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Selenium - What Why How

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Selenium is an essential trace mineral and like a lot of essential nutrients, a small amount does a lot of good but it doesn’t mean that more will do more good, in fact too much is actually toxic. Overdose of selenium causes effect that were originally called “alkali disease”. On the other hand selenium deficiency has been shown to create white muscle disease. 

Selenium is an essential component of the enzyme Glutathione Peroxidase, one of the body’s natural anti-oxidants.  As an antioxidant It protects the cells from damage by the waste products (known as free radicals) of cell metabolism.  In particular selenium destroys peroxides and prevents them from causing a loss of cell membrane integrity.


There is also a link between selenium and muscle integrity separate from its anti-oxidant properties. It has been used in conjunction with vitamin E to help prevent muscle disorders (tying-up) in some horses. The link between selenium and vitamin E is interesting in that if one is deficient it can be compensated for by an adequate supply of the other. Vitamin E in the cell membrane will reduce the formation of lipid peroxides, selenium in the cell fluid will remove any that do form. Low vitamin E means selenium is needed more to deal with the peroxides and the contra is true. 


Selenium is also necessary for the development of the acquired immune system.



Many areas have sufficient selenium naturally available in the soil and hence transfers to the horse via the grass intake.  However there are large areas of New Zealand soils that are deficient in selenium so supplementation is essential. Low selenium will affect the efficient function of the horse’s muscular system.


Horses with low selenium exhibit negativity to being ridden or worked, over reactive to stimuli, “hot “and difficult with lowered performance. It results in inadequate antioxidant levels with initially mild symptoms that include the horse being stiff and sore after exercise and prone to infections, from a depressed immune system, also possibly prone to tying up syndrome. 


Symptoms of too much selenium include gradual weight loss, a loss of hair from the mane and tail and a break in the hoof wall at the coronary band.  The changes to the feet can cause a lameness similar to that seen with laminitis. Acute poisoning occurs when large amounts of selenium are ingested over a short period of time, signs involve most of the body systems and death usually results.

In breeding horses , low selenium can be a cause of low fertility, the most severe problems are seen in newborns and can be fatal. 


 More intense symptoms include the development of myopathy (white muscle disease), impaired movement, difficulty in suckling and swallowing, respiratory distress and impaired heart function.

Performance horses have high energy demands when they are training and competing. During exercise, muscle cells require antioxidants that need elements such as selenium to deal with free radicals, the by-products of muscle energy use. The higher the workload the more antioxidants are needed. A performance horse deficient in selenium is at risk of muscle damage created by free radicals leading to muscle pain and stiffness and slow muscle recovery following strenuous exercise. 




Supplementation should never be given with first obtaining a blood test, a horse may even exhibit the same behaviour symptoms if they are high or low  in selenium, however over supplementing selenium can be toxic. 


Selenium deficiency is seen as work intolerance , poor hair and coat , muscle inflammation , weak pale muscles ( white muscle disease) . So outward symptoms can be confusing to determine a high or low level horse . Therefore the blood test is an absolute necessity and should be repeated at regular intervals on a horse receiving supplementation. 

As well as a blood test, the local soil status should be known and the levels in any prepared feeds being used in the diet.  An assessment should be made to add up the sources .


Horses require between 1-3mg of selenium on a daily basis, even 2-3 times the required daily amount over a long period of time, will cause chronic selenium poisoning.  


Selenium can be administered either in a liquid inorganic form, or in a organic form, or in a organic or chelated form.  The latter is by far the preferred option.  Vetpro’s Selenium K is a powdered chelated (organic) form of selenium.  A daily 15 gram dose provides 1.66mg of available selenium.  

It is safer and more convenient to use a daily dose of Vetpro Selenium K  as  the 15gram daily dose is exactly one scoop.


Using Selenium in the liquid form of the 5mg per ml concentration, you have to feed a daily dose of only 0.33ml, which requires very careful measurement.


A weekly dose of 2ml also requires great care and management so there is a real vulnerability to creating an overdose.




Left and Centre: Signs of Selenium over supplementation.

Right: Anxiety and tension can be a symptom of low and high selenium levels.



Read more about Selenium K here - a safe way to supplement Selenium.


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