Dietary Nonenriched and Iron‐Enriched Yeasts Improve Hematological and Antioxidant Parameters in Rainbow Trout, <i>Oncorhynchus mykiss</i> , Fed on Diets Containing Cottonseed Meal

This study aimed to evaluate the effects of dietary cottonseed meal (CSM) as a partial substitute for soybean meal, along with iron-enriched and nonenriched <i>Saccharomyces cerevisiae</i> (PTCC 5052) on growth performance, anemia, iron and gossypol retention, and hepatic antioxidant and histological characteristics in juvenile rainbow trout, <i>Oncorhynchus mykiss</i>. Fish (31.6 ± 0.33 g) were distributed across 16 tanks in four quadruplicate treatment groups. A control diet without CSM, a diet containing 15% CSM (CSM), a diet with 15% CSM plus 1 × 10⁸ cfu/g of <i>S. cerevisiae</i> (CSMY), and a diet with 15% CSM plus 1 × 10⁸ cfu/g of iron-enriched <i>S. cerevisiae</i> (CSMYFE) were fed to the treatment groups. After 8 weeks of feeding, there were no significant differences in growth performance, feed efficiency, total/differential leukocyte counts, plasma iron concentration, alanine aminotransferase, aspartate aminotransferase, and hepatic antioxidant and histological characteristics among the treatment groups. Blood erythrocyte counts and hematological indices were similar across treatments, but the CSM group exhibited significantly lower blood hematocrit (<i>p</i> = 0.005) and hemoglobin (<i>p</i> = 0.002) levels compared to the other treatments. Hepatic iron concentration was significantly higher in the CSMYFE group than in the other treatments (<i>p</i> = 0.001). Hepatic gossypol concentrations in the CSM, CSMY, and CSMYFE treatments were similar and significantly higher than that of the control treatment (<i>p</i> < 0.001). Plasma total antioxidant capacity (<i>p</i> = 0.002) and ascorbate level (<i>p</i> = 0.025) were significantly elevated in the CSMY and CSMYFE groups compared to the other treatments. In conclusion, a dietary inclusion of 15% CSM as an alternative to soybean meal does not negatively affect fish growth performance, hepatic histology, or antioxidant parameters; however, it does result in decreased hematocrit and hemoglobin levels while increasing hepatic gossypol levels. The inclusion of <i>S. cerevisiae</i>, whether iron-enriched or nonenriched, mitigates the decreases in blood hematocrit and hemoglobin levels and enhances hepatic antioxidant parameters.