Selenium and the Horse

Selenium is a trace element required by the horse.

Selenium is not manufactured by plants, but is absorbed by them from the earth. If the soil is low in selenium then it is likely that both hay and oats grown on it will also have a low content . Most soils are not low in Se but some are.

Main electrolytes : Na K Ca Mg Chloride Phosphate

Trace Elements : Fe Cu Zn Co selenium S I

To check on the electrolyte levels of a horse is not straightforward; Blood Plasma Concentration gives an overall guide to electrolyte/mineral levels, but will only detect large deficiency or excess. However horses are very capable of maintaining plasma levels to the detriment of tissue levels therefore a combined blood urine analysis may be better. Even so it is not easy to analyse out a trace of selenium in comparison to a main electrolyte such as Sodium. Possible but difficult, therefore probably expensive!

Most equine diets have a natural excess of electrolytes.

BUT soils deficient in selenium may cause problems for horses especially broodmares youngstock .

Selenium is a key part of the blood cell enzyme glutathione peroxidase which in conjunction with vit E, removes free radicals {very reactive damaging substances that are sometimes formed within cells, see below} from the body. Selenium and/or Vit E deficiency have been shown to cause muscle disease in young horses.

Substantial excess Selenium is TOXIC.

Toxicity is partly due to selenium replacing sulphur in proteins, typically causing diseased hoof horn, known as alkali disease. If poisoning is acute then death occurs due to organ damage, especially the liver. Some plants e.g.. vetch can contain selenium at up to 6000ppm, this is considered sufficient to cause acute poisoning if eaten in any quantity.

Quotes for absolute daily requirement appear contradictory, but in the UK, where Se additives are not necessary under normal conditions the herbage typically contains 0.1 mg per kg of dry matter. Therefore it doesn't seem unreasonable to conclude that a possible inclusion rate for an adult horse would be 1mg per day as a feed supplement. Youngstock typically require a greater level of trace elements than adults, but given the nature sensitivity of Selenium in the diet, I would not be inclined to exceed this level unless professionally advised to do so.

Selenium requirement action is known to vary with VitE levels, therefore a supplement that contains both VitE Selenium would seem sensible. A few years ago I did feed one such supplement to help a mare recover from a skin hair infection. My observations aren't scientifically based, but it did seem to help her.

Did you know that Brazil Nuts are a good source of Selenium? Just 1½ Brazils will supply a human with their daily requirement.

Free Radicals - these are substances that are highly reactive due to the fact that they have an unpaired electron. They typically only exist for a millisecond but in sufficient quantity they can do immense damage in this time. It has been shown that people with abnormally high free radical levels are very prone to heart disease. It is also suggested that the natural ageing process of the body, is strongly linked to the cumulative effect of free radicals, over the years.

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Updated: October 2005.