If there's one mineral that is contentious among Kiwi equestrians, it has to be selenium! Most horse owners are aware that selenium can be toxic to horses, but also it is an essential trace mineral for horses in New Zealand where the soil is often low in selenium.

Some horse supplements contain selenium and others don't, and often horse owners are unsure which their horses need. This can lead to "robust" debate between well-meaning equestrians!

Our Digestive VM (vitamins and minerals) horse supplement contains the macro minerals Calcium, Phosphorous and Magnesium as well as trace minerals Copper, Zinc, Selenium, Manganese, Iodine and Chromium - and a complex vitamin premix including Vitamin E, plus the amino acids Threonine, Lysine and Glutamine.

Digestive EQ (a gut balancer for horses) doesn't contain any selenium.

To clear up the debate, we asked an expert New Zealand Equine Nutritionist to explain.

Our expert:

Nikita Stowers MSc (Nutrition) BSc (Animal Science) BBS
Equine Nutritionist – Veterinary and Nutritional Integration LTD

What is Selenium and why does my horse need it?

Selenium is a trace mineral that is required by the horse in much lower amounts than the likes of Calcium and Magnesium, however it is essential for many bodily functions. It works in conjunction with Vitamin E as an anti-oxidant. Together with Vitamin E it protects cell membranes from free-radical damage. Selenium and vitamin E work best when both are sufficient in the horse’s diet.

Are NZ Soils truly low in Selenium?

Traditionally NZ soils have been known to be low in Selenium. This translates to low Pasture Selenium values often less than 0.01mg/kg Dry Matter. According to the National Research Council the requirement for a 500kg horse is approximately 0.1mg/kg DM, or 1mg/daily and others suggest a recommended range of 1-3mg/horse/day.

The horse is often not able to get enough Selenium from pasture alone and they require supplementation. This is usually done through Selenium supplements obtained from your Veterinarian and is also included in most concentrate feeds and broad spectrum vitamin and mineral supplements on the market today.

How do I know if I’m feeding enough Selenium?

The only accurate way to know if you are feeding your horse the correct amount of Selenium is by taking a blood test. Both Serum and Whole Blood can be tested however we prefer to test whole blood as the acceptable range for horses is more established and it shows the Selenium picture over time as opposed to Serum which gives us a snapshot of the last 24-48 hours.

If your horse tests in the acceptable range you don’t need to change anything in their diet with regards to Selenium. However, if your horse tests outside this acceptable range you will either need to supplement them (if below 1600nmol/L) with Selenium or reduce the Selenium in the diet (if it’s above 3200nmol/L).

What does Selenium deficiency look like?

Selenium deficiency is most commonly seen in nursing foals, especially where their Dam had inadequate Selenium supplementation during pregnancy. Affected foals have malformed muscles, are weak, have difficulty standing and this can result in death due to heart failure.

Broodmares suffering from deficiency may have decreased fertility and risk retained placenta, so if you are breeding horses on your property and have increased incidence of retained placenta it may be an indication of Selenium deficiency.

Symptoms of deficiency in adult horses include a depressed immune system, poor performance, stiffness after exercise, more frequent episodes of recurrent exertional rhabdomyolysis (RER or tying up), impaired cardiac function, difficulty swallowing and paralysis.

If your horse has a low Selenium level in their blood (<1600nmol/L) then your Veterinarian or Nutritionist will advise you to add Selenium to their diet. This can be done with inorganic or organic Selenium, however it should be noted that organic forms of Selenium have a higher safety margin than others although Organic Selenium does tend to be better absorbed by the horse.

Can my horse have too much of a good thing?

We have come a long way with what we feed our horses in the last 20 years. As horse owners we often add supplements to our already balanced feeding regimes and this usually results in excesses being excreted but we need to be careful with trace minerals like Selenium because Selenium toxicity appears to be increasing in NZ.

Added to this, many of our horse properties are on ex-dairy or vegetable growing land and this has often been heavily fertilised with added Selenium meaning our pasture levels may not be as low as we think.

The below graph shows Selenium levels over the year on a property in New Zealand taken from two different paddocks. It can therefore be really useful to have your pasture Selenium tested so you know how much your horse is getting, especially if you are adding Selenium to their diet.

Using balancer programmes like FeedXL or an Equine Nutritionist can be really helpful to know if we are overdoing it with Selenium.

Symptoms of Selenium toxicity include Mane and Tail hair loss, crumbly or cracked hooves, increased salivation and respiratory rates.

Whilst the recommended rates for Selenium are usually between 1-3mg/day, the threshold for Selenium is around 5mg/day so it’s easy to see how we can overdo it.

To put it into perspective, in our clinic (vani.nz) we see around the same amount of toxic selenium levels in blood tests as we do deficient - so it is a real problem that we need to be aware of.

If your horse presents with a high blood Selenium level it is best to get your diet assessed by a Nutritionist who can advise on next steps. Essentially you need to reduce the amount of Selenium in the diet and/or reduce the absorption of the Selenium you are feeding. This is sometimes required if a horse isn’t receiving supplementary Selenium but is still testing high. Sulfur based supplements can be used, such as Copper Sulphate, that will reduce Selenium absorption.

If you take away two things from this article they would be to check how much Selenium you are actually feeding your horse and ensure you are blood testing them annually to make sure you are continuing to get it right. Sometimes levels in feeds and supplements may change over time so it pays to check yourself once a year for peace of mind for you and for the health of your horse.