| الملخص: | A 7-week rearing trial was designed to investigate the effects of <i>Eucommia ulmoides</i> leaf extract (ELE) on growth performance, body composition, antioxidant capacity, immune response, and disease susceptibility of diet-fed GIFT. The results showed that dietary ELE did not affect growth performance or whole-body composition (<i>p</i> > 0.05). Compared with the control group, plasma ALB contents increased in the 0.06% dietary ELE group (<i>p</i> < 0.05), and plasma ALT and AST activities decreased in the 0.08% dietary ELE group (<i>p</i> < 0.05). In terms of antioxidants, compared with GIFT fed the control diet, 0.06% dietary ELE upregulated the mRNA expression levels of Nrf2 pathway-related antioxidant genes, including <i>CAT</i> and <i>SOD</i> (<i>p</i> < 0.05), and 0.06% and 0.08% dietary ELE upregulated the mRNA levels of <i>Hsp70</i> (<i>p</i> < 0.05). In terms of immunity, 0.06% dietary ELE suppressed intestinal <i>TLR2</i>, <i>MyD88</i>, and <i>NF-κB</i> mRNA levels (<i>p</i> < 0.05). Moreover, the mRNA levels of the anti-inflammatory cytokines <i>TGF-β</i> and <i>IL-10</i> were upregulated by supplementation with 0.04% and 0.06% dietary ELE (<i>p</i> < 0.05). In terms of apoptosis, 0.06% and 0.08% ELE significantly downregulated the expression levels of <i>FADD</i> mRNA (<i>p</i> < 0.05). Finally, the challenge experiment with <i>S. agalactiae</i> showed that 0.06% dietary ELE could inhibit bacterial infection, and significantly improve the survival rate of GIFT (<i>p</i> < 0.05). This study demonstrated that the supplementation of 0.04–0.06% ELE in diet could promote intestinal antioxidant capacity, enhance the immune response and ultimately improve the disease resistance of GIFT against <i>Streptococcus agalactiae</i>.
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