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Iron: An Important Trace Mineral in Equine DietsBy Kentucky Equine Research Staff · February 21, 2013

Horses need calcium, phosphorus, and a number of other minerals in their diets. Trace minerals are those that are required in smaller amounts than the major minerals. Though the quantities are not large, these trace minerals play vital roles in the health and development of horses. Many minerals are supplied in forage (grass and hay) or grain products, and others may be added in supplements that supply one or more specific nutrients. In order to get the best performance from their horses,
owners and trainers may begin by adding one or two nutritional supplements, including others from time to time as they hear of new products and the athletic successes they seem to guarantee. In some cases, high levels of some minerals are not only unnecessary, but can actually be harmful to the horse.

Iron is usually, at least from the standpoint of the layman, the first trace mineral that is considered in terms of supplementation. A survey conducted at a California race track indicated that a large majority of trainers had their horses on some type of iron supplement. This concern with iron stems from the well-known function of iron as part of the heme molecule. This is but one of the important functions of iron, but is the basis of the interest in iron for the performance horse.

The ancient Greeks recognized iron deficiency anemia and were said to have consumed water in which an iron sword had been placed in order to correct the symptoms of iron deficiency. As early as 1886 it was shown that pure crystals of horse hemoglobin contained 0.33% iron. Cytochromes, catalases, and peroxidases are other iron-containing compounds that were discovered as early as the late 1800’s. The cytochromes such as hemoglobin are involved in oxygen utilization via electron transport.

Hemoglobin, the component of the red blood cell that allows oxygen to be carried to the tissues, requires iron for its synthesis. It appears that the cytochromes and enzymes associated with tissue metabolism as well as myoglobin take precedence in the supply of iron in the body and as such the first deficiency symptom associated with iron is anemia. The anemia associated with iron deficiency is hypochromic microcytic anemia. There are few instances when practical diets for horses would result in iron deficiency anemia. In the previously mentioned study, horses that received supplemental iron had iron levels that fell into the normal range for iron in adult horses. Very few of the horses examined had any evidence of anemia, and those with  resting hematocrits below 34% (defined as anemia) showed no evidence of impaired iron status. This scenario is frequently the case, and it is rare that a horse with lowered hematocrit responds to supplemental iron with a concomitant increase in hematocrit. Clinically significant anemia in the athletic horse is indeed a rare entity.

One of the significant aspects of iron metabolism that buffers the horse and other animals from iron deficiency anemia is the ability of the body to conserve iron. Approximately 67% of the iron in the body is contained in the red blood cells in the form of hemoglobin. Red blood cells are formed in a process known as erythropoiesis and remain in the circulation for about 150 days.

When these red cells die, iron contained in the heme molecule is recycled and utilized to synthesize new heme molecules and red blood cells. As such, there is rarely a net loss of iron from the body. It is interesting to note that exceptions to this are severely parasitized horses, horses with gastric ulceration that leads to blood loss, and possibly horses that suffer from severe exercise-induced pulmonary hemorrhage.

One study indicated that more than 80% of the horses at the racetrack had some degree of gastric ulceration or at least some erosion of the gastric mucosa. Whether this problem has resulted in net blood (and therefore iron) loss is still subject to some speculation. The fact that iron recycling is effective and iron deficiencies are rare is apparent from the lack of effectiveness of some of the iron tonics in increasing packed cell volume and hemoglobin concentrations in the blood of horses in training. More times than not, low hematocrits are an indicator of infection, low-grade systemic disease, or even perhaps marginal B-vitamin status brought about by stressed large colon or cecal microbial populations rather than a deficient dietary intake of iron.

Iron appears to be efficiently absorbed at low levels of intake. Iron absorption is decreased as iron intake increases and also with the presence of high concentrations of copper, zinc, manganese, cadmium, and cobalt. Additionally, due to the effective iron conservation mechanism, net iron excretion is low, and iron appearing in the feces appears to result primarily from unabsorbed dietary iron rather than from net endogenous fecal losses. Research has failed to show an increase in hemoglobin, packed call volume, or serum iron when ponies were supplemented with high levels of iron.

There appears to be some iron lost in the sweat. The concentration of iron in horse sweat is estimated to be about 21 mg/l. Horses in intense exercise may lose as much as 25-30 liters of sweat per day. and this sweat loss would represent a net iron loss in the sweat of 500 mg/day. With these sweat losses and the possibility of greater synthesis of hemoglobin and myoglobin (muscle iron containing compound) in the athletic horse, there may be reason to believe that the athletic horse has greater requirements for iron than does the mature sedentary horse.

Practical dietary constituents contain from 40-50 ppm iron for the cereals, 100-150 ppm iron for the oilseed meals, and 200-1,000 ppm iron for many of the forages. Soil in forage samples results in abnormally high iron values and may lead to errors in assessing the iron status of the diet, so care must be taken in sampling of pastures to avoid soil contamination. Another factor to consider in assessing the iron concentration in a manufactured feed is the amount and source of any dicalcium phosphate in the diet. Rations containing much dical, especially supplements or fortification pellets, may contain as much as 1,000 ppm iron. It is probable that most of this is iron oxide that is poorly absorbed. Questions remain, however, as to the impact of this iron on the efficiency of absorption of other minerals.

So, does the average horse need iron supplementation beyond what is ingested in forage and grain products? The answer is that most horses probably don’t need to consume extra iron as long as they are being fed plenty of forage and the recommended amount of a commercial feed designed for the level of exercise the horse is asked to perform. Oversupplementation of iron rather than a low dietary level is more likely to be a problem in performance horses. Excessive dietary iron in horses, can lead to liver failure, especially in young foals.