ABSTRACT
A 70-day feeding trial was conducted to study the effects of dietary nucleotide, yeast cell wall (containing 20% ß -glucan) and their combination on growth performance, feed utilization and immune response of grass carp (Ctenopharyngodon idella) with 69.97 ± 0.05 g of initial body weight. Four isonitrogenous (about 38% crude protein) and isolipidic (about 5% crude lipid) diets were established. Based on the control diet (CD), the other three experimental diets were prepared by adding 0.01% of nucleotide (NT), 0.1% of yeast cell wall (YCW) and NT (0.01%) +YCW (0.1%), respectively. Results showed that no significant difference was found in survival of grass carp ranging from 94.44% to 97.78% among all the groups (P > 0.05). Compared with the control group, weight gain rate, muscle crude protein content, serum protein, trypsin and chymotrypsin activities in midgut, lysozyme and immunoglobulin M in serum significantly increased in fish fed the YCW diet (P < 0.05). The significantly highest weight gain rate, villus height and digestive enzyme activities in midgut and innate immune parameters in serum were found in fish fed the NT + YCW diet (P < 0.05). The gene expressions of ß-defensin, hepcidin, il-10 and tgf-ß1 in the midgut, and tor and s6k1 in liver significantly increased in fish fed the NT + YCW diet. Meanwhile, the gene expressions of il-1ß and tnf-α in the midgut decreased significantly (P < 0.05). The liver histology showed the better development in dietary NT and/or YCW supplemented groups than those in the control group. In conclusion, combination of dietary NT and YCW had significantly synergetic improvements on the growth, feed utilization, digestive enzymes, innate immunity and histology of midgut and liver of grass carp.