Soybean meal level and probiotics in first feeding fry diets alter the ability of rainbow trout Oncorhynchus mykiss to utilize high levels of soybean meal during grow-out

Abstract

Inclusion rates of soybean meal in salmonid diets are currently kept low to minimize detrimental effects on growth, enteritis and immune responses. Probiotics have been used to treat both infectious and noninfectious enteritis in humans and other terrestrial animals and may represent a feasible method for increasing soy utilization in soy-sensitive aquatic species. To test the hypothesis that probiotics incorporation in rainbow trout starter diets can induce immune-mediated soybean tolerance, a two-phase experimental design was employed. In the starter phase (first feeding, 0.13 ± 0.01 to 6.5 ± 0.32 g fish − 1 ), a practical-type diet was formulated to contain 48% crude protein and 20% crude fat containing either 0 (S0), 10 (S10) or 20% (S20) soybean meal (SBM) and supplemented with (S0P, S10P, S20P) or without a commercially available probiotic (Mycolactor Dry Probiotic ®) in a 3 × 2 factorial design. Diets were fed to four replicate tanks of fish per treatment (300 fish tank − 1 ; House Creek strain) for 8 weeks. Trout were reared in 150 L tanks supplied with 4 L min − 1 of constant temperature (14.8 °C) flow-through spring water. Potentially soy tolerant rainbow trout produced by feeding probiotics and increased levels of soybean meal in starter diets as described above were then fed the industry standard level 15% (G15) or a diet with a challenge level of 43% (G43) of soybean meal during a 12 week grow-out and digestibility trial. Pathological changes were observed in intestines of fish fed the 43% SBM during grow-out; however, these changes were less severe when fish had been exposed to soybean meal in starter diets. The addition of probiotics to starter diets appeared to improve soybean meal utilization by first feeding rainbow trout, but probiotic use had only limited benefits when they were not continuously provided in the diet.