Effect of soybean meal replacement by cottonseed meal and iron supplementation on growth, immune response and resistance of Channel Catfish ( Ictalurus puctatus) to Edwardsiella ictaluri challenge

Three basal diets containing 0%, 27.5% and 55.0% solvent-extracted cottonseed meal (CSM) as replacements of 0%, 50% and 100% of solvent-extracted soybean meal (SBM) on an equal nitrogen basis were each supplemented with three levels of iron (40, 336 and 671 mg/kg) from ferrous sulfate heptahydrate (3×3 factorial experiment). Each diet was fed to juvenile channel catfish in triplicate aquaria twice daily to apparent satiation for 10 weeks for subsequent determination of growth response, hematology, specific and non-specific immune response, and mortality following Edwardsiella ictaluri challenge. Fish fed diets containing 27.5% CSM as a replacement of 50% of SBM had improved weight gain (WG) and feed efficiency ratio (FER). Total replacement of SBM by 55.0% CSM decreased WG, feed intake (FI) and FER. Total cell count (TCC), red blood cell count (RBC), hematocrit (Ht) and hemoglobin (Hb) were not affected by dietary levels of CSM. Iron supplementation significantly affected TCC and RBC and maximum values of these parameters were obtained at 336 mg of iron/kg diet. However, Ht and Hb were not affected by increasing levels of supplemental iron. Values for TCC, RBC and Hb were significantly affected by the interaction between dietary levels of CSM and iron. For fish fed the diet containing 0% CSM (SBM-based diets), these parameters increased linearly with increasing dietary levels of iron. When CSM levels were increased to 27.5% or higher, 336 mg supplemental iron was sufficient for maximum hematological values. Macrophage chemotaxis in the presence of exoantigen was significantly higher for fish fed diets containing 55.0% CSM as compared to those fed the lower CSM diets. Agglutinating antibody titers were also significantly higher for fish fed diets containing CSM, but the values did not differ for those fed the 27.5% or 55.0% CSM diets. Dietary levels of iron, and interactions between dietary levels of iron and CSM had no effect on macrophage chemotaxis and antibody titers. Cumulative mortality at 15 days post-challenge was significantly higher for fish fed the SBM-based diet (0% CSM) at 54.4% as compared to 35.0% and 21.6% for those fed the 27.5% and 55.0% CSM diets, respectively. No differences were observed among mortality of fish fed the CSM-containing diets. Dietary levels of iron supplementation, and the interactions between dietary levels of iron and CSM had no effect on post-challenged mortality of fish.