The present study investigated the effect of iron supplementation on thyroid hormones concentrations, oxidative stress characteristics, complete blood count (CBC) and serum biochemistry, and performance and health in calves of two groups: test group (n = 17, intramuscular injection of 500 mg iron-dextran on the 2nd day of life) and control group (n = 17, intramuscular injection of normal saline at the same time). Blood samples were taken from all of the calves instantly after birth and at 7, 14, 21, 28, and 90 days of age. Duration of treatment for diseases, body weight, height, and length were measured at each sampling time to evaluate growth and health. The results indicated that age (sampling time) had a significant effect (p ≤ 0.05) on the values of all measured variables except for ferric reducing ability of plasma (FRAP) amount and band neutrophil number. The treatment (group) had no significant effect on the amount of measured variables. Significant interactions between sampling time and group were found for white blood cell count (WBC), adult neutrophil, and bilirubin amounts (p ≤ 0.05). Significant higher amounts of hematocrit (HCT) and hemoglobin (Hb) at day 28 and mean corpuscular volume (MCV) at day 7 were seen in the test group compared with control group. Test group in comparison with control group had significantly lower amount of WBC at day 7; mean corpuscular hemoglobin concentration (MCHC), band neutrophils, and lymphocytes at day 14; and number of platelets at day 28. Growth and total weight gain and mean daily gain had no significant differences between the two groups. The duration of treatment for pneumonia and diarrhea was not significantly different between trial groups. The results show that parenterally administration of iron to newborn calves had no effect on improving erythropoiesis, performance, thyroid metabolism, oxidative/antioxidative situation, and health of dairy calves.

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