A 56-day feeding trial was conducted on the juvenile rainbow trout (11.5 ± 3.64 g) to assess the effect of giant milkweed (GMW) in the diet of on innate immune responses including lysozyme activity (LYZ), total immunoglobulin (Ig) and alternative complement activity (CoM). Five levels (10, 20, 30, 40 and 50 g kg-1) of GMV together with a control diet were used (n=3) at 18 raceway systems at a density of 200 fish per tank. Fish fed with the diet containing 40 g kg-1 GMV displayed significantly (p<0.05) higher activities of LYZ (5.12 ± 0.59 µg/ml), Ig (3.67 ± 0.72 µg/ml) and CoM (4.15 ± 0.12 U/ml) in the serum compared to other treatments. The results showed positive effects of GMV as a potent immunostimulant on non-specific immune system of rainbow trout.

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