kg diet) were prepared and fed to respective groups of fish, twice a day (8:00 AM and 4:00 PM) at 4 % body weight. During the trial, changes in the level of DO and temperature ranged from 5.5 to 6.1 mg L(−1) and 21.5 to 23.5°C, respectively. At the end of the feeding experiment, all fish in each tank were weighed and counted to determine growth parameters, while for the study of other indices, nine samples/treatment group were selected. The results of the study indicated a positive effect of both sources and dosage levels of β-mannanase supplementation on all studied indices, that is, significantly improved (P < 0.05), growth performance (%weight gain, specific growth rate), survival %, hematological indices (RBC, Hb, HCT, and MCHC), immunological indices (lysozyme activity, WBC, respiratory burst activity, and phagocytic activity), improved apparent digestibility of nutrients (crude protein, crude fat, and carbohydrates), and digestible energy. Furthermore, higher activity (P < 0.05) of the digestive enzymes (cellulase, lipase, and protease) and upregulation of MyoD gene in muscle and TNF-α gene in liver, intestine, and muscle were also observed, while the activity of serum AST (serum aspartate aminotransferase) and ALT (alanine transaminase) as compared to control group was significantly decreased (P < 0.05). Based on the results, β-mannanase supplementation (500 U/kg) could be recommended for obtaining better carp production when low-cost plant protein-rich diets are used.