Comparative evaluation of processed soybean meal (EnzoMealTM) vs. regular soybean meal as a fishmeal replacement in diets of rainbow trout (Oncorhynchus mykiss): Effects on growth performance and growth-related genes

A 12-week feeding trial was conducted to compare low and high inclusions of conventional soybean meal (SBM) or enzyme-treated SBM (Enzo) as fishmeal replacements in the diets of rainbow trout, Oncorhynchus mykiss. Six isonitrogenous (43% crude protein) and isolipidic (19% crude lipid) diets were produced: a control diet (fishmeal-based) and five experimental diets replacing fishmeal by 8% or 16% with SBM (low or high SBM, respectively); 8% or 16% with Enzo (low or high Enzo, respectively); and 16% with SBM + Enzo blended with 50:50 ratio (Enzo + SBM). Triplicate tanks, each containing 40 fish, were hand-fed to apparent satiation. Compared with the control, all groups that consumed SBM or Enzo diets had lower growth performance (P < 0.05). However, fish fed Enzo showed significantly higher growth performance and protein retention than fish fed SBM at each replacement level. Whole-body crude protein content showed a decreasing trend with increasing dietary SBM and Enzo level, with significant differences detected between the control and the highest SBM or Enzo + SBM levels. However, fish fed control or low Enzo diets had no significant differences (P > 0.05). Whole-body content of some essential amino acids (e.g. isoleucine, leucine and valine) showed a significant corresponding decrease with high dietary SBM inclusions. Trypsin activity in the proximal intestine decreased significantly with increasing dietary SBM and Enzo levels. Hepatic expression of GHR1 was greater in high SBM and high Enzo + SBM treatments compared to the control diet, which exhibited the highest expression of IGF-II (P < 0.05). Expression of IGFBP-1b1 was lowest in the control treatment and highest in the high SBM diet (P < 0.05). Expression of myogenin and myoD, two myogenic proteins, was also affected by diet (P < 0.05), with highest expression in the slowest growing treatment group (high SBM). The inferior performance of rainbow trout fed SBM compared to Enzo was likely due to reduced feed intake and nutrient availability leading to alternations in the somatotrophic axis. Thus, Enzo shows promise as an alternative to SBM in rainbow trout diets.