Effects of discontinuous administration of β-glucan and glycyrrhizin on the growth and immunity of white shrimp Litopenaeus vannamei

A 6-week growth trial was conducted to compare the effects of different feeding strategies of dietary immunostimulants on the growth and immunity of white shrimp Litopenaeus vannamei (4.70 ± 0.20 g). Six feeding strategies were set, including feeding immunostimulants-free diet continuously (control), feeding dietary β-glucan or glycyrrhizin continuously, feeding dietary β-glucan discontinuously, feeding dietary β-glucan and glycyrrhizin alternately. The results showed that compared with glycyrrhizin, β-glucan could maintain the immunity of shrimps at a higher level during the experimental period. However, continuously applying β-glucan or glycyrrhizin into the diet caused immunity fatigue in L. vannamei. On the 27th day, the total haemocyte count (THC), superoxide anion and superoxide dismutase (SOD) activity of the shrimps fed with β-glucan continuously were no longer significantly higher than those in the control group. Meanwhile, phenoloxidase (PO) activity was no longer significantly higher on the 35th day. THC, PO activity and SOD activity of the shrimps fed with glycyrrhizin were no longer significantly higher than those in the control group on the 25th, 37th, 29th day, respectively. Discontinuous administration of β-glucan or glycyrrhizin could eliminate the immunity fatigue. Shrimps fed with dietary β-glucan 2 days followed by the basal diet for 5 days showed the highest specific growth rate (SGR). It was concluded that this feeding strategy is most suitable for L. vannamei.