Effects of dietary licorice (Glycyrrhiza uralensis) supplementation on growth performance, muscle quality, and immunity in the common carp (Cyprinus carpio haematopterus)

The study aimed to investigate the effects of dietary licorice (Glycyrrhiza uralensis) on the growth performance, muscle quality, and immunity of common carp (Cyprinus carpio haematopterus). Five diets were formulated to contain 0.0%, 0.5%, 1.5%, 2.5%, and 4.0% licorice, respectively. Each diet was randomly distributed to triplicate groups of 60 juvenile (average weight 21.27 ± 0.13 g) per aquarium. After 15 weeks of feeding, the licorice had positive effects on body growth and feed utilization, the FBW, WGR, PER and SGR reached a maximum in the 4.0% treatment group (P < 0.05. Moreover, the qualities of muscle were also improved by licorice including crude protein, the composition of amino acids and fatty acids, especially the highest proportions of DAAs, FAAs and PUFAs were in 0.5% treatment groups (P < 0.05). The activities of liver function enzymes (AST and ALP) in serums did not influence (P > 0.05) by licorice except for significant lower alanine transaminase in 0.5% and 1.5% groups (P < 0.05). However, other serum indices in terms of TG, TC, and HDL were down-regulated or up-regulated after licorice dealing, of which the lowest TG, TC, and the highest HDL were in 0.5%, 1.5%, and 4.0% groups, respectively. In addition, the immunity and oxidant resistance also improved as well as growth performance and muscle quality. Among them, the survival rate of 0.0% and 1.5% groups was the lowest and highest after A. hydrophila dealing, respectively. The licorice diet reduced the mRNA levels of pro-inflammatory cytokines (IL-6, IL-1β) and INOS, but enhanced IL-10 and hepcidin expression (P < 0.05). Meanwhile, the antioxidant enzyme genes (GPX1 and SOD) were also strongly increased by licorice (P < 0.05). Subsequently, the abilities of licorice in pathogenic and oxidant resistance were further checked by Aeromonas hydrophila infection. The levels of cytokines, INOS, hepcidin, GPX1, and SOD were obviously lower than that of the 0.0% groups (P < 0.05), which echoed the decreasing mortality in other licorice treatment groups during A. hydrophila infection. Similarly, the villi height and myenteron thickness were significantly increased with licorice addition (P < 0.05). In summary, the study proves that long-term licorice (5–15 g/kg) addition can improve growth performance, muscle quality, non-specific immunity, antioxidant activity, and A. hydrophila resistance of the common carp. The results lay a foundation for the application of licorice as a safe and effective feed additive in aquaculture.