To study the different inclusion levels of giant milkweed (GMW) in the diet of juvenile rainbow trout (11.5 ± 3.64 g) on growth performance and biological indices, five levels (10, 20, 30, 40 and 50 g kg-1) of GMV together with a control diet were used (n=3) for 56 days in 18 raceway systems with a stocking rate 200 fish per pond. Specific growth rate (SGR) of fish fed the diets containing 40 g kg-1 GMW (2.36% day-1) increased significantly (p≤0.05) compared to all inclusion levels of GMW (mean: 2.13% day-1) and control diet (1.92% day-1). Feed conversion ratio (FCR) of fish fed the diets containing 20 g kg-1 GMW (1.31% day-1) decreased significantly (p≤0.05) compared to all inclusion levels of GMW (mean: 1.57) and control diet (1.71). With an increase of GMW in the diet from 10 to 30 g kg-1, protein efficiency ratio (PER) as one of the most important of nutritional efficiency indices increased significantly (p≤0.05) from 2.36 to 3.48 compared to control (2.17) and then it remained insignificantly constant. With an increase in GMW dose from 10 to 30 g kg-1, the levels (ng ml-1) of plasma triiodothyronine (T3) of fish fed the diets of GMV increased significantly (p≤0.05) from 2.3 to 3.5 ng ml-1 compared to control (2.1 ng ml-1). The results of present study showed it is possible to include 40 g kg-1 GMV as a supplement in the diet of juvenile rainbow trout.

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