Effects of replacing wheat bran by fermented palm kernel meal on growth performance and intestinal health of grass carp (Ctenopharyngodon idellus)

Non-grain feed ingredients that have poor palatability or contain anti-nutritional factors can be used to replace grain ingredients in fish feed after fermentation. This is significant for enhancing the quality of feed ingredients and reducing aquaculture's reliance on traditional grain sources. The utilization of palm kernel meal in fish feed has garnered considerable attention, yet the potential of fermented palm kernel meal remains relatively unexplored in this context. In this study, different levels (0 %, 5.1 %, 10.2 %, 15.3 %, 20.4 %, and 25.5 %) of fermented palm kernel meal (FPKM) by enzymes (protease, non-starch polysaccharide enzymes) and probiotics (yeast, lactic acid bacteria, Bacillus) to replace varying proportions (0 %, 20 %, 40 %, 60 %, 80 %, and 100 %) of wheat bran. Fish breeding experiment was conducted in flowing net cages, with feeding twice a day for 84 days. Substituting wheat bran with FPKM in the diets of grass carp ( Ctenopharyngodon idellus ) can yield positive outcomes. When wheat bran was replaced with FPKM, the growth performance of grass carp improved, achieving optimal results at an 80 % replacement level (Δweight gain +8.5 % vs control; 95 % CI +8.1 to +8.9 %; P < 0.001). Notably,this substitution led to a remarkable enhancement in digestive enzyme activities, including trypsin, amylase, and lipase, indicating better nutrient absorption and utilization (trypsin +11.4 %, 95 % CI +10.2 to +12.9 %; lipase +29.2 %, 95 % CI +20.8 to +37.5 %; amylase +37.7 %, 95 % CI +30.8 to +44.6 %, all P < 0.001). But high levels (≥40 %) replacement significantly reduced plasma diamine oxidase activity, lipopolysaccharide, D-lactic acid, and endothelin-1 levels ( P < 0.05). High levels (≥60 %) of substitution significantly increased midgut lysozyme, complement 3, immunoglobulin M, complement 4, poly immunoglobulin R, and nitric oxide levels ( P < 0.05). Similarly, complete replacement of wheat bran by FPKM significantly up-regulates the relative expression of zo-1 , occludin and il-10 genes in the midgut ( P < 0.05). In conclusion, replacing wheat bran with FPKM can enhance the growth performance (an 80 % substitution level is the best) and intestinal health of grass carp, which are closely related to probiotic enzyme synergistic fermentation. • Adding 25.5 % FPKM to completely replace wheat bran did not negatively affect the growth performance of grass carp. • The optimal growth rate was observed at a replacement level of 80 %, which corresponds to a dietary inclusion of 20.4 % FPKM. • The partial substitution of wheat bran with FPKM improved intestinal health by enhancing the activity of digestive enzymes and strengthening both the chemical and immune barriers of the intestine.