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Sign of selenium over supplementation.


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


Signs of selenium over supplementation.


All horse owners need to know about the intake of this mineral.

All horses will need a certain amount of it. If the horse is on pasture with soil that contains normal levels then supplementation is not necessary and over supplementing will be very negative to the health and well being of the horse.

If the soil level is too low, or the horse is stabled without any supplementation – then the health of the horse again suffers and supplementation is necessary.

HOWEVER it is vital to have a blood test done before considering supplementing this mineral – for full details on whether you should give it to your horse, read our article on Selenium.

Selenium – What, Why, How


What is Selenium? Is it Good for a Horse or is it Toxic?

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 the effects 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 is used in conjunction with vitamin E to help prevent muscle disorders such as 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.

Why do Some Horses Need Selenium and Some Not?

Many areas have sufficient selenium naturally available in the soil and hence it transfers to the horse via the grass intake. However there are large areas of New Zealand soils that are deficient in selenium so soil testing is useful. If the soil is low supplementation will be necessary, either directly to the horse or by adding to the soil, often called prilling. However it is very important that organic or inorganic selenium is never given to a horse without firstly having a blood test carried out by a veterinarian.

What Are the Symptoms of Too High or Too Low Selenium


Some symptoms of high or low levels are similar in that the attitude and behaviour of the horse is affected mainly due to pain and discomfort. But in particular 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. Internally low levels result 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. Continual of low levels will be seen in poor hair and coat, muscle inflammation, weak pale muscles (white muscle disease).

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.

With performance horses, they 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.

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. There have been cases in New Zealand where the person feeding the horse has not understood the need for correct dosing, especially when using the inorganic liquid selenium, and inadvertently overdoes resulting in tails falling off, feet totally breaking down to the point where some horses had to be put down.

How to Supplement Selenium

As discussed 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 since over supplementing selenium can be toxic, it is important to accurately know the levels in the horse’s system.

The blood test should be repeated at regular intervals on a horse receiving supplementation. A horse that is high will need an assessment of the soil and how much selenium is already in the feeds and supplements, particularly since some premix feeds may contain it. If necessary it may mean finding alternate grazing that has not been prilled with selenium.

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 (chelated) form. The latter is by far the preferred option, as the liquid form is only a tiny dose, given once a week or once a fortnight and that is difficult to measure and also to remember if more than one person is responsible to administering. There is another source of selenium  as a combined form as sodium selenite. This is found in many standard mineral supplements and will provide a small daily amount for horses that are on normal soils and are therefore neither low or high in their blood levels. It is just a daily maintenance amount.

Vetpro’s Selenium-K is a powdered chelated (organic) form of selenium. A daily 15gram 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, whereas using selenium in the liquid form of the 5mg per ml concentration, you may have to feed a daily dose of only 0.33ml, which requires very careful measurement, even a weekly dose of 2ml also requires great care and management hence there is a real vulnerability to creating a toxic overdose.

Toxicity cannot occur with the daily amount of Vetpro Selenium K chelated selenium.

Copper, Chromium & Selenium – Three Essential Trace Elements


Trace elements are essential for normal body function. They are involved in the enzyme systems that control the many biochemical reactions that take place within the cells.

Three trace elements that are particularly important to the performance horse are Copper, Chromium and Selenium.



Copper has many functions. It is essential for bone, cartilage and tendon formulation, the utilization of iron in the formation of haemoglobin and in the formation of melanin, the black pigment in hair.

Copper deficiency can cause bones to become weak and brittle, hair colour to fade. It is especially important in young growing horses as the development of bone collagen relies on sufficient presence of copper.

In fact, lack of dietary copper has been associated with the development of O.C.D.

Anaemia may develop with very low levels of copper due to its effect on the mobilisation of iron.

The daily dietary requirement for an adult horse is between 80-115mg. High levels of other trace minerals can interfere with the uptake of copper, for this reason some mineral supplements have more copper than the recommended daily requirement. Toxicity from overly high levels is very rare.

It is important that horses receive a daily mineral supplement and that it contains copper.

Note: Copper and zinc need to be balanced. Too much of one can interfere with the uptake of the other. The ideal copper to zinc ratio is 1:3. Most feeds and pasture contain sufficient zinc (a horse needs 40 mg per day) but it is important to be aware that too much zinc will affect the uptake of copper. High zinc intake can occur from water obtained from some bores, a horse can tolerate high levels of zinc as toxicity is very rare but it is the effect on copper that is important.

Also a high concentration of iron in the diet can interfere with both zinc and copper absorption, making already low levels of these minerals even less available to your horse. Check that you are not feeding no more than 8 times more iron than zinc.

For example the most common mineral imbalance found in hay is too much iron combined with low zinc and copper levels.


Chromium exists in two forms, the inorganic metallic form (Cr6) which is toxic to animals, and the organic form (Cr3) which is non-toxic and therefore is able to be fed as a supplement. Chromium is a constituent of a biochemical involved in the metabolism of carbohydrates and fats.

The benefits of chromium as an essential nutrient have been recognized by human athletes for years. Studies have shown how daily supplementation increases muscle gain and reduces body fat when combined with a physical training programme.

It is only since 1992 that equine researchers have clearly defined chromium’s role in improving health and performance in horses. Studies showed that horses on diets supplemented with 5mg of organic chromium per day had significant lower blood lactic acid levels after exercise when compared with control horses. Lactic acid is a by-product of energy metabolism. During intense exercise it accumulates in muscles and causes muscle fatigue. By supplementing with organic chromium, the length of time a horse can sustain a maximum effort during exercise is increased. Another benefit of chromium supplementation is the reduction in blood cortisol levels. Cortisol is commonly termed the “stress hormone” – Levels increase during stressful periods. Increased cortisol levels lead to a reduction in energy utilization and protein synthesis, important factors in the athletic horse.

More recent studies have shown that 5mg per day lowers blood insulin, especially in horses on high grain or high starch diets – such as performance horses, for example in racing. This means a more efficient use of glucose (ie energy) in the metabolism of the horse. Another study showed higher triglyceride values during exercise, possibly indicating more efficient fat mobilisation. Horses use fat for sustained energy.


Selenium’s most important function in the body is as a component of an enzyme called Glutathione Peroxidase – a powerful antioxidant.

As part of normal metabolism, cells produce waste products called peroxides. These substances are potentially dangerous to the integrity of the cell wall and must be neutralized quickly and efficiently. Glutathione Peroxidase is able to do this and hence protects the cell from the destructive effects of peroxides.

Because muscle cells metabolise large amounts of energy, they produce large amounts of peroxides. Selenium deficiencies usually manifest as muscle problems.

Large areas of New Zealand soils are deficient in selenium. Farmers often add Selenium in the form of pills. With horses, supplementation is a good idea, particularly horses in full training. Breeding horses have also been shown to benefit from regular selenium supplementation.

Before supplementing with selenium – it is important to take a blood test to assess the levels in the horse as too much selenium will be toxic and cause ill health, excessive in a short period may prove fatal. 

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