A 7-week rearing trial was designed to investigate the effects of Eucommia ulmoides 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 (p > 0.05). Compared with the control group, plasma ALB contents increased in the 0.06% dietary ELE group (p < 0.05), and plasma ALT and AST activities decreased in the 0.08% dietary ELE group (p < 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 CAT and SOD (p < 0.05), and 0.06% and 0.08% dietary ELE upregulated the
mRNA levels of Hsp70 (p < 0.05). In terms of immunity, 0.06% dietary ELE suppressed intestinal TLR2, MyD88, and NF-κB
mRNA levels (p < 0.05). Moreover, the
mRNA levels of the anti-inflammatory
cytokines TGF-β and
IL-10 were upregulated by supplementation with 0.04% and 0.06% dietary ELE (p < 0.05). In terms of apoptosis, 0.06% and 0.08% ELE significantly downregulated the expression levels of FADD
mRNA (p < 0.05). Finally, the challenge experiment with S. agalactiae showed that 0.06% dietary ELE could inhibit
bacterial infection, and significantly improve the survival rate of GIFT (p < 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 Streptococcus agalactiae.