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At birth, 24 Standardbred foals were assigned at random to 1 of 2 groups and were given a placebo supplement (group 1) or an iron supplement (248 mg of iron/treatment; group 2). Foals were given iron supplement or placebo 4 times during the second and third weeks after birth. Hematologic variables and general health were monitored until foals were 4 months old. Mean pcv in foals of both groups decreased during the first 2 weeks after birth, but values remained within adult horse reference ranges. During the first 6 weeks after birth, foal erythrocytes were smaller than adult horse erythrocytes, but foal erythrocyte glucose-6-phosphate dehydrogenase activity was greater than that in adult horses. At every measurement, indices of anisocytosis were lower in foals, compared with adult horse reference values, suggesting that foals have a homogeneous population of microcytic erythrocytes during early foalhood. In 2-week-old foals of both groups and in 4-week-old placebo-treated foals, mean serum iron concentration was lower than that in adult horses. In foals at birth and during the first 4 months, total iron-binding capacity values were above the adult reference range. In newborn foals, transferrin saturation percentage values were above the adult reference range; however, values decreased to below the reference range in foals from 2 weeks to 4 months after birth. When foals were born, serum ferritin concentration values were above the adult horse reference range, but decreased to within the reference range by the time foals were 1 day old. From 2 through 6 weeks after birth, foal ferritin concentration values were below the adult reference range. There were no significant differences between treatment groups in any variable measured. All foals remained healthy and grew as expected on the basis of comparison with the previous year's foal crop.

The development of low serum ferritin and iron concentrations, high serum total iron-binding capacity, low transferrin saturation percentage, and microcytosis in conjunction with decreasing pcv during the first 2 weeks after birth might be explained by foal depletion of iron stores. However, beneficial effects on hematologic variables were not observed in response to oral administration of 4 doses (248 mg each) of iron during the second and third weeks after birth. Apparently, most foals have sufficient body iron stores at birth and sufficient iron intake to support demands for iron during early foalhood.

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in American Journal of Veterinary Research