Channel catfish (Ictalurus punctatus) fingerlings (average initial weight, 1.70g) were fed casein-based purified diets supplemented with 0, 0.02, 0.06, 0.20 or 0.40 mg of selenium per kg from sodium selenite (Na2SeO3), selenomethionine (Se-M) or selenoyeast (Se-Y) for 9 weeks. Data for weight gain and glutathione peroxidase activity were subjected to regression analysis to determine dietary requirements and relative bioavailability for the various selenium sources. Broken-line analysis showed that minimum supplemental dietary selenium requirements as Na2SeO3, Se-M and Se-Y for weight gain were 0.28, 0.09 and 0.11 mg kg−1, and for liver glutathione peroxidase activity were 0.17, 0.12 and 0.12 mg kg−1 diet. Relative bioavailability values of Se-M and Se-Y compared to Na2SeO3 were 336 and 269% for growth, and 147 and 149% for glutathione peroxidase activity, respectively. Selenium from Se-M and Se-Y showed significantly higher rates of accumulation in liver and muscle than selenium from Na2SeO3. This study indicates that selenium allowance in diets of channel catfish can be reduced when selenomethionine or selenoyeast replaces inorganic selenium.

Fingerlings of channel catfish Ictalurus punctatus (average initial weight, 1.70 g) were fed a casein-based purified diet supplemented with 0, 0.02, 0.06, 0.20, or 0.40 mg of selenium/ kg from sodium selenite (Na2SeO3), selenomethionine (Se-M), or selenoyeast (Se-Y) for 9 weeks. Subsequently, fish were challenged by immersion with a virulent strain of Edwardsiella ictaluri (105.9 cells/mL), and mortalities were recorded during a 14-d period. Antibody titers of surviving fish were measured, and macrophage chemotactic activity was determined in fish from each treatment. Dietary concentration and source of selenium significantly influenced weight gain and immune responses of the fish. Dietary selenium concentrations for maximum weight gain were 0.20 mg/kg from Se-M and Se-Y and 0.40 mg/kg from Na2SeO3. Dietary selenium concentration for maximum survival from E. ictaluri challenge was 0.20 mg/kg for fish fed Se-M and 0.40 mg/kg for fish fed Se-Y and Na2SeO3. Antibody production generally increased as dietary concentration of selenium increased. Antibody titer was highest for fish fed Se-Y, intermediate for fish fed Se-M, and lowest for fish fed Na2SeO3. Macrophage chemotactic response was the same for fish fed the control (0 selenium) and Na2SeO3 diets and was significantly lower than responses by fish fed Se-M and Se-Y. This study indicates that channel catfish challenged with E. ictaluri are responsive to dietary selenium and that organic selenium sources have greater potency than inorganic sources.