Soy-based ingredients are essential in salmonid diets, providing a sustainable, cost-effective alternative to fishmeal. The primary soy-based products used are soybean meal (SBM) and soy protein concentrate (SPC). Optimizing the SBM-to-SPC ratio is crucial for reducing feed costs and improving performance and gut health resistance in trout. In this study, we conducted a feeding trial to optimize the SBM-to-SPC ratio across two commercial strains of rainbow trout to assess strain-specific responses in growth, feed utilization, gut health, and gene expression.
In 8 weeks feeding trial, 1500 fish (4.2 g) were distributed into 10 treatments in triplicates (50 fish/tank) following a 2 ´ 5 factorial design, where two types of rainbow trout strains, i.e., commercial strain 1 (CT1) vs. commercial strain 2 (CT2) were fed with 5 different isonitrogenous (44% crude protein) experimental diets with varying ratios of SBM to SPC for 8 weeks. Experimental diets were Diet 1 (control), 0% SBM/SPC + 30% fishmeal; Diet 2, 11% SBM + 18% SPC; Diet 3, 22% SBM + 13% SPC; Diet 4, 33% SBM + 6% SPC; and Diet 5, 44% SBM + 0% SPC. Fish were fed twice daily at apparent satiation. All the data were subjected to two-way ANOVA to determine the main and interaction effects of strains and diets and Tukey’s HSD for pairwise comparison using R programming.
Results showed that all the growth-related parameters exhibited significant interaction of ‘Strain ´ Diet’. Based on the growth performance, increasing the SBM up to 33% with reduced SPC of 6% (Diet 4) showed no significant (p > 0.05) difference with control (Diet 1). Although CT2 strain showed higher specific growth rate (SGR) (Figure 1) than CT1, final body weight (FW) and weight gain (WG) showed no significant (p > 0.05) difference between the strains. Daily feed intake (DFI) was significantly (p < 0.05) higher in CT1 than CT2 strain, but feed conversion ratio (FCR) and protein efficiency ratio (PER) showed the opposite trend. Interestingly, CT1 showed a significantly (p < 0.05) lower viscerosomatic index (VSI) (11.30) than the CT2 strain (15.94). Gut histology and gene expression parameters are being analyzed.
Overall, although CT2 strain performed better than CT1 strain in terms of SGR, TGC, FCR, and PER, opposite trend is true in terms of DFI and VSI. Among the experimental diets, reduction of SPC from 18 to 6% with SBM increment from 11 to 33% showed similar performance like control fishmeal-based diet.