Horses constantly ingest dirt when they graze. While excessive dirt consumption sometimes causes problems such as sand accumulation in the large intestine, the dirt that horses normally consume while grazing supplies some essential nutrients–most notably iron. Forages and grains contain additional dietary iron for horses.

Iron is important for hemoglobin, the pigment in red blood cells that allows them to transport oxygen. Storage and transport proteins account for 20% of the iron in the body. Iron also is found in myoglobin, a muscle pigment, and cytochromes, enzymes that produce energy for cells. An iron deficiency can lead to anemia, the most common cause of which is anemia of inflammation (a normal response of the body to chronic inflammation or infection in which 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.

A team of researchers in the Netherlands conducted a study in which they investigated the effects of stall confinement on iron status in foals. 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. Blood work showed 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 iron source, since all foals were eating the same grass and only the pasture group had access to soil