Iron in the Equine Diet

Iron is an essential trace mineral for horses, meaning that it is required in the diet in small amounts. These minerals are required in mg/kg or parts per million (ppm) as opposed to grams.

Iron plays a critical role in a variety of body processes, most notably is the role that it plays in oxygen transport. Hence, for optimal health, it is required, but recently there has been increasing concern across the industry about iron overload in horses. As a preface to this article, please note that the research on this topic in horses is limited.

Dietary Iron Requirements

The NRC, 2007 (Nutrient Requirements of Horses), details established iron requirements to prevent deficiency. For an average sized (500 kg) horse that is not in work it is recommended that they receive 400 mg of iron daily. The amount required does increase with workload and physiological demands, as a 500 kg lactating broodmare requires 625 mg of iron daily.

Most of the time, the forages that our horses are consuming will provide them with adequate dietary iron. Additionally, it is present in all vitamin and mineral supplements. So, it is rare to have dietary iron deficiencies. If we look at the Balanced Bay averages for Ontario hay tested in 2023, the iron content averaged 139 ppm on a dry matter basis. With the range being 21 – 553 ppm.

If we assume that a 500 kg horse is consuming 10 kg of their body weight in hay the horse would be receiving 1390 mg of iron daily, from forage alone. This is before any supplemental feeds which will also contain iron, even if it isn’t listed on the guaranteed analysis. It should be noted that it is estimated that only 20% of dietary iron is absorbed by the horse.

Sources of Iron in the Diet

Almost everything that is fed to our horses contains iron. Hay comprises most of their diet, therefore, when concern is raised about dietary iron, having the hay tested is critical. Iron content in hay is influenced by the plant species, soil composition and harvesting conditions. Factors like the height of the baler and soil contamination can largely influence mineral content of the hay. When taking a hay sample, it is important to be cautious of soil contamination from bales being stored on the ground. Hence, why it is preferable to use core samples.

Commercial feeds, vitamin and mineral supplements, and products such as beet pulp all contain iron. In fact, it is impossible to have a vitamin/mineral product without any iron – even if the label does not state iron content. For example, monocalcium phosphate is a common feed ingredient and adds iron to the product. Additionally, pelleting feeds is another source of iron. Salt sources such as Himalayan also contain iron, the pink colour is from the iron content of the salt.

Excess Iron

It is well reported that equine diets typically oversupply iron, but horses are generally quite good at regulating their iron levels closely. When it is ingested from hay and feed, the acid in the stomach will convert it into a soluble form. The digesta will then be passed into the small intestine where most of the iron absorption takes place. Once absorbed by the intestinal cells the iron is then available to be transported into the bloodstream for use.

Hepcidin is a hormone that is a key player in iron regulation in the body as it controls how much is absorbed. Essentially, when too much iron is present, the hepcidin will inhibit the release of iron from the intestinal cells into the bloodstream. This is done through the degradation of ferroportin which is the gatekeeper between the intestinal cells and the bloodstream. Thus, trapping the iron in the intestinal cells to prevent use and absorption via the bloodstream. These cells are eventually sloughed off and the excess iron will present in the fecal material.

Iron toxicity is rare, but more concern is being raised about iron overload in horses. According to the NRC, 2007, no reported cases of iron toxicity from consuming high iron forages have ever been reported in horses. The concerns around overconsumption of iron include an increase in oxidative stress as well as liver damage. There is also concern of high dietary iron negatively impacting the absorption of zinc.

Iron Deficiency

Dietary iron deficiency is extremely rare in horses as forages generally contain an excess supply already, and if not, our feeds supply iron as well. Low iron, specifically anemia, can present in horses. Anemia is when there are inadequate red blood cells (RBC) or hemoglobin circulating in the bloodstream. This is generally due to health issues such as loss of blood or an infection causing the destruction of red blood cells at a greater rate than the body can replace them. In these cases, an iron supplement may be added, however, a veterinarian must be consulted prior to any decision being made.

Hemoglobin is a protein present in red blood cells that transports oxygen throughout the body. The hemoglobin will contain about 60% of the body’s iron. RBCs will circulate for roughly 150 days, at the end of their lifespan the cells will die. The iron from these cells is recycled which means that there is very little net loss of iron from the body. This is part of the reason why the dietary requirements to prevent deficiency of iron are low.

Mineral Interactions

When balanced nutrition is discussed, mineral ratios are mentioned. This is because minerals can impact each other as metabolic pathways are shared. For example, iron has a close relationship with zinc as they rely on the same transporter protein. When there is an over abundance of one mineral, it is thought that the transport protein can become saturated and inhibit the absorption of the other mineral. So, in theory, if iron is oversupplied, zinc may not be adequately absorbed resulting in a deficiency.

Although there is very little concrete research, many nutritionists will calculate the iron to zinc to copper to manganese ratio to ensure that iron does not exceed about 10:1:4:4. Please note that this is not an established ratio by the NRC and more information is required to be able to definitively state an optimal range for this mineral ratio.

A Nutritionist’s Top Tips

• Test your hay (yes, this is #1 as always)
• Feed a quality ration balancer or vitamin/mineral premix that does not contain added iron, or that has a guaranteed low iron level. Note that iron will always be present but avoid ingredients that indicate added iron such as iron oxide.
• Consult a nutritionist to ensure your horse’s diet is balanced.
• Only add iron supplements under close supervision by a veterinarian if the horse is anemic.

Take Home Message

The science is lacking as there is so much we simply do not yet know when it comes to iron in the equine diet. When issues with iron arise, and suspected overload is present, it is encouraged that you work closely with an equine nutritionist. They will be able to take the numerous factors into account including, accuracy of the hay analysis, potential soil contamination, iron content in the water, content in commercial products that may not be listed on the guaranteed analysis, bioavailability of the iron in the feed ingredients etc.

By: Madeline Boast, MSc. Equine Nutrition

References:

Frape, D. (2010). Equine Nutrition and Feeding Fourth Edition. Chapter 3: Trace Elements. Wiley-Blackwell.

Inoue, Y., Matsui, A., Asai, Y., Aoki, F., Matsui, T., & Yano, H. (2005). Effect of exercise on iron metabolism in horses. Biological trace element research, 107, 33-42.

National Research Council. (2007). Nutrient Requirements of Horses.