Abstract The current study aimed to investigate the effects of different sources of organic iron (native product in comparison to foreign ones) in the milk on plasma mineral concentration (Fe, Cu, Ca, P, and Zn), body weight, starter intake, feed conversion ratio (FCR), health condition, body measurements, and fecal score. Female calves (n=36) were randomly allocated to 3 groups (12 calves per treatment) and fed control milk (no iron supplement) (C), a low dose of iron (LF) consisting of 400 mg of Fe supplementation with the purity of 10% iron (native) and high dose of iron (HF) which had 200 mg of Fe supplementation with 20% purity (foreign). All animals were kept in individual pens and had ad libitum access to starter feed and water during the experimental period. From the first day of age to weaning (56 days), starter intake was measured daily. Body weight, skeletal measurements, fecal score, and health parameters were monitored weekly until weaning age. Calves that received the iron supplements had higher plasma Fe concentration, average daily gain (ADG) and starter intake, in contrast, they had low Cu, Ca, and P content and FCR than the control group. Supplementing milk with iron resulted in improved health status. Results showed that native Fe had the same impacts as the foreign product. There were no notable effects of treatment on skeletal parameters, fecal score, heart rate, respiratory rate, and rectal temperature. According to this experiment, applying iron supplements in diets requires taking into account the iron supply via other sources such as milk and or solid feeds. According to this experiment, although calves’ health and performance improved regardless of iron sources, the amount of iron supplied through other sources such as milk or solid feed should also be considered, when using iron supplements in the diet. Keywords: calf health, iron supplement,

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