Brassica by-products in diets of rainbow trout ( Oncorhynchus mykiss) and their effects on performance, body composition, thyroid status and liver histology

Abstract

An experiment was designed to assess the inclusion rate of cabbage meal in diets for juvenile rainbow trout ( Oncorhynchus mykiss). Cabbage meal was evaluated at 320 (C4), 250 (C3) and 160 g kg −1 (C2) inclusion rate and compared to a control diet (C1) without cabbage meal. Triplicate groups of trout (initial body weight: 16.6 g) were fed by hand with isonitrogenous and isoenergetic experimental diets, twice a day for 10 weeks. The results showed very high significant differences ( P<0.001) between diets on final body weight, specific growth rate and voluntary feed intake and a linear effect of substitution. Diets effected a highly significant difference ( P<0.01) on feed gain ratio and protein efficiency ratio with the substitution being explained by a quadratic contrast. The diets also promoted very highly significant differences ( P<0.001) in all coefficient of total tract apparent digestibility (CTTAD) except for crude protein in which differences were of smaller magnitude ( P<0.01). The partial replacement of gelatinised starch and fish meal by cabbage meal seems to have a negative influence on all CTTAD except ash. Diets containing cabbage meal were associated with significantly lower plasma T 3 levels ( P<0.05) than diet C1 due to the presence of anti-nutritional factors. Thus, inclusion of cabbage meal in diets for rainbow trout is acceptable up to a certain level.