Submitted to: Aquaculture
Publication Type: Peer reviewed journal
Publication Acceptance Date: 7/3/2001
Publication Date: N/A
Citation: Interpretive Summary: Previous studies have shown that the amount of cottonseed meal (CSM) that can be included in catfish feeds depends on the level of free gossypol and available lysine. Dietary free gossypol at 900 ppm or higher has been shown to be toxic to channel catfish. Iron from ferrous sulfate has been used to counteract the toxicity of gossypol in feeds of warm-blooded animals. High levels of iron used to detoxify gossypol may be harmful to fish because a delicate balance exists between the need of iron for host defense mechanisms and the need of iron to sustain bacterial growth. Free gossypol in cottonseed meal may also affect fish immunity and disease resistance. Thus, this study was conducted to evaluate the effect of iron supplementation to diets containing different levels of CSM on hematology, immune response and resistance of catfish to Edwardsiella ictaluri challenge. Results show that iron content in practical diets (204 to 283 mg/kg) may be insufficient for blood cell formation due to compounds or factors which inhibit iron absorption. The level of supplemental iron needed appeared to vary with the composition of the diet. For maximum hematological values, a higher level of supplemental iron (671 mg/kg) was needed for soybean meal-based diet compared to that (336 mg/kg) required for diets in which 50% or more soybean meal was replaced by CSM. However, iron present in these diets was probably sufficient to maintain normal immune function and protection against E. ictaluri. Gossypol or other compounds in CSM may have beneficial effect by improving the immune response and resistance of channel catfish against E. ictaluri infection. The higher palatability of the CSM diets may also have contributed to the improved survival of challenged fish in these treatments.
Technical Abstract: Three diets containing 0, 27.5 and 55.0% cottonseed meal (CSM) as substitutes for soybean meal (SBM) were each supplemented with three levels of iron. Each diet was fed to juvenile catfish in three aquaria twice daily to satiation for 10 weeks. Hematology immune response and mortality following Edwardsiella ictaluri challenge were determined. 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. Ht and Hb were not affected by dietary levels of iron. TCC, RBC and Hb in response to dietary levels of CSM was influenced by supplemental levels of iron. For diets containing 0% CSM, these parameters linearly increased with increasing dietary levels of iron. When CSM levels were 27.5% or higher, 336 mg iron/kg was sufficient for maximum hematological values. Macrophage chemotaxis was significantly higher for fish fed diets containing 55.0% CSM as compared to those fed the lower CSM diets. Antibody titers were also significantly higher for fish fed diets containing CSM, but the values did not differ for those fed the 27% or 55.0% CSM diets. Dietary levels of iron, and interactions between 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. The values were lower and did not differ for fish fed the 27.5 and 55.0% CSM diets. Gossypol or other compounds in CSM may have immunostimulatory or antibacterial effects leading to the improvement of resistance of catfish to E. ictaluri infection.