Soybean meal (SBM) is a core ingredient in aquafeed. However, high SBM inclusion in salmonids feed induces enteritis (SBMIE) ultimately reduces nutrient utilization, and decreases the growth performance of carnivore species, including rainbow trout (Oncorhynchus mykiss). Whole 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, nutrient utilization, and distal intestinal histology of trout.
Nine experimental diets (42% crude protein and 20% lipid): fish meal-based diets (FM, control), SBM-based diets (30%SBM and 40%SBM), 30%SBM+5% BSFL, 40%SBM+5% BSFL, 30%SBM+0.2%SB, 40%SBM+0.2%SB, 30%SBM+5% BSFL+0.2%SB, and 40%SBM+5% BSFL+0.2%SB are being fed twice at satiation level for 16 weeks. Each diet was fed to five replicated 19 fish (~30 g) per tank. 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 is no significant difference in average weight (AVG) and weight gain (%) at the 5th, 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 intake, 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 (Fig 1.)
Shortening and thickening of the folds were most apparent in the SBM40 + I5, the SBM40 + SB, and the SBM30 + I5 + SB, but the SBM30 + I5 appeared similar to the control diet. Based on the results obtained in the present study, it is suggested that 5% insect meal or 0.2% sodium butyrate may be used as a feed additive in high soybean meal diets for rainbow trout.