This is research for human pregnancies, but is of interest.

From: http://www.acu-cell.com/femn.html

"Iron & Manganese: Both elements share left-right-sided cell receptors and are considered essential to human health. As is the case with all other associated mineral pairs, the absorption of iron (Fe) is dependent on manganese (Mn), however with manganese being frequently lower than iron, and since
iron can provoke a number of problems when supplemented (constipation, gastric upset), the addition of manganese, when low also, is vitally important. This not only prevents further ratio conflicts between the two, but also substantially reduces the amount of iron needed when manganese is supplemented at the same time.

While some sources claim that manganese lowers iron levels, this is mostly a theoretical consideration that would only happen under unusual circumstances. In actual clinical settings, I have not seen a single incidence of a patient's iron (ferritin) levels decline as a result of taking manganese, even when doses
as high as 150mg per day were supplemented on an ongoing basis. In fact, most minor iron-deficiency situations can be dealt with by using manganese alone - without any iron - which reduces any possible adverse effects that can be part of routine iron supplementation.

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Pregnant women are a most vulnerable target for
either insufficient, or excessive iron supplementation, with the latter being able to trigger 'toxemia of pregnancy' (high iron causes excessive sodium retention), and in which case higher amounts of folic acid should have been given instead.
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Also

http://www.umm.edu/altmed/ConsSupplements/Ironcs.html
... absorption of dietary iron, pregnancy, and the rapid growth ... Severe iron overdose occurs when amounts of iron ... Chromium, Copper, Iodine, Iron, Manganese, Molybdenum, Nickel, Silicon ...

and
http://www.osti.gov/energycitations/product.biblio.jsp?osti_id=20487510

I know the position you are in. I was in an area where the well system was "improved", resulting in a bout of birth defects in puppies in my litters, as well as a neighbor whose bitches started having "split seasons" and could not be bred. Both of us went to reverse-osmosis water purification systems for our show/breeding dogs, and the problems stopped immediately. This is not a good possibility for you, considering the amount of water each horse needs per day.

I found a little more

http://www.redwings.org/HTMLarts/ironsta.htm

"Animals have developed sophisticated mechanisms to conserve this essential element and at the same time to maintain iron in a safely bound or chelated form. Iron is one of the few elements for which internal balance is maintained by controlling intestinal absorption of dietary intake, rather than regulating excretion. The body does not have a regular means of excreting or eliminating excess iron when iron accumulates as a consequence of excessive iron administration or blood transfusion. Iron overload has been shown to cause acute massive hepatic necrosis in certain circumstances in calves, lambs and foals. Iron accumulation as the result of hemochromatosis in humans is reported to result in chronic liver damage. A critical balance must be maintained between the body's absolute requirement for iron and protection of the body from the potential adverse effects of iron overload."

"Carlson heads a research project where data was collected from a large group of horses in training. Most of the horses were relatively young, with maidens accounting for nearly one-third of the horses sampled. Over half of the horses had received intravenous iron during the 3 months prior to sampling. There was no significant relationship between the trainers perception of performance and serum iron. Interestingly, there were no significant differences in serum iron between horses which had never received injectable iron and horses that occasionally or regularly received parenterally administered iron.

Overall the serum iron and iron binding capacity of these horses in training were remarkably similar to values reported for clinically normal sedentary horses. Only a few horses were judged to be anemic, and none had low serum iron. Dietary iron supplementation was practiced by most trainers, and in many instances this supplementation seemed excessive. However, iron overload, as judged by serum iron concentration, did not appear to be a common problem in the group of horses sampled."

http://horse.purinamills.com/bulletins/musthaveminerals.htm

Iron (Fe)
Fast fact: Corticosteroids can dramatically increase the level of iron in a horse's blood 48 to 72 hours after administration.

How your horse uses it: In oxygen transport and hemoglobin production, making iron critical to a horse's endurance potential. Although it is common practice to give iron supplements to equine athletes to enhance performance, additional iron provides no benefit unless the horse is deficient.

Where it's found: In most forages and grains in ample amounts for horses in all stages of work and development. The horse has the ability to absorb more iron from his diet in times of need.

Dietary requirements: The iron requirement for most horses is 40mg/kg of diet.
If he doesn't get enough: With rare exceptions in foals, iron deficiency occurs only in horses who have had chronic or severe blood loss or long-term parasite damage. When iron stores are depleted from the liver, spleen and bone marrow, the affected horse becomes anemic with a marked decrease in exercise tolerance.

If he gets too much: Iron toxicosis is caused almost exclusively through injected supplementation, which bypasses the normal absorption process. Horses with high levels of iron in their blood are more prone to bacterial infection. Foals who receive too much iron may become comatose and die.