Dirt an Important Source of Iron for Horses

Horses constantly ingest dirt when they graze. Excessive consumption of dirt, especially when pastures become short, sometimes causes potential problems such as sand accumulation in the large intestine. However, the dirt that horses normally consume while grazing supplies some essential nutrients, notably iron. Forages and grains contain additional dietary iron. Young foals can ingest iron when they begin to consume their mare’s manure (coprophagy).

Iron is important for hemoglobin, the pigment in red blood cells that allows them to transport oxygen. A deficiency of iron can lead to anemia. Storage and transport proteins account for 20% of the iron in the body. Iron is also found in myoglobin, a muscle pigment, and cyotchromes, enzymes that produce energy for cells. The most common cause of anemia is called anemia of inflammation, which is a normal response of the body to chronic inflammation or infection. The body will sequester its iron stores at its own expense to help fight infection because bacteria also require iron for their metabolism and growth. Other common causes of anemia are chronic blood loss and parasitism.

Researchers in the Netherlands investigated the effects of stall confinement on iron status in foals.¹ The study involved three groups of foals: foals kept only in stalls, foals kept in stalls and allowed to exercise 45 minutes per day, and foals kept on pasture. The foals kept in stalls were fed freshly cut grass from the same pasture where the pastured foals were kept.

At 1-3 months of age, the foals kept in stalls became listless and blood work showed that they were anemic and had low blood iron concentrations. All foals were then started on an oral iron supplement. Iron supplementation significantly improved the anemia and serum iron concentration in stalled foals, and they became more active. The authors concluded that soil from the pasture was an important source of iron, since all foals were eating the same grass and only the pasture group had access to soil.

Iron deficiency and iron-deficiency anemia are very rare in horses, especially in grazing animals. Anemia can be caused by a wide variety of illnesses, drugs, and other nutrient deficiencies. The diagnosis of iron deficiency can be more difficult that it seems because of the effects of any concurrent illness and unreliable serum iron concentrations. Measurement of blood transferrin, an iron transport and storage protein, is more reliable to diagnose iron deficiency. If the horse has access to pasture, iron deficiency is unlikely.

Excessive iron supplementation in horses, especially foals, can be dangerous and lead to liver failure. Most anemic animals are anemic for reasons other than iron deficiency, especially if they have access to pasture. Iron supplements should only be given for conditions known to respond to supplemental iron, and the manufacturer’s recommendations should be followed to avoid possible toxicity.

1) Brommer, H., and M.M. Sloet van Oldruitenborgh-Oosterbaan. 2001. Iron deficiency in stabled Dutch warmblood foals. Journal of Veterinary Internal Medicine 15:482-485.