Evaluation of growth, nutrient utilization, and metabolic function in rohu, Labeo rohita (Hamilton), fed diets incorporated with fermented Saraca asoca leaf meal.

The present study evaluated the potential of Ashoka, Saraca asoca leaf meal (SLM), in carp diets following fermentative processing with a tannase-producing fish gut bacterium, Bacillus subtilis (KP765736). The processing of SLM led to a significant (P < 0.05) reduction in major anti-nutrients (tannin, trypsin inhibitor, and crude fiber), while crude protein content increased. Seven sets of isonitrogenous (35% crude protein) and isocaloric (18.82 kJg^-1) diets were prepared using raw (R1, R2, R3) and fermented SLM (F1, F2, F3) at 10%, 20%, and 30% levels by weight replacing fishmeal and de-oiled rice bran in the reference diet (RD). Diets were fed to rohu, Labeo rohita fingerlings (4.01 ± 0.08 g), for 70 days in triplicate. Fish fed diets containing 30% fermented SLM (F3) exhibited significantly (P < 0.05) better growth (241.25%), improved nutrient utilization, and enhanced activities of digestive enzymes compared to raw SLM-fed groups. Furthermore, tannin accumulation in the liver and muscle was significantly lower (P < 0.05) in fish fed fermented SLM diets compared to those fed raw SLM diets. Additionally, tannin contents in the diets were noticed to be positively correlated (P < 0.05) with tannin accumulation in fish tissues and negatively correlated (P < 0.05) with growth. Hepatic and muscle enzymes associated with carbohydrate metabolism in fish fed RD performed similarly to those reared on fermented SLM diets. Conversely, key enzymes involved in protein metabolism, hexose monophosphate shunt, and the tri-carboxylic-acid cycle showed increased activities in fish fed raw SLM diets, indicating dietary stress and a shift from carbohydrate metabolism to protein catabolism. Moreover, protein, glycogen, and amino acids in hepatopancreas and muscle showed a progressive increase with the gradual inclusion of fermented SLM in the diets. In conclusion, this study might suggest incorporating 30% (w/w) fermented SLM in the diets of rohu without interfering with growth, feed utilization, and metabolic function.