Supplementation of Zn, Cu, and Fe in a commercial-type, plant-based diet for rainbow trout

A three-variable, central composite design coupled with response-surface regression analysis was used to examine the effects of dietary iron (Fe), copper (Cu), and zinc (Zn) on growth and physiological performance of juvenile rainbow trout fed a plant-based diet. Each micromineral was tested at five levels for a total of fifteen dietary combinations (diets) and fed for 12 weeks. Growth performance as measured by weight gain (WG) and feed conversion (FCR) were significantly affected by dietary supplementation of all three microminerals. Significant interactions between Cu x Fe for WG and Cu x Fe, Cu x Zn, and Zn x Fe for FCR were also observed. After removing non-significant terms, response-surface regression modeling showed that for optimal growth performance, rainbow trout require approximately 3-4 mg/kg Cu, 80-90 mg/kg Fe, and 35-40 mg/kg Zn supplemented in a plant-based diet. Supplementation of Zn and Cu but not Fe also affected survival of rainbow trout, with similar added concentrations of Zn (4 mg/kg) and Cu (35 mg/kg) providing the best predicted survival. Dietary supplementation of Zn and Cu also exhibited an interactive effect on whole body (WB) deposition, where increased dietary addition of one reduced WB concentration of the other. Alkaline phosphatase (Zn-requiring) and ceruloplasmin (Cu-requiring) but not catalase (Fe-requiring) enzyme activities were affected by their native, active site-requiring microminerals, but activities were not affected by interactions between the supplemented microminerals. Recommended dietary supplementation made by the National Research Council (NRC) for Fe (60 mg/kg), Cu (3 mg/kg), and Zn (15 mg/kg) for rainbow trout were determined using semi-purified or fishmeal-based diets. The optimal, predicted supplementation level for a plant-based diet in this study was similar for Cu but higher for Fe and Zn. These differences are largely due to the observed interactions among these microminerals with regard to growth performance. Moving to an all-plant feed for rainbow trout will require updates to supplemented micromineral concentrations for the best growth performance.