Iron exists in two forms; ferric and ferrous ions. In the soil they are found mainly as inorganic, ionic salts such as ferric oxide and ferrous sulphate, although complex salts and organic chelates (usually two organic chains surrounding the iron molecule) can be found. However, the main criteria for absorption by the roots of plants is the solubility of these compounds. Iron oxide, for example, is insoluble and therefore unavailable whilst one of the many hydrated forms of iron sulphate is reasonably soluble and is potentially bioavailable.

The Plant

There are two strategies for the absorption of iron (Hell & Stephan 2003). One, for the higher plants, is a creation of an iron chelate that changes ferric iron to ferrous and also increases the solubility of ferrous iron (by increasing level of hydration). The second – exclusive to the grasses – is the formation and absorption of a ferric chelate. Both strategies end up with both ferric and ferrous iron being incorporated in a chelate with Nicotianamine, which is the major compound for transporting iron to the shoots, leaves and storage area s via the phloem and xylem. Phloem and xylem forms may be different but the target tissues will receive iron as nicotianamine (Fe-NA), and then become incorporated in protein, heme, phytoferritin, iron sulphide clusters or precipitate. Phytoferritin and precipitate are the only two fates if iron is absorbed in excess, and the precipitate by its very nature – insoluble – will be unavailable.

Phytoferritin is a complex of excess iron and tends to accumulate in the chloroplasts and plastid stomata of leaves. Basically large quantities of iron are bound in protein sinks and are released in times of iron deficiency to avoid reduction of photosynthesis and chlorosis.

Because iron is relatively unavailable to the plant it has developed a sophisticated absorption, transport and storage system to:

• Ensure there are reserves for the plant to continue to photosynthesise during iron deficiency
• Keep the iron in a stable form so it does not destroy cell membranes.

Because an approximate 3,000,000,000 people are defined as anaemic (WHO 1998) a tremendous amount of work has been conducted to try to increase iron concentrations in plants and this is leading to genetic modification of mainly cereal crops, with variable success (Fossard et al. 2000, Zehng et al. 2010). Under general circumstances, however, plants tend to absorb sufficient iron for their immediate and potential needs irrespective of the iron content of the soil. Studies on South African soils, which have iron contents in excess of those theoretically needed for wheat growth (Materachera 1999), showed weak correlation between iron levels and iron uptake. In house data at British Horse Feeds show that South African Teff, Lucerne and Oat straw all have iron levels that are lower than NRC (and therefore presumably U.S.) quoted values despite high iron levels in the soil. It is not a simple matter to equate iron levels in the soil with those in plants and illustrates one reason why iron deficiency is a greater problem than iron excess.

The ability of any plant to absorb and utilise iron is dependent on the form and solubility of that form in the soil.

The high levels of iron in beet root systems are stored as an insoluble precipitate and are maintained to transport to leaves and shoots.

Plants – with different strategies for iron accumulation between grasses/cereals and higher plants – will have iron present across all tissues but the form and availability of this will vary between plants and areas. Leafy material contains the more “active” types (heme, iron clusters) and its storage mechanism involves phytoferritin. Seed heads tend to have more iron protein chelates and roots more insoluble iron.

The Animal

Bioavailability is a term used to describe the amount of a mineral or trace element that can be incorporated into an organic system. Bioavailability of iron for plants will be different to bioavailability of iron for animals and this will vary between species and individuals. However, the mechanism that determines bioavailability is the same across animal species and this is down to the capability of the gut to absorb minerals.

Much is known about the sites and processes of absorption. Mineral ions can be absorbed both actively and passively along the length of the gut. These absorption sites are areas of conflict as some minerals compete with each other and high levels of one will reduce the absorption of the other. For example high levels of copper will reduce the absorption of iron, although the reverse does not occur. Zinc, manganese, phosphorus and copper all affect iron absorption, while iron affects phosphorus and possibly manganese. In addition organic minerals – such as chelates – can be absorbed actively along the small intestine (the molecules tend to be too large to cross the hindgut membrane) and by pass the sites of competition. Chelated minerals, increasingly found in commercial mineral premixes, capitalise on this effect to allow more precise administration of essential elements. Work on human subjects, for example, have shown that soya ferritin is absorbed with almost the same availability as ferrous sulphate – the most soluble of the iron salts (Davila-Hicks et al. 2004), although both crossed the gut by different pathways.

Two other factors affect the bioavailability of the minerals and these are the physical conditions of the gut - its acidity along its length – and the presence of micro-organisms in the gut. Plant chelated iron is at its most stable in neutral conditions whilst soluble iron salts will likely dissociate in the stomach acid. Insoluble iron salts, however, will only slightly dissociate – if at all – in the stomach, which is why iron oxide does not become a more available iron chloride.

The second factor for bioavailability is the presence of the gut microflora. Generally speaking, as iron deficiency is more common than iron excess in all environments, there is competition for iron. Animal iron sources have been shown to be bound by gut microflora with varying degrees on strength (Styriak 2004)and it has been shown that fungi have a number of strategies to capture the iron from plant chelates (Nevitt 2011). Microbes, including fungi can absorb and utilise free iron, as in iron salts, whilst utilising chelates is a specialist function where the microbes break down the chelate and use the iron. However some chelates have antimicrobial properties and so can affect microbial populations( Holbein et al. 2010) . This occurs in all environments including the soil, plants the gut and within the animal itself (Jung et al. 2008).

The animal gut can absorb iron in the form of soluble salts and some chelates, although the latter only in the small intestine. The stomach may improve the solubility of some iron salts but there will be competition along the gut from the microflora. Chelated iron has a more complex interaction; some will be broken down and utilised by microbes, some will be absorbed. However insoluble iron salts will not be absorbed and will pass through the gut unchanged.

The Horse

Although the mechanisms described above hold true for all species of animals there are variations between species. Carnivores for example do not usually suffer from deficiencies as they can readily absorb the heme and animal ferritin chelates, whilst herbivores such as the black rhino (selective herbivore) has developed strategies to utilise the reasonable levels of iron in sub-tropical leaves. In comparison the horse appears to have a higher efficiency of absorption, but grazes on forage which has lower iron content (Clauss et al. 2007).Tapirs have a lower efficiency (Clauss et al 2009).

Foals, as with all mammals, receive iron from their mother’s milk as lactoferrin, which is assumed to be easily digested. As it is based around the ferritin chelate (similar to both phytoferritin and animal ferritin) it probably has similar bioavailability to iron sulphate.

When it comes to the weaned horse the source of iron is more varied. Phytoferritin, iron clusters etc. are all ingested and so the type of feed will impact on the proportion of these forms. In addition the horse will also ingest iron salts from the soil. Iron deficiency has been observed in stabled Dutch Warmblood foals fed fresh cut grass, but not in their grazing counterparts. The soluble iron salts in the soil would be the only difference (Brommet et al. 2001).

Another source of iron that is perhaps overlooked is that found in mineral premixes. Traditionally this will be an iron salt – usually ferrous sulphate – although increasingly manufacturers are using chelated iron (in this case a specific chelate based on amino acids that can be actively and efficiently absorbed).

So when it comes down to it, what can the horse utilise, and why is iron deficiency not a major problem? Firstly it is down to the efficiency of iron metabolism within the horse which is very good at keeping iron in the body. Unlike humans, the exercising horse does not lose increasing amounts of iron in sweat and urine, and the exercise itself improves iron absorption (Inoue et al. 2005). The horses in that trial were fed a ration of Lucerne, oats, timothy hay and a vitamin/mineral premix. Measurement of iron availability was 17% which was higher than previous reports of 12% (Schriver et al. 1986).

Trials on horses fed similar diets of alfalfa, fescue and bluegrass , but no supplements, gave iron digestibility of a negative value (Crozier et al. 1997). Presumably this is down to experimental error but the point to make is that without supplementation absorption of iron decreased.

The NRC quote bioavailability of feeds at around 15% which is similar to those values quoted in technical research for horses.

Iron is bound, to various degrees, with various fibre sources. It has a strong binding with cellulose and hemicellulose, and a relatively weak bind with pectins. However these ligands (binding arrangements) are acid dependent and iron is less susceptible to acid than other minerals such as calcium, zinc, copper and manganese. It means that the stomach acids will not release any appreciable iron from the pectins. As iron absorption mainly takes place in the foregut, very little is available if bound as fibre ligands.

The horse ingests feeds through its diet, from the soil and via any supplementation. Deriving information from various sources it appears there is a scale of absorbability across species. Between species there is variation but for the herbivore absorbability and therefore bioavailability is, in descending order:

• Protein bound chelates, such as supplements, heme, lactoferrin.
• Soluble iron salts (Ferrous or ferric ions)
• Phytoferritin
• Insoluble Precipitates such as Iron Hydroxide.

As this passes through the tract of the horse soluble iron salts may become more soluble by conversion to iron chloride in the stomach. Whilst chelates, phytoferiin and insoluble precipitates are mainly unaffected.

In the small intestine there is passive absorption of the soluble salts, depending on the level of competition from e.g. copper and manganese and gut microflora, whilst chelates are actively absorbed. As the iron bound in fibre is released by gut fermentation there will be limited absorption – depending on the form. However in the case of the insoluble precipitates there will be little, if any absorption.

NRC have set maximum tolerable levels of 500mg/kg ration. As forage levels are around 100-250 mg/kg, and bioavailabilty is around 15%, 15- 35.5 mg/kg is absorbed. 40 mg/kg of iron is the NRC recommendation, so there may be some shortfall. Modest supplementation is advised. One Cup delivers 175mg; for a 500 kg horse this equates to ~ 17mg/kg diet, which is a sensible level of supplementation.

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