Influence of fermentation conditions for soybean meal in a non-fish meal diet on the growth performance and physiological condition of rainbow trout Oncorhynchus mykiss

Defatted and heat-treated soybean meal (SBM) was fermented using compound bacteria predominantly Bacillus spp. under two different fermentation conditions until the material temperature reached 80 °C: initial water addition of 30% (fermented for 7 h, FSBM-1) and 45% (10 h, FSBM-2). A fish meal-based diet and three non-fish meal diets containing SBM, FSBM-1 or FSBM-2 each at a 47.6% level with a 21.0% level of corn gluten meal were fed to rainbow trout (initial BW, 14.7 g) for 10 weeks. The growth and feed efficiency ratio of fish fed SBM diet and FSBM-1 diet were inferior to those of fish fed FM diet and FSBM-2 diet. Lower digestibility values of carbohydrates and lipids, especially of polar lipids observed in fish fed SBM diet were improved by the fermentation (SBM < FSBM-1 = FSBM-2). Biliary bile acid content, proportion of cholyltaurine in the biliary bile, and bile acid concentration in the intestinal digesta increased by the fermentation (SBM < FSBM-1 < FSBM-2). Morphological abnormalities in the distal intestine observed in fish fed SBM diet were moderately improved in fish fed FSBM-1 diet and were not observed in fish fed FSBM-2 diet. Morphological abnormalities in the liver of fish fed SBM diet were also improved in both FSBM diet groups. These results indicate that fermentation of soybean meal under appropriate conditions is beneficial for preventing various physiological abnormalities that occur in rainbow trout fed soybean meal. We conclude that properly fermented soybean meal is a promising ingredient as the main protein source in a non-fish meal diet for rainbow trout.