Supplementation of glycine, prebiotic, and nucleotides in soybean meal-based diets for largemouth bass (Micropterus salmoides): Effects on production performance, whole-body nutrient composition and retention, and intestinal histopathology

Abstract Aiming to optimize plant-based diets for largemouth bass (LMB; Micropterus salmoides), a 10-week feeding trial was conducted to evaluate effects of supplemental glycine, prebiotic, and nucleotides in soybean meal (SBM)-based diets formulated to contain 50% SBM, 40% crude protein, and 12% lipid. Four test diets were formulated by supplementing glycine at 2% (GLY), GroBiotic®-A at 2% (GBA), a nucleotide mixture at 0.15% (NTMX), or all three test ingredients combined (COMB) to the unsupplemented control diet (CTL). Supplementation replaced wheat flour and aspartate in CTL as needed. The sixth diet (FMR) comprised a SBM-free formulation containing 44% fish meal and served as an standard reference. Each experimental diet was randomly assigned to four groups of twenty feed-trained LMB (6.0 ± 0.1 g/fish) stocked in 24, 110-L glass aquaria operating as a recirculating aquaculture system. Statistical significance was considered at P ≤ 0.05. Upon conclusion of the feeding trial, survival of LMB averaged 98.8% and was unaffected by diet. Final weight ranged from 48.5 to 60.6 g and weight gain from 712 to 913% of initial weight. GLY supported faster growth of LMB than GBA, NTMX and COMB, and was the only SBM-based diet to support growth and feed efficiency similar to FMR. Concentrations of isoleucine, phenylalanine, proline, and valine in LMB were influenced by diet. Lower retention efficiency of protein and most essential amino acids was observed in COMB- compared to CTL-fed groups, whereas FMR outperformed all SBM-based diets. COMB lowered plasma alkaline phosphatase activity among diets and, along with NTMX, increased plasma transaminase activity relative to CTL and FMR groups. Liver malondialdehyde increased from CTL to GLY and was higher in all groups fed the SBM-based diets compared to FMR. Highest liver superoxide dismutase activity was observed in fish fed GLY, while glutathione peroxidase activity was unaffected by diet. Histopathological analyses of LMB intestine revealed signs of mild enteritis in all fish. Height of folds in both anterior and posterior intestinal segments increased from CTL to GBA and FMR-fed fish, while the height of enterocytes was unaffected by diet. Based on our results, i) supplemental glycine may improve growth performance of LMB when dietary FM is almost completely replaced and dietary glycine level is reduced by SBM; ii) similar responses of LMB gastrointestinal tract to SBM- and FM-based diets indicate tolerance to the former; iii) additional investigations on the role of supplemental nutrients/additives in optimizing plant-based diets for LMB are recommended.