Should you worry about iron overload?

By Shelagh Niblock, PAS

Horse owners who are interested in equine nutrition, and actively involved in planning the composition of their horse’s diet, will know that iron intake has become a subject of much discussion. Terms such as “iron overload” are easy to find using a Google search, and the risks associated with “free radicals” and “oxidative stress” are often coupled to the amount of iron in the equine diet. Iron levels in typical equine diets have been targeted as the reason for many equine health issues, including metabolic conditions, reduced immune function, poor hair coat and hoof wall, and developmental disease in growing horses. The internet has several popular websites available for the average horse owner to “educate” themselves about iron in the equine diet. Frequently, these sites also have products offered for sale or have links to sites that sell products that are supposed to help the horse with “iron overload.” So just what is iron overload in the equine diet, and do horse owners really need to worry about it?

Iron Metabolism in Horses

Iron is a trace mineral which, in small amounts, is essential for life but can be toxic if overfed. Iron uptake from the digestive tract of horses is very tightly regulated by a system of hormonal controls. Iron is primarily responsible for oxygen transport in the blood and muscle, in the form of compounds called hemoglobin and myoglobin, respectively. Three-quarters of a horse’s total iron stores are located in the liver, spleen, and muscles. Within the liver and spleen, a large percentage of the iron is located in the macrophages (large white blood cells), making iron an important part of the horse’s immune system. Iron is a constituent of some enzymes and, therefore, responsible for several metabolic processes in the equine body. And finally, as a component of lactoferrin found in mare’s milk, it is essential for its bactericidal role in the mammary gland of the mare, as well as the gut of the newborn foal.

How Iron Uptake is Regulated in Horses

Iron is transported from the small intestine into the enterocytes (cells lining the gut wall), by transporter proteins responsible for the transfer of several different trace minerals. Horses control iron uptake in their digestive tracts through a hormone called hepcidin, so that as more iron is transported through the gut wall, more hepcidin is produced, thereby down-regulating the transfer of iron into the horse’s circulatory system. Hepcidin also increases the amount of iron sequestered, or captured, by the macrophages in the blood stream. Circulating free iron Fe2+, liberated from spent red blood cells or macrophages, is transferred back into the small intestine via bile. This process limits any excess of oxidative free iron Fe2+ in the bloodstream that might accumulate as a result of normal red and white blood cell turnover. All of these processes serve to limit and regulate the amount of iron actually present in the horse’s body to what the horse actually needs metabolically, and to prevent excessive free iron from accumulating.

Related: Your Horse and B Vitamins

Although the typical equine diet often supplies high levels of iron, the actual incidence of true “iron overload” is rare thanks to the natural ability of horses to safeguard themselves from it by regulating their iron uptake. Photo: Shutterstock/Oksana Perkins

Once the iron absorbed from the small intestine is actually in the bloodstream, it is transported by a protein called transferrin to the liver and spleen for production of hemoglobin, myoglobin, or a storage form of iron called ferritin. Ferritin, stored in the liver and spleen, keeps extra iron not immediately needed by the horse in a safe non-reactive form, so it is available for use by the horse when needed.  Excessive free iron Fe2+ in the circulatory system of the horse will cause oxidative stress and tissue damage consistent with what is called iron overload, but fortunately, the equine metabolism is well-equipped to ensure that this rarely ever happens.

Facts About Iron and Horses

There is plenty of misinformation on the internet about the effects of excessive iron in equine diets. Always look for peer reviewed research if you are in any doubt about anything you read online. The reality is that horses do need iron in their diets. According to the National Research Council (NRC) 2007 Nutrient Requirements of Horses, mature horses require a daily intake of 40 mg of available iron per kg of dry matter intake. That means a mature 500 kg horse eating 2 percent of his body weight in dry matter will eat 10 kg of dry matter per day, thus 40 mg/kg x 10 kg = 400 mg of available iron is necessary daily for normal metabolism in our horse. As the workload of a horse goes up, the dry matter intake also increases, as does the requirement for available iron. NRC has established that growing foals and pregnant or lactating mares need 50 mg/kg of dry matter intake. Your horse’s diet probably provides iron in excess of that, but it has been well-established in research that not all of that iron is available to horses, and in addition to this, horses have an excellent ability to regulate the uptake of iron in the gut. The more iron in the diet, the less they actually absorb. Research in horses has established that on average only 20 percent of the iron they consume is actually available. A great deal of the iron they consume is not digestible and will be excreted in the manure.

Horses or foals that are given either injectable iron or large oral doses of easily digestible sources, like iron sulphate, can suffer from an iron overload syndrome, as sources such as these may overwhelm their natural iron regulatory mechanism. Toxic levels of available iron can overwhelm the iron transfer process in the circulatory system of the horse, resulting in free circulating iron, which could cause severe oxidative stress and potentially liver damage; however, iron levels of this magnitude are rarely found in typical equine diets. Although some equine diets are naturally high in iron due to the forage iron content, the availability of the iron for absorption in the small intestine of the horse is low and most of it will, consequently, be excreted.

Main Sources of Iron in the Horse’s Diet

The typical equine diet will provide at least 100 mg of iron per kg of dry matter daily. That means that our 500 kg horse eating 2 percent of its body weight in dry food per day (10 kg) will consume at least 100 mg x 10 kg = 1,000 mg of iron per day. In many cases, however, this amount may be much higher because of soil contamination in the forages our horses eat. It’s not unusual to see iron analysis on hay samples between 200 to 500 mg per kg of dry matter (of the forage). Some of this iron will be part of the grass plant, but a substantial proportion of it will be from the soil contamination inherent in stored forages for horses. Soil is loaded with iron, and despite our best efforts, horses inadvertently consume it. Even horses on pasture will be consuming some soil, but the good news is that much of the iron in soils is not available for absorption in the small intestine of a horse. If the iron consumed cannot be broken down to the absorbable form Fe2+ in the acidic environment of the stomach, it will pass through the digestive tract of the horse and into the manure.

Horses inadvertently consume some soil while grazing, but most of the iron in soils is not available for absorption in the small intestine, and will pass through the digestive tract and into the manure. Photo: Dreamstime/Zbynek Pospisil

Iron in Forages or Feeds Like Beet Pulp

The iron inherent in grass plants, such as pasture or hay, could be from either digestible or indigestible iron sources. Plants use iron in photosynthesis and store iron as a substance called phytoferritin. This stored iron is usually, but not always, available for uptake in the small intestine. The digestibility of iron in plants is a function of the maturity and fibre content of the plant.

Fermentation of the forage fibre in the hindgut will release any iron associated with it, but iron cannot be readily absorbed by the horse once it passes from the small intestine. Beet pulp is an excellent example of this. An analysis of beet pulp will show that it often contains high iron levels, but two factors significantly reduce its potential impact on the iron status of the horse. Firstly, the iron in beet pulp is largely in a form unavailable to horses, and therefore will pass through the gut into the manure. Secondly, beet pulp is loaded with a hindgut fermented complex carbohydrate called pectin. The iron in beet pulp tends to be associated with these complex carbohydrates and will only have the potential to become free iron once the beet pulp has undergone fermentation in the hindgut. Because the horse’s gut absorbs iron primarily in the small intestine, the chance of free iron in the hindgut being absorbed is greatly reduced, if not entirely eliminated.

Commercial Mineral Supplements

Iron coming from your mineral supplements and manufactured feeds will have varying availability depending on the source. Iron oxide is completely unavailable to the horse, while iron sulphate may be more available depending on other factors in the equine diet. Many equine mineral supplements utilize dicalcium phosphate (DCP, often referred to as “Dical”), as both a calcium source and a phosphorous source. Along with calcium and phosphorous, DCP carries about 1000 mg/kg (parts per million or ppm) of iron as an inherent part of the rock it came from. Manufacturers often call this iron “background noise” as it is present but was never part of the formulation of the mineral supplement. Most equine supplement manufacturers include this iron on the product tag as part of total iron. If you are trying to source a low iron mineral, the “background noise,” iron might be a source of alarm; however, there is no research that demonstrates that the iron present as a contaminant in dicalcium phosphate is at all available to horses.

There are mineral products available that have been formulated to be VERY low in total iron. These supplements will be using a phosphorous source other than DCP. Be sure to check the tag of any products claiming to be very low in iron, as they may also be low in calcium depending on the phosphorous source used. A low calcium mineral is not necessarily a bad thing for a horse getting calcium from something like alfalfa hay, but if you have a performance horse or, in particular, a growing horse, you should consult with a qualified equine nutritionist or your veterinarian as to whether the mineral is suitable for your horse.

Iron in Water

Water can actually be a more potent source of free iron in your horse’s diet than any feed because frequently the iron in water is in the soluble ferrous (Fe2+) form, as opposed to the more stable ferric (Fe3+) form. Horses can utilize iron in water in the ferrous form much more easily than iron in the ferric form. Water can be a more insidious source of available iron in the equine diet, and should be tested if you have concerns.

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Because water can be a more significant source of free iron in the horse’s diet than any feed, have it tested if you have concerns about the amount of iron in your water. Photo: Shutterstock/Marie Charouzova

Use Caution When Analyzing Hay for Iron

A forage analysis is always a good place to start when evaluating your horse’s ration, but checking for trace mineral content requires extra care in order to get reasonably accurate results. The potential for sampling error is sizeable when acquiring a forage sample, and because of normal lab deviations in results, all trace mineral analyses should be evaluated cautiously.

Trace mineral analysis must be done using wet chemistry methods, rather than near infrared reflectance (NIR) spectroscopy (see Hay Testing with NIRS: Does It Make Sense for You This Year? CHJ Summer 2020 issue article explaining NIR lab analysis versus wet chemistry lab analysis). NIR analysis is not an accurate way to analyze for trace minerals, including iron in forages. Because of the very high risk of soil contamination in stored forages, like hay or haylage, the best way to get an accurate sample is to core your bales and to avoid bales that have been sitting on the ground for any length of time. Still, even with the utmost care in sampling, it is inevitable that some of the iron reported in your analysis will be due to soil contamination, and your evaluation of the forage for iron content should take that into consideration.

Normal Lab Error

It is difficult to get an accurate result when analyzing a forage sample for iron. Statistically, the repeatability of iron results is very poor. In other words, you could test the same sample for iron ten times and get ten widely different results. This is a function of sampling error and laboratory error, but the bottom line is that an iron analysis on your hay is an educated guess only.

A good example of this is at www.equi-analytical.com — Common Feed Profiles tab. A quick look at the iron analyses for MMG (mixed mainly grass) hay in the Equi-Analytical database shows that, over the last 16 years, 25,767 samples have been analyzed and the average result for iron was 342.776 mg/kg of MMG hay on a dry matter basis. BUT the range in iron content was anywhere from 0 mg/kg up to 12,833.906 mg/kg, and the standard deviation, a statistical measurement calculated to indicate the extent of the potential deviation of your sample from the reported amount, is 12,491.130 mg/kg! Any statistician will tell you that results like these suggest that it just is not possible to accurately assess iron content in MMG hay.

The legume hay database is not quite so dramatic but still astounding in its range — 52,623 samples were analyzed for iron over 16 years, and the average iron content was 400.913 mg/kg, but the range of results were from 2.599 to 799.227 mg/kg, with a standard deviation of 398.314 mg/kg. These numbers do not instill confidence in the minds of equine nutritionists when they are trying to make a realistic assessment of the danger of iron overload in an equine diet.

How to Assess the Results of Blood Analysis

If you are concerned about the risk of iron overload in your horse, you may ask your veterinarian to draw a blood sample for analysis. Your veterinarian can help you assess the results when they come back, as your lab report will show a number of iron related terms: iron content or serum Fe, ferritin, and two measurements called total iron binding capacity (TIBC) in mg/l, and transferrin saturation index (TSI) in percent. The serum Fe is not a good indicator of iron status in your horse’s body, but it is useful when it is used with the TIBC to calculate the TSI percent, which is an important measurement. The serum ferritin is not very sensitive in terms of excessive iron intake, but can be a useful measurement in determining low iron status in horses.

The Actual Risk of Iron Overload in Horses

Although the typical equine diet often supplies high levels of iron to our horses, the actual incidence of true iron overload is rare because of the low digestibility of iron in sources like forage in equine diets, and the excellent ability of horses to regulate iron uptake in the small intestine. The horse with ulcerative conditions in the small intestine could have their ability to regulate iron uptake impacted, and so could be at more risk of iron overload. There has been speculation, driven by some websites, that the metabolic horse — in particular, the horse with equine metabolic syndrome (EMS) or pituitary pars intermedia dysfunction (PPID) — could be at greater risk of iron overload, and that many of their health issues are driven by excessive iron in their diets. This has never been established in peer reviewed research. The fact that iron overload in horses is very rare is likely because of the innate ability of horses to safeguard themselves from the abundant sources of it in their environment. Horses have evolved in a high iron environment over millennia and, as the excellent adapters they are, have demonstrated that they have the situation under control. Characteristics such as poor hoof wall, poor hair coat, reduced immune function, the incidence of insulin resistance and of PPID are all valid concerns for horse owners, but research has demonstrated that they are not often the result of iron overload in equine diets.

The Bottom Line

High iron in forages is frequently a function of soil contamination. Soil in forage brings its own set of quality problems, so if possible, avoid buying hay that is dusty or dirty. If you have a metabolic horse or one with health issues, ask your veterinarian to do a blood panel and ask for their help in assessing the results.
Remember - association is not causation. In other words, just because you have a horse with a poor hair coat, and your lab analysis suggests your hay may be high in iron, does not mean the poor hair coat is caused by the iron in your horse’s total diet. Buy the best mineral you can find that meets the needs of your horse. Many good equine minerals have no added iron because it is simply not necessary. Don’t worry about “background noise” iron because it is probably not available to your horse’s metabolism anyway, and research has never established that it causes a problem for horses.
Remember that the actual incidence of iron overload in horses is rare, and equine health issues can be multifaceted. If you have concerns about a health problem in your horse, contact your veterinarian and establish a plan to help diagnose the issue. If you want to evaluate your horse’s diet for its realistically available mineral status, consult with a qualified equine nutritionist. And finally, if you want good information on iron in your horse’s diet, make sure you consult websites that base their information on peer reviewed research.

Related: How to Choose Feed Supplements

Information for this article was sourced from summaries of peer reviewed research provided in the National Research Council 2007 Nutrient Requirements of Horses and by Equine Applied and Clinical Nutrition, edited by R.J Geor, P.A. Harris and M. Coenen. Both these resources are available for purchase online.

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Photo: iStock/Middelveld