Co‐fermentation of Bacillus subtilis and Bacillus licheniformis and its application in the feeding of Koi

The aim of the present study was to optimize the solid state co-fermentation requirements of soybean meal (SBM) by Bacillus subtilis and B. licheniformis and assess the application effects on the feeding of koi carp. The soluble protein conversion rate (SPCR) was the highest when co-fermented by B. subtilis and B. licheniformis with the mixed ratio 1:2 (V/V, marked as SL1:2). As the results of response of surface methodology showed, the optimal co-fermentation requirements by SL1:2 were at inoculums size 5% under the duration of 60 h at 30°C and with the substrate/water ratio 1:1. The highest SPCR was 50.22% ± 2.06%, and the anti-nutrient factors in fermented soybean meal (FSBM) decreased significantly (p < 0.05). Six isonitrogenous diets were formulated. Fish meal (FM) was partially replaced by 0%, 30% and 60% of FSBM in the treatment groups, and FSBM replaced by SBM as controlled group. When FM was replaced by 0% and 30% of FSBM, the effects of FSBM on protease activity, immune activity and antioxidant activity of Koi were higher than that of the same proportion of SBM. Meanwhile, the effects of FSBM on the feeding of koi carp could reduce the dosage of 10% SBM at the same proportion. As the results show, FSBM used in aquaculture effectively reduced feed expenditure, improved economic and environmental benefits, and prevented the deterioration of water quality.