Dietary glycine supplementation enhances the growth performance of hybrid striped bass (Morone saxatilis ♀× Morone chrysops ♂) fed soybean meal-based diets

This study was conducted to test the hypothesis that supplementing 1% and 2% glycine to soybean meal (SBM)-based diets can improve the growth performance of juvenile hybrid striped bass (HSB). The basal diets contained 15% fishmeal and 58% SBM (DM basis). Alanine was used as the isonitrogenous control in different diets. All diets contained 44% crude protein and 10% lipids (DM basis). There were four tanks (15 fish per tank) per dietary group, with the mean of the initial body weight (BW) of fish being 5.3 g. Fish were fed to apparent satiation twice daily, and their BW was recorded every 2 wk. The trial lasted for 8 wk. Results indicated that the BW, weight gain, protein efficiency ratio, and retention of dietary lipids in fish were enhanced (P < 0.05) by dietary supplementation with 1% or 2% glycine. In addition, dietary supplementation with glycine did not affect (P > 0.05) the feed intake of fish but increased (P < 0.05) the retention of dietary nitrogen, most amino acids, and phosphorus in the body, compared to the 0% glycine group. Dietary supplementation with 1% and 2% glycine dose-dependently augmented (P < 0.05) the villus height of the proximal intestine and reduced the submucosal thickness of the gut, while preventing submucosal and lamina propria hemorrhages. Compared with the 0% glycine group, dietary supplementation with 1% or 2% glycine decreased (P < 0.05) the proportion of skeletal-muscle fibers with diameters of 40 to 60 µm but increased (P < 0.05) the proportion of skeletal-muscle fibers with diameters of 80 to 100 µm and > 100 µm. Collectively, these findings indicate that glycine in SBM-based diets is inadequate for maximum growth of juvenile HSB and that dietary supplementation with 1% or 2% glycine is required to improve their weight gain and feed efficiency. Glycine is a conditionally essential amino acid for this fish.Animal agriculture (including aquaculture) provides high-quality protein for improving human nutrition and health. The United States is the top producer of hybrid striped bass (HSB) in the world as both food and sport fish. Fishmeal has traditionally been used as the major protein feedstuff in HSB diets, but feeding fish with fishmeal is not sustainable in the industry. Over the past four decades, there have been extensive studies to replace fishmeal with plant-sourced feedstuffs (mainly soybean meal) in aquafeeds at variable success. It has now been recognized that the content of glycine (the most abundant amino acid in the animal body) in soybean meal is only about half of that in fishmeal. Results of this study indicate that glycine is inadequate for normal intestinal structure or maximum growth in HSB fed soybean meal-based diets. Supplementing 1% or 2% glycine to these diets increased protein accretion, skeletal-muscle hypertrophy, and weight gain in HSB, while improving their intestinal structure. These findings indicate an important role for a sufficient provision of dietary glycine in the optimal nutrition, health, and growth of HSB, and have broad implications for developing low-fishmeal diets to enhance fish production and sustain animal agriculture.