Trace Mineral Basics: Iron

It’s not just the heart and lungs that help circulate oxygen in the body. Iron (Fe) is a key component of hemoglobin (a blood protein) and myoglobin (a muscle protein), both responsible for oxygen transport in their respective environments. It is also found in macrophages (white blood cells that engulf and remove foreign particles), linking Fe to immune function, as well.

Interestingly, Fe utilization increases in deficient diets and decreases when horses consume excess cadmium, cobalt, copper, manganese, and zinc. Newborn animals absorb Fe more efficiently than mature animals.

Requirements and Sources

The National Research Council’s Nutrient Requirements of Horses (NRC 2007) states that an average 1,100-pound (500-kilogram) mature horse requires 400-500 milligrams of Fe per day. Intake requirements vary for pregnant and lactating mares and growing foals.

Common feedstuffs typically meet a horse’s Fe needs. The NRC estimates that cool season grass hay (e.g., Kentucky bluegrass, tall fescue, and perennial ryegrass) contains about 180-199 milligrams of Fe per kilogram, while legume hay (e.g., alfalfa) is estimated to contain 207-250 milligrams of Fe per kilogram. Even if forage makes up as little as half of a horse’s diet, good-quality hay should generally be sufficient to meet dietary Fe needs.

Deficiency and Excess

The principal sign of Fe deficiency is anemia (decreased red blood cells and/or hemoglobin in the blood). Anemia results from major blood loss. By and large, Fe deficiency is not a major problem for healthy horses at any life stage, especially if they have access to soil and pasture. However, foals might be more susceptible to Fe deficiency—mares’ milk is low in Fe and foal growth rate is high, but clinically recognized deficiency is rare.

Data on maximum tolerable Fe intake in horses is derived from what is known in other species, and has been set at 500 milligrams of Fe per kilogram of the ration. Excess Fe is stored in the liver, too much of which can cause tissue damage. Again, foals are especially susceptible to Fe toxicity in the first few days of life due to high plasma Fe concentration at birth, and high absorption rate during this period.

Research has shown that Fe supplementation has no effect red blood cell production or oxygen carrying capacity and is of little to no benefit to the horse. Iron injections can be dangerous to horses, potentially resulting in severe reactions or death.

Take-Home Message

Most forage-based diets provide sufficient Fe to meet the horse’s needs, and grain concentrates are generally fortified appropriately. Additional Fe supplementation is not warranted in healthy horses. Dietary mineral balance is key to ensure proper Fe status in the body.