Effects of Astragalus membranaceus by-product on pellet quality, mold growth and resistance of Crucian carp (Carassius auratus) against Aeromonas hydrophila

Abstract

This study investigated the effects of Astragalus membranaceus by-product (AP) on feed quality, mold growth, and fish disease-resistant ability. Seven fish diets were prepared with gradient levels of AP (0, 1.0, 2.0, 4.0, 8.0, 16 and 32 %). A total of 420 juvenile Crucian carp (7.17 ± 0.05 g) were randomly assigned and fed for 60 days. Results showed that AP supplementation decreased pellet hardness, and increased pellet durability index, as well as inhibited mold growth of pelleted fish feed (P<0.05). Fish weight gain (WG), feed efficiency (FE), content of reduced glutathione, and the activity of anti-hydroxy radical and anti-superoxide anion, as well as enzymes such as trypsin, amylase, alkaline phosphatase, Na⁺/K⁺-ATPase^, superoxide dismutase, glutathione reductase, lipase, and glutathione-S-transferase in fish digestive organs were all improved (P<0.05), while malonaldehyde levels were decreased by dietary AP supplementation (P<0.05). The optimal AP supplementation levels for Crucian carp were 1.02 % and 1.03 %, as determined by broken-line regression analysis of WG and FE. Furthermore, dietary AP supplementation decreased the mortality of crucian carp after challenge with A. hydrophilia and increased lysozyme activity, and levels of total amino acids and protein in the plasma, and decreased content of ammonia, and activities of glutamate-oxaloacetate transaminase and glutamate-pyruvate transaminase in the plasma (P<0.05). Our study suggested that AP supplementation improves feed quality and inhibites mold growth. The beneficial of AP on fish growth performance may be associated with improved absorption and digestion. Dietary inclusion of AP enhanced non-specific immunity and antioxidant status so as to improve fish disease-resistant ability.