Iron in Horse Feed

Iron as a trace element is found in the soil and in plants only in small quantities, but it has an important role in equine health, as it is involved in metabolic processes and enzymatic reactions in the body. But how much iron is found in horse feed and why does my horse need it?

Why do horses need iron?

As a trace element, iron is responsible for several important functions in the equine organism. About 60% of the iron requirement is for the formation of the red blood pigment haemoglobin, and 20% for the formation of myoglobin. Haemoglobin is found in the red blood cells (erythrocytes) and can bind oxygen. A sufficient supply of oxygen forms the basis for a consistent supply of erythrocytes. The presence of iron is also essential for a functioning blood and muscle metabolism. This trace element is also needed for the formation of various enzymes.
 

Iron’s path through the horse’s body

Iron as a vital and essential nutrient is taken in through feed and absorbed in the entire gastrointestinal tract – particularly in the small intestine. There it enters the cells of the intestinal epithelium and subsequently into the blood, where it is bound and transported by the protein transferrin. Over 70% of this bound protein is needed for the creation of haemoglobin in the bone marrow. The rest is stored in the iron deposits in the liver and spleen (in the form of haemosiderin and ferritin). Iron can also partly be stored directly in the cells of the mucosa of the small intestine.

Iron can generally be present in different valences. Divalent iron (Fe2+ or also haem iron) is dominant in animal products and has good availability with a resorption rate of 15-40%. Plant feeds, however, mostly contain trivalent iron (Fe3+ or non-haem iron), with a resorption rate of only 1-15%. If the body’s iron reserves run low, plant iron absorption can still be upregulated, whereby higher amounts of Fe3+ are reduced to Fe2+ and thus made available. The absorption rate is therefore essentially dependent on the iron’s valence. However, other nutritional components can also have an influence. Grains, brans, soya products and oil seeds (i.e. their tannins, oxalic acid, phosphates, polyphenols and phytic acid) can hinder absorption. The iron’s availability is increased through gastric acid, as this reduces the Fe3+ to Fe2+ and thus into a soluble state.

Adult horses have a total of 95 mg iron per 1 kg body mass (32% in muscle, 36% in the blood, and 13% in the bones). Of note here is the high iron content in the bodies of foals, whereby 80% of this iron is found in the blood. This is because growing foals need more iron while blood is being created, and come into the world with higher iron reserves to compensate for the minimal amounts of iron they get through milk.
 

Can too much iron be toxic?

All iron metabolism is subject to a strict control mechanism, as iron performs vital functions, yet can be toxic in high concentrations. The peptide hepcidin, which is synthesised in the liver, is also known as a protein metabolism hormone. If the iron level is too high, the production of hepcidin is increased in the liver, which decreases iron absorption via the bowel. If the iron level falls too low, less hepcidin is produced and more iron is reabsorbed. Excess iron can also be excreted in small amounts through the bile, faeces, and urine. This protective function, however, only works sufficiently if iron intake is not excessive. Over time, large amounts of iron can have a negative impact on the utilisation of phosphorus, copper, manganese and zinc in the organism. A guideline value in the literature is given as a maximum of 1000 mg iron per 1kg dry matter in the total ration.

Iron in feed

Looking at the iron content of hay, one notices that it fluctuates greatly not only between years, but also within the year. Looking, for example, at hay analyses from 2016 to 2021, the range for iron fluctuates between 31 and 6.219 mg per 1kg of dry matter. The type and pH level of soil, climate, fertilisation, botanic composition of the crops, vegetation stage, and methods used for haymaking and preservation can all have an impact on trace element content. Herbs and legumes, for example, are richer in trace elements than grasses are. In addition, an earlier cutting tends to mean a higher iron content. Of note is that iron is always abundant in the forage.

Iron is often found in abundance in the soil, so that the iron content in hay may rise in the case of feed contamination. That may be why there is a correlation between the hay plant’s length at the time of cutting (or the type of machine used) and the hay’s iron content. The lower the height of the cutting, the higher the hay’s iron content.

In the forage drying processes, warm-air aeration systems brought significant improvements with regard to feed contamination and parallel to iron content.

Iron deficiency

An iron deficiency may sometimes be unrelated to nutrition, for example when a horse loses lots of blood due to an injury. To be able to make a definite assessment of a possible iron deficiency, one would have to determine the serum ferritin level in the blood. Overall, it is vital to find the cause, as secondary iron deficiencies can be the result of infections or parasite infestations.

In addition, some horses require more iron; namely, those with higher sport performance demands, such as race horses. The expansion in the blood volume leads to a slight increase in the iron requirement. Nevertheless, targeted iron supplementation is not recommended, as a healthy, balanced diet already supplies sufficient quantities of the trace element. Horses with severe parasite infestations can also have iron deficiencies.

Low haematocrit and haemoglobin levels in the blood may point to an iron deficiency as well. Nevertheless, these parameters are nonspecific because such low levels may have other causes, such as infection, parasites, or other nutritional deficiencies. The serum ferritin value should be determined, whereby this also acts as an inflammation marker, which must be kept in mind when interpreting blood counts.

Excess iron

Mammals do not have the ability to actively secrete excessive amounts of iron. Nevertheless there are very few cases of acute iron toxicity after the consumption of feed supplements containing iron.

In a 2001 study, horses were fed high amounts of iron over an eight-week period, with no changes detected in the liver. In a more recent study, horses were continually fed high concentrations of iron over a considerably longer period (9 years). The iron was not present in their feed, but rather in their drinking water. Subsequently, this long-term supply of iron led to an observance of an accumulation of iron deposits in the tissue (haemosiderosis). In conclusion, long-term excessive consumption of very high iron concentrations in the drinking water can lead to physical damage. In previous studies, high amounts of iron in feed have not been associated with organ damage from excessive iron storage. A very recent study also showed that excessive iron consumption (4400 mg per day on average for a 500 kg horse) through feed does not – as is sometimes assumed – lead to insulin resistance in the horse. This theory was originally put forth because research in humans showed that iron can have an impact on insulin resistance. Increased iron serum as well as increased transferrin values can be indications of an oversupply of iron in the horse's body. However, a liver biopsy would be required for a conclusive diagnosis.

 

Janina Beule, M.Sc. Equine science
Januar 2023, © AGROBS GmbH

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