High inclusion of soybean meal (SBM) in aquafeed induces enteritis (SBMIE) and ultimately reduces nutrient utilization and decreases the growth performance of carnivore species, including rainbow trout (Oncorhynchus mykiss). Based on the available literature, insect meal (black soldier fly larvae, BSFL) and organic acid (sodium butyrate, SB) exhibited positive effects in terms of improving the growth performance and health of fish. This study aimed to investigate the effects of supplementation of BSFL meal and SB in SBM-based diets on growth performance, feed conversion ratio, and distal intestinal structure of trout.
A total of 855 fish (30 g) were distributed into 9 treatments with five replicates (19 fish/tank). Nine experimental diets (42% crude protein and 20% lipid): fish meal-based diets (FM), SBM-based diets (SBM30, 30%SBM and SBM40, 40%SBM), SBM30+BSFL, 30%SBM+5%BSFL, SBM40+BSFL, 40%SBM+5%BSFL, SBM30+SB, 30%SBM+0.2%SB, SBM40+SB, 40%SBM+0.2%SB, SBM30+BSFL+SB, 30%SBM+5% BSFL+0.2%SB, and SBM40+BSFL+SB, 40%SBM+5% BSFL+0.2%SB are being fed twice at satiation level for 16 weeks. Fish were sampled every four weeks for 16 weeks. All the data were subjected to one-way ANOVA using R programming.
At the end of the feeding trial, the growth performance, feed utilization, and distal intestinal histology of the trout were measured. The results showed that there was no significant difference in average weight (AVG) and weight gain (%) at the 4th, 8th, and 16 weeks among the dietary groups (p > 0.05). However, in the 12th week, the trout fed SBM40 registered a significantly lower value, while those fed SBM30+SB recorded a higher response (p<0.05). The feed conversion ratio (FCR), protein efficiency (PER), and HSI value did not differ (p > 0.05) among the dietary groups. The histological results did show that the trout fed soy diets appeared to have a loss of vacuolization in the villi and increased lamina propria thickness within these folds. Shortening and thickening of the folds were most apparent in the SBM40+BSFL, the SBM40+SB, and the SBM30+BSFL+SB, but the SBM30+BSFLappeared similar to the control diet. Genes associated with growth performance, oxidative stress, gut barrier integrity and acute inflammatory-related cytokines and chemokines, NF-kB and TNF-α-related genes, and regulators of B and T lymphocytes function are being analyzed. Additionally, gut microbiome data are being analyzed.
Based on the results obtained in the present study, it is suggested that 5% whole larval insect meal or 0.2% sodium butyrate may be used as a complementary additive in high soybean meal diets for rainbow trout.