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For a few years now, the excess of iron in horse feed has increasingly come to the attention of concerned horse owners in countries outside Germany.
Overloading of the metabolism with the excessive iron present in the feed is held responsible for many physical symptoms such as a weak immune system, metabolic disorders as well as developmental disorders in young horses.
The main symptom however, of iron overload is supposed to be poor hoof horn.

As a result, more and more horse owners are trying to feed their horses with the lowest possible iron content in mineral feeds as well as their daily basic feed intake.
Let’s take a closer look at this trace element, which occurs naturally in large quantities in all green plants.

Iron is an essential trace element

Iron is an essential trace element that is needed by the body in the mg range per day but can actually cause serious physical symptoms if it is overloaded or oversupplied.

The absorption of iron from the feed via the intestinal wall is therefore very strictly controlled by a hormone called hepcidin.

In the body, iron is primarily responsible for transporting and storing oxygen in the blood and muscles, in the form of haemoglobin and myoglobin. Liver, spleen, and muscles – the organs that, among other things, store blood and can make it available to the body when required – account for three quarters of a horse’s total iron pool. A considerable proportion of iron is present in the macrophages in the liver and spleen, making it an important element of the horse’s immune system.
Iron also plays a role in some enzymes and is therefore responsible for various metabolic processes in a horse’s body. As lactoferrin, for example, it has a bactericidal effect, which in turn demonstrates the link to immune defence.

Only 10% of the iron found in food is absorbed

Iron is generally absorbed in the small intestine. Here, Fe3+ is first reduced to Fe2+ before it is transported via the iron transporter resp. the metal transporter 1 DMT-1 into the enterocytes, the wall cells of the small intestine. There it initially ends up in the labile iron pool, which acts as a temporary store. If required, iron is then transferred from the enterocytes into the blood via the transporter ferroportin.

The absorption of iron from the unstable iron pool of enterocytes is regulated by the hormone hepcidin, which is produced in the liver. The liver is therefore predestined to control iron absorption, as it is one of the largest iron stores in the body and is therefore the first to receive the information when the iron stores are depleted. If there is enough iron in the body, more hepcidin is formed and iron absorption is stopped.

If the body is deficient in iron, hepcidin production in the liver is reduced and the absorption of iron from the enterocytes is promoted via ferroportin. In the blood, the iron is bound to transferrin transported and is now ready for uptake into the cells, for example the hepatocytes of the liver.

Only 10% of the iron contained in the feed is absorbed by the horse. This is therefore sensible because green plants contain large amounts of iron. They need it to build up the green leaf pigment chlorophyll, which in turn enables the plant to convert sunlight into chemical energy (sugar), a process known as photosynthesis. In a natural (grazing and foraging) diet, horses therefore absorb significantly more iron from their basic feed than their own bodies require. In the course of evolution, mechanisms have been established that prevent the metabolism from being overloaded with this trace element.

Horses require between 400 and 500 milligrams of iron per day, depending on their age and use. The recommended intake is therefore 40 milligrams of iron per kilo of dry feed.

Horses can regulate high iron levels in their feed well

If we look at the values in the basic feed, we see that the iron content fluctuates greatly and often far exceeds 400mg per day. Horses have always been used to coping with high levels of iron in their diet. The absorption of iron is so strictly regulated that an excess of iron from the feed does not occur under normal conditions, provided the horse is healthy.

It can become problematic if iron is fed by humans, for example as iron chelate (i.e. in organic form). This is because such organic trace elements bypass the natural regulatory mechanisms for mineral absorption. Iron chelate is therefore not absorbed via the normal route taken by the Fe3+ or Fe2+ found in plants. Instead, it is absorbed via the amino acid transporter in the intestine, so that a higher release of hepcidin practically comes to nothing, as the amino acid transporter does not react to hepcidin.

Excess (organic) iron cannot be excreted again!

Unlike other trace elements such as zinc or copper, the body cannot remove excess iron from the body via the kidneys or intestines. Once absorbed, it is stored in the liver or used for the corresponding metabolic processes. If organic iron (iron chelate) is fed, this storage in the liver eventually leads to liver inflammation due to the constant excess of iron. If no action is taken, liver failure will occur followed by the death of the horse.

In the case of iron poisoning, the feed containing the organic iron must be immediately stopped and iron chelators prescribed by the vet should be administered. Such iron chelators are able to bind the iron in the blood. It is extremely rare and are usually caused by the supply of organic iron by a mineral balancer or supplementary feed. Well water in areas where there is a lot of iron in the soil often has very high iron levels, often as a result of the fertilisation of surrounding agricultural land.

Sometimes horses are given an iron supplement as their blood test results suggests the horse is suffering from anaemia. In this case it would be advisable to first find the cause of the anaemia and differentiate an actual iron deficiency from a low number of erythrocytes and/or low haemoglobin levels, which can also have causes other than an iron deficiency.

In addition, chronic and acute inflammation can also lead to low iron levels in the blood test result. In this case, the body often tries to eliminate the inflammation through a self-induced iron deficiency, which results in anaemia. If the inflammation is treated, the anaemia resolves itself. Iron supplementation would therefore be counterproductive.

Poor horn quality is not a consequence of iron overload

The widespread assumption that excess iron in the diet leads to poor hoof horn quality in horses is certainly a twisted misinterpretation. Ferritin levels in the blood are often elevated in cases of equine metabolic syndrome and insulin resistance. This increase results from a derailed metabolism and not from excessive iron intake. In metabolic syndrome cases, the reduced iron intake is also not the way forward to help the horse to better health, instead an appropriate diet and increased exercise will help regulate the metabolism.

If the insulin resistance or metabolic syndrome is brought under control with the appropriate therapeutic measures, the ferritin values will also return to a normal level – quite independently of the iron values in the feed (provided no feed with organic iron chelate is given).

Only an increased intake of haem-iron, which is only found in meat products, has been shown to increase the risk of type II diabetes, which of course has no relevance for horses as herbivores.

Poor hoof horn quality can have many causes, none of which have anything to do with iron intake via the feed. These would include disorders that are associated or even followed by metabolic syndrome and insulin resistance; the feeding of organic selenium (selenocysteine, selenomethionine, selenium yeast) or a lack of sulphur, which in turn is often linked to incorrect fermentation (dysbiosis) in the large intestine, intestinal mucosal inflammation or detoxification disorders in the form of Kryptopyrrole (KPU).

Zinc deficiency also contributes to poor horn quality, as does improper hoof trimming, unfavourable ground conditions (e.g. constant moisture or prolonged drought) or lack of exercise. Excess iron, which can be provoked by feeding organic iron from a mineral balancer or supplementary feeds, primarily leads to liver failure, but not to poor hoof horn.

Iron chelate, i.e. organically bound iron, generally has no place in horse nutrition. You should therefore pay attention to this when selecting mineral feed and other supplements.
© Adobe Stock / pimmimemom

Normal nutrition does not lead to excess iron

If the horse is fed a normal diet of hay or grass (which you can also have tested for iron content if in doubt but should not be surprised at the possibly high values), gets tap water or well water of impeccable quality and is fed a mineral feed with an inorganic iron source, you are definitely on the safe side.

Well water should be checked for quality and mineral content at regular intervals regardless – tap water from the public supply system is automatically monitored by the water suppliers in most countries.

Conclusion

Iron chelate, i.e. organically bound iron, has no place in horse nutrition due to its inability to remove iron from the body. The regulation of iron levels in the horse’s blood and tissues takes place via strictly regulated uptake and not via increased or reduced release, so the uptake regulation must never be tricked by the administration of organic variants.

If the horse is given a lot of mineralised feed supplements, it may be worthwhile to calculate what and how much is given to the horse every day. Often enough, the findings are that there are completely different minerals in excess or in deficiency or in forms that are not well tolerated (especially organic compounds), which can upset the metabolism and cause a number of visible symptoms.

Team Sanoanimal