Reining in nutritional issues: selenium imbalance in horse diets

There are about 50 thoroughbred horses per 10,000 people in Ireland. This staggering number is far more than most racing nations, and 10 times higher than in the UK, France, and the US. The Irish horse racing industry is worth €1 billion per annum, and so keeping horses strong, healthy, and happy is incredibly important to the Irish economy. Selenium imbalance in horse diets can have worrying consequences.

Selenium is an essential trace element for many animals, including horses, cows, and humans. It is an vital component of many enzymes and proteins that serve to make DNA, protect against infections and cell damage, and support reproductive systems.

Trace elements are required in very small amounts, and horses require particularly low levels of selenium. A total dietary intake of 3 milligrams per day is more than sufficient. Selenium availability is determined by the presence of antagonists principally sulphur. High levels of sulphur in the diet will reduce selenium availability. A dairy cow, on the other hand, will require up to 5 times that amount, as requirements are species dependent. Selenium has a very narrow range of nutritional safety, meaning that ensuring levels are just right is crucial for the health of horses on stud farms.

Read on to find out more about how selenium imbalance affects horses, selenium levels in Ireland, and how to ensure optimal selenium levels with IAS.

Selenium imbalance in horse diets: why does it matter?

Horses get their daily dose of selenium from grazing hay or grasses that grow in selenium-containing soils. However, if you live in a region with low selenium levels in the ground, then your forage may be deficient.

Selenium deficiency in horses can be easily missed as its initial effects present below the surface. Low levels can result in white muscle disease, foetal reabsorption, and retained placenta at birthing.  Studies have also indicated that a low selenium intake can result in a suppressed immune system.

If horses are fed to excess, it can result in selenium toxicity, which is also known as selenosis. Whilst this condition is relatively unusual, OCAE Agricultural Consultants have worked on an increasing number of stud farms over the last few years with horses displaying clinical symptoms of selenium toxicity. These symptoms may include loss of mane and tail hair, cracked and brittle hooves with abscesses at the coronary band, or, in extreme cases, laminitis. This, in turn, causes severe pain and lameness.

Be aware that affected animals do not necessarily exhibit the full range of symptoms. On occasion, the loss of mane and tail hair has been blamed on companion animals. So, accurate, reliable analysis of both the grass and the animals themselves is vital to recognising and mitigating issues before it is too late.

What are the selenium levels in Ireland?

It is estimated that 95% of Ireland’s pastures are deficient in selenium, with the remaining 5% providing sufficient or excess levels. This has been exacerbated over the summer due to the increased temperatures and prolonged sunny weather. Long dry periods can increase soil selenium availability by its oxidation to the selenate form, which is a more soluble form of the element.

Typical grass in Ireland will supply one third of a horse’s daily requirement, with the balance provided by concentrate feed or additional supplementation such as free access mineral licks. Excess selenium is generally associated with lands located in a flood plain, fields close to watercourses, and reclaimed land. Counties Dublin, Meath, Westmeath, west Offaly, east Galway, and north Tipperary are documented as having the highest incidence of soils with excess levels of selenium, although pockets of land with high selenium do occur in other counties. Horses grazing in pastures with grass selenium readings in excess of 0.5 mg/kg are at significant risk.

Herbage selenium concentrations are also influenced by climate, with higher concentrations occurring during dry years. This was reflected in elevated selenium readings from herbage samples processed by IAS Laboratories during the 2013 grazing season. Levels were also elevated during the summer of 2018 and again during August of 2022, where we once again experienced particularly dry spells.

How do I ensure optimal selenium levels?

Grass analysis is an important tool in establishing the overall mineral profile of a farm, including the selenium distribution. In addition, the daily mineral supply of essential minerals such as calcium, phosphorous, copper, and zinc can be calculated, and accurate mineral supplementation recommended.

Nutrient interactions are an important factor to be considered when determining the extent of a mineral deficiency or imbalance. For example, sulphur and copper interact with selenium, and pastures with high sulphur levels will have reduced selenium availability.

Farms with high background levels of selenium require specific management, especially during the grazing season (April to September). Grass samples should be taken each year from individual paddocks and sent for laboratory analysis at the beginning of the grazing season. Horses should not remain in paddocks with elevated readings for a prolonged period (more than 3 weeks). Supplemental selenium through concentrate feeds, free access minerals, and other supplements may need to be reduced.

Specific fertiliser programmes should also be formulated with particular attention paid to the application of sulphur. The use of fertilisers with additional sulphur may be appropriate, but if the farm has inherently high sulphur levels, further applications may induce copper deficiency.

The individual mineral status of the horses can also be determined through blood and hair analysis. Blood analysis will determine the current profile, whilst hair analysis will establish exposure over a period of time.

Further information is available from the Environmental Protection Agency (EPA) and the Geological Survey of Ireland (GSI). The EPA has produced a soil geochemical atlas which provides information on the mineral profile of soils throughout Ireland. The Geological Survey of Ireland (GSI) has also published the results of its “Tellus Border” regional mapping project which collected geochemical data from soils in the six border counties.

This data, which provides information on the bedrock geology and baseline mineral status for farms across the whole of Ireland, is of immense value to the agricultural sector. Understanding and benchmarking the status of these soils helps to indicate soil health across the nation to help farmers and advisors determine how well they are performing, as well as to measure change over time.

How can IAS help me avoid selenium imbalance in horse diets?

Our grass mineral analysis services quantify deficiencies and imbalances in major and trace minerals, helping stud farmers and advisors consider all the dietary needs of their animals and decide the appropriate supplement required. We also offer blood, milk, and hair mineral analysis to give you a full mineral profile for a number of important elements to help keep your animals strong and healthy.

Contact IAS today – [email protected] (059 9721022)