"The NRC Nutrient Requirements of Horses 2007 suggests that possible adverse effects of excessive iron might outweigh any supposed advantages, and warns that supplemental iron can be toxic to foals and that iron injections often result in severe reactions and death in horses".
"The cause of the anemia must be identified and correctly treated - it's treating the primary disease, not feeding an iron supplement, that eliminates anemia in most cases."
Read full article here: https://www.thelaminitissite.org/articles/anaemia-and-iron-supplements