I know, the above is certainly not an image of iron oxide. But it is a lump of magnesium, and while this blog is about the role of iron in the equine diet, there's an interesting connection to magnesium oxide further on, which as we know is a hot topic, featuring highly in the equine diet and in our EquiVita range.

We recently had a technical note sent out to us by Premier Nutrition, who produce our bespoke EquiVita premix, written by Chloe Poolman, one of their Equine Nutrition team. It makes for very interesting reading and may put to rest the iron-jitters that seem to be out there. Especially - and I get questioned on this a lot! - the statements "not all iron is considered bioavailable to the horse" and specifically "iron oxide, commonly found as background iron in minerals such as ... magnesium oxide, has limited bioavailability and therefore is not considered an iron source in the diet."

I had to double-check with the Equine Team at Premier that I'd read this right, that any iron residue in good old magox wasn't likely to load up the iron level in the equine diet, and was assured that I'd understood correctly. Which kind of turns the rumour out there that "iron in magox is badbadnotgood" on its head. I think ...

So, as I don't particular want to be shot down in flames by those out there who know better than me, I'm not going to summarise the technical note but put it here in full. It's a bit science-y in places but you'll get the gist, and I have permission to pass it on to you. PS - the tables didn't copy/paste very well so bear with my limited formatting skills 😉

THE ROLE OF IRON IN EQUINE DIETS

Introduction

The role of the mineral iron and the quantity consumed by horses comes under scrutiny from time to time. Some believe excess iron in the diet is harmful, whilst other opinions regarding supplementary iron range from desired to wasteful. This review summarises the role of iron in the body and its control, dietary sources, animal requirements and regulatory controls.

The primary function of iron is the oxygen transport of haemoglobin, myoglobin and cytochromes (Halliwell and Gutteridge, 1989). It also has other roles as an enzyme activator and in erythropoiesis. (Munoz et al., 2009; Piccione et al., 2017). The majority (67%) of iron within the body is contained in red blood cells (Jackson, 2007). This figure rarely deviates significantly as iron form haem-carrying cells, mostly erythrocytes, and is “recycled” during erythropoiesis in the bone marrow (Pearson and Andreasen, 2001). It is also found in skeletal muscles within myoglobin, within the blood serum as transferrin and in the storage forms ferritin and haemosiderin.

Controlling iron in the horse

The horse is highly able to conserve iron via complex homeostatic mechanisms. Iron is absorbed in the duodenum and upper jejunum in the digestive tract, and rapidly coupled to the transferrin, by which it is transferred to the tissues.

The peptide hormone hepcidin, synthesized by liver hepatocytes, plays a pivotal role in iron status. Hepcidin acts to suppress iron absorption within the gut or initiate an iron influx from stored iron to mediate levels as required.

In times of increased erythropoiesis or during inflammation or hypoxia, hepcidin upregulation acts to conserve iron within the cells and reduce free iron in the blood. It does this via its control of ferroportin, a membrane transporting protein that moves iron from inside the cell to outside and into the bloodstream.

In fact, low iron and high fibrinogen in plasma are both sensitive indicators of systemic inflammation in horses. Rapid development of hypoferremia is particularly valuable during the earliest phases of infection to help inhibit bacterial growth.

Strenuous physical activity commonly leads to iron losses through sweat (Meyer and Coenen 2002) and increased red blood cell turnover. The concentration of iron in horse sweat is estimated to be about 21 mg/l (Jackson, 2007). This sweat loss would represent a net iron loss in the sweat of approximately 500 mg/day.

Considering these, there is the possibility of greater synthesis of haemoglobin and myoglobin (muscle iron containing compound) in the athletic horse, giving good justification that their requirements for iron would be higher than for mature or sedentary horses. However, these are met by uprated hormonal control mediated by hepcidin, by generally increased intakes of iron-containing feedstuffs. Studies from Piccione et al., (2017) and Inoue et al., (2004) both demonstrated how exercise significantly increased iron balance and the natural increase in iron absorption compensates for the adverse effect of exercise on iron status.

Iron requirements

The NRC (2007) recommended levels for an adult horse is 400mg per day (estimated weight 500kg on a maintenance diet), rising to 500mg/kg for horses in hard work. Updated values (Geor et al 2013) suggest 400-450mg per day for maintenance requirements and 500-625mg per day for horses in training (also based on 500kg estimated weight).

Iron deficiency and excess

Iron deficiency is rare. Deficiencies are only seen in cases where there has been extreme blood loss, rather than a lack of iron within the diet. However, anaemia, commonly associated with low iron status, is a common haematological sign in competition and racehorses. This is often considered an apparent anaemia, in which the reduction in blood parameters related to iron (packed cell volume, red blood cell count), are related to the natural hepcidin-mediated reaction of the body to conserve iron stores as a result of increased erythropoiesis or inflammation.

Iron toxicosis is rarely reported in adult horses. As grazing herbivores, horses are adapted to consumption of iron-rich forages. With further investigation, Pearson et al., (2001) assessed the oral administration of 50mg/kg ferrous sulphate to adult ponies for 8 weeks. Results showed that hepatic iron concentrations, serum iron concentrations and transferrin saturation were increased compared to the baseline and control concentrations; but they were not reported as being significantly increased. This study gives evidence supporting the effect of natural homeostatic controls of absorption, and the data did not appear to consider alternative factors contributing to the iron intake from soil.

It should be noted here that newborn foals have limited ability to regulate intestinal iron absorption and cases have been reported of death by acute liver failure in foals given an oral intestinal inoculum containing ferrous fumarate during the first days of life.

Comments on the relationship between Iron and EMS

Links in humans between insulin resistance and iron excess have led to concerns in horses. However, a recent study (Kellon and Gustafson (2019)) showed no statistically significant associations between insulin indices and iron indices which indicates a poor, or non-existent, relationship. The lack of a significant relationship between insulin and ferritin is not unexpected considering both iron overload & hyperinsulinemia are multifactorial (Nielson et al., (2012))

Cases of iron overload have occurred in horses without evidence of metabolic syndrome; indeed - iron is supplied at luxury levels in general daily diets, again suggesting at best an indirect and non-linear relationship between high iron and EMS.

Iron in feedstuffs

Iron is naturally abundant in many feedstuffs consumed by horses. Good sources are typically green leafy plants and seed coats. Table 1 shows iron levels found in feedstuffs typically fed to horses.

Table 1: Iron content of feed materials commonly used in equine diets

Feedstuff - Typical iron content (mg/kg)

Grass - 100-700 (DM basis)1

Hay/haylage - 100-700 (DM basis)2

Dried lucerne - 600

Oats - 76

Barley - 85

Sugar beet pulp, molassed - 190

Sugar beet pulp, unmolassed - 460

Soya bean meal (hipro) - 150

Limestone - 0

Linseed (whole) - 140

Dicalcium phosphate - 4300

Notes:

1 Varies according to soil type, soil pH, leaf content, and grass length

2 Varies according to soil type and soil contamination and maturity

Iron in proprietary feed products typically range from 50-200 mg/kg as fed; levels in supplements depend on the form and function of individual products.

Typically, the largest source of iron in the equine diet is forage, either preserved or fresh. In general, the iron content of most forages is >100mg/kg on a dry matter basis but this can vary depending on both soil type and the amount of soil ingested as a result of “soil splash” on the leaves.

However, not all iron is considered bioavailable to the horse. Certain iron salts and chelates are thought to have good-high levels of bioavailability, whereas iron oxide, commonly found as background iron in minerals such as calcium carbonate, magnesium oxide, monocalcium phosphate and dicalcium phosphate, has limited bioavailability and therefore is not considered an iron source in the diet. As a result, iron oxides are no longer permitted feed additives in the EU.

Typical dietary supply of iron

The following example diets give indicative dietary iron intakes and compare these to published requirements and EU regulatory maximums.

Table 2: Indicative dietary iron intakes

Diet (500kg horse) / Hard work / Light work

Hay (kg) / 8.0 / 12.5

Hard feed (kg) / 6.5 / 2.0

Total (kg) / 14.5 / 14.5

Iron content of feedstuffs:

Hay (mg/kg) / 150 / 150

Hard feed (mg/kg) / 200 / 150

Total dietary iron supply (mg/d) / 2500 / 2175

Published requirements:

NRC (2007) (mg/d) / 500 / 400

ECAN (2013) (mg/d) / 500 - 625 / 500 - 625

Comparison with permitted EU trace element maximums:

Dietary iron concentration (mg/kg) / 172 /150

EU MPL (mg/kg) / 750 / 750

Feed regulatory considerations for iron - maximum permitted levels

In the EU, maximum permitted levels (MPLs) are set out for the various trace elements in the diet of animals. These are expressed in mg/kg in the complete diet (corrected to 88% dry matter).

The iron MPL is 750mg/kg of complete diet (see EU Regulation 2017/2330), so that (based on a 500kg horse eating 2.5% of bodyweight), the MPL in grammes per day can be calculated as follows:

• (500 x 2.5%) = 12.5kg total dietary dry matter intake for a 500kg horse

• 12.5kg / 88% = 14.2kg dietary intake corrected to 88% dry matter

• 14.2 x 750mg = maximum permitted iron intake per day = 10,653 mg (10.7g) of iron per day.

Feed additives authorised for use in the EU

Table 3: Feed additives containing iron authorised for the use in the EU (pursuant to EU Regulation 1831/2003).

Additive / % iron / EU Authorisation Number / Comments

Iron (II) sulphate monohydrate / 30 / 3b103

Iron (II) sulphate heptahydrate / 30 / 3b104

Iron (II) fumarate / 30 / 3b105 / Used in pastes only

Iron (II) chelate of amino acid hydrate / 9 / 3b106 / Example brand name Availa®

Iron (II) chelate of protein hydrolysate / 15 / 3b107 / Example brand name Bioplex®

Iron (II) chelate of glycine hydrate / 18 / 3b108 / Example brand name B-Traxim®

Labelling of iron additives

When iron is added to an equine product formulation via one or more authorised additives, it must be listed on mandatory labelling particulars as the amount of added elemental iron (mg/kg), together with the source of the iron and its additive authorisation number (EU Regulation 767/2009 as amended by EU Regulation 2017/2279). Only iron added from an additive source should be labelled in this way. Total iron can be voluntarily declared under the analytical constituents section.

For example: Nutritional additives / Trace elements / Iron: 50mg/kg as iron (II) sulphate heptahydrate (3b104)

Labelling of iron under analytical constituents

If declaring total iron content of the compound feed voluntarily under Analytical Constituents, this value will be the added amount plus the background amount supplied (naturally occurring) by the other ingredients in the product.

CONCLUSION

In general, equines receive luxury intakes of iron in their diet. As a component of that most fundamental function, oxygen transport, iron is tightly conserved within the body using complex homeostatic mechanisms. This, together with varying iron content and bioavailability of the ingredients used in equine diets, mean high safe limits are given for iron in equine diets (NRC and EU regs). Reported cases of iron toxicity are very low, thus giving the regulators no justified reason to alter the current maximum recommended level.

References available on request.

Author: Chloe Poolman, Equine Nutritionist, Premier Nutrition