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 more is poisonous.

Selenium is an essential component of the enzyme Glutahione Peroxidase, one of the body's natural anti-oxidants.  It protects the cells from damage by the waste products of cell metabolism.  Horses require between 1-3mg of selenium on a daily basis.  Large areas of New Zealand soils are deficient in selenium so supplementation is essential.

Supplementation should not be by a slap-dash approach.  The local soil status should be known as well as the levels in any prepared feeds being used in the diet.  A Blood test will determine a horse's blood level which will determine what level of supplementation is necessary.

Even 2-3 times the required daily amount over a long period of time, will cause chronic selenium poisoning.  Symptoms 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.

Selenium can be administered either in a liquid inorganic form, or in a organic form, or in a organic or chelated form.  The later is by far the preferred option.  Vetpro's Selenium K is a powdered chelated 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 organic or chelated Selenium (like 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 there is real vulnerability to creating an overdose.