Selenium is an essential micro-mineral (also called trace element) which, along with vitamin E, is an important component of the body’s antioxidant system. It also plays an important role in thyroid hormone metabolism and growth.
Selenium intake of horses on forage-only diets is always too low in the UK, because our forages contain marginal levels of selenium. This is simply the nature of grass! Fertilisation of the soil with selenium can raise levels, but supplementary feeding is a more reliable method of meeting requirements.
Soil selenium levels and soil pH can alter the selenium level of grass, but often not enough to bring it up to the level required by a horse. For that reason, selenium should always be supplemented in the diet.
Selenium requirement is 1-2 mg daily for resting horses and those in light work, eating about 10 kg of dry matter daily. The requirement increases in harder working horses, up to about 3 mg daily. Elevated levels are also recommended for breeding horses, and 2-3 mg daily is often fed. Adequate selenium intake is vital for pregnant and lactating mares, because milk selenium content is directly related to the selenium status of the mare, and the growing foal relies on adequate selenium for optimal, healthy growth.
Selenium can be supplemented in the diet in the inorganic salt forms; sodium selenite or selenite, or organic selenium bound to amino acids e.g. selenium-enriched yeast. Organic minerals, which may be called ‘chelated’ are thought to be more bioavailable.
Selenium deficiency causes myopathy (muscle disorders) resulting in weakness, poor locomotion, heart dysfunction, breathing and swallowing difficulties. Blood plasma levels can show extremes of selenium status i.e. very deficient or toxic, and hair can also reflect longer term intakes, but values are difficult to interpret.
Maximum tolerable levels of selenium are relatively lower than for other essential micro-minerals, at about 20 mg in a daily intake of 10 kg of dry matter. In practice, aiming for a maximum intake of 5 mg daily is advisable. Selenium toxicity causes disturbances in keratin formation, possibly by a disruption of sulphur use and/or oxidative damage, and hair loss, hoof horn defects and marked lameness.
All horses fed UK grass-based forage require selenium supplementation to make up for shortages in the forage. Supplementation can be done via a vitamin and mineral supplement, a mineralised salt block (but be sure the horse takes in enough), a balancer or a compound feed (but be sure the full recommended amount is fed).