Tryptophan can alleviate the inhibition in growth and immunity of tilapia (GIFT Oreochromis spp.) induced by high dietary soybean meal level

A 85-day feeding trial was designed to verify the effects of soybean meal on growth, feed utilization, antioxidants, and immune response of tilapia (GIFT Oreochromis spp.), and evaluated the effects of dietary tryptophan on alleviating the negative responses induced by high inclusion of soybean meal. Five experimental diets, two of which substituted 15 % and 45 % of the fish meal with soybean meal, the others supplemented with 0.2 %, 0.4 % and 0.6 % L-tryptophan on the basis of 45 % soybean meal, respectively, were fed 400 juvenile fish (initial weight: 2.98 & PLUSMN; 0.04 g). The results showed that the crude ash content, red blood cell count (RBC), lipase (LPS) and glutathione peroxidase (GSH-Px) activities of tilapia significantly decreased when the soybean meal replaced fish meal by 45 % (p < 0.05). Although the growth performance values of replacing 45 % soybean meal were slightly lower than that of replacing 15 % soybean meal, while there was no significant difference between them (p > 0.05). However, supplementation with tryptophan in the high soybean meal replacement groups significantly improved the final weight, weight gain rate (WGR), specific growth rate (SGR), phagocytosis index (PI), RBC, trypsin (TPS), LPS and GSH-Px activities of tilapia (p < 0.05), and the apparent digestibility and feed coefficient of tilapia were significantly decreased (p < 0.05). Taken together, these results suggested that excessive soybean meal inhibited the feed utilization and immunity of tilapia to a certain extent, while the inclusion of 0.2 %-0.4 % tryptophan in the diet could alleviate the negative effects of high soybean meal diet in tilapia. This study will provide ideas and a theoretical basis for further expanding the application of soybean meal in tilapia feed.