Selenium is an essential trace element but, unlike most nutrients, it has a low-toxicity threshold. Care is required to achieve the right balance between the correct amount and over-supplementation. The authorities also take a prudent view and list only certain forms of selenium for use in feeds and supplements.
Selenium is best known as an antioxidant, but it has other roles including iodine metabolism, repair of DNA and in the immune system (although this may be linked to its antioxidant properties).
As an integral part of the enzyme glutathione peroxidase, selenium’s antioxidant role is within the cells of the body to prevent the formation of free radicals. It is often mentioned with vitamin E because both nutrients work within the cells to prevent free radical damage. A deficiency of one increases the requirement for the other, and vice versa.
Horses typically require 0.1mg/kg selenium in the total diet when inactive, increasing to 0.3mg/kg in exercising horses and broodmares.
UK soils, and the plants that grow in them, are generally low in selenium, so forages and cereals typically have low selenium contents (0.1 mg/kg). As a result it is often necessary to add selenium to the diet via hard feed or supplements.
A classic sign of selenium deficiency is white muscle disease in foals and young stock. The mare transfers selenium to the foetus during pregnancy and passes more to the foal through her milk. She must receive plenty of selenium (via a dietary supply above 0.05 mg/kg) throughout her gestation to set the foal up for early life.
At the other end of the spectrum, the first signs of toxicity are poor hoof quality and pain around the coronet band, although the classic signs are the sloughing off of hooves and loss of mane and tail hair.
- Compound feeds are generally supplemented with selenium at levels of 0.2-0.3mg/kg feed. The selenium source is sodium selenite. The EU has recently banned selenium yeast
- Supplements, particularly those supplying antioxidant or immune system support, can typically supply 1mg selenium per dose
- A healthy adult horse eating 10-12kg of compound feed and forage per day will consume about 2mg of selenium per day, before supplement addition
- The toxic threshold for selenium is as low as five times the requirement. Double-dosing — giving supplements and feed that both contain selenium — may approach toxic levels