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Selenium Deficiency in Australia - What It Means for Your Horse

Selenium is a trace mineral that plays critical roles in the horse's antioxidant defence system. Learn why selenium is important for your horse.

What is selenium?

Selenium is an essential element of the diet and is available via plants from the soil. In conjunction with vitamin E, selenium has antioxidant properties and may help to protect cells from damage. It is also involved in immune function and is also necessary for growth and fertility.

Why is selenium important for your horse?

Oxygen is required by the horse to fuel chemical reactions that produce energy. These chemical reactions produce oxidative by-products (often referred to as “free radicals”) which can damage cell proteins and DNA if they are not kept in balance with antioxidants. The production of these free radicals increases when a horse’s activity increases and its body consumes more oxygen as fuel.

Antioxidants are produced to inhibit the detrimental effects of these free radicals. The inability of the horse to produce enough antioxidants can result in the accumulation of free radicals causing oxidative damage to cells. Along with vitamin E, selenium is considered an antioxidant because it prohibits free radicals from damaging cell membrane

Selenium requirements, deficiency and toxicity in the horse

The selenium requirement of the horse has been estimated at 0.1 mg/kg ration (dry matter) which equates to 1 mg per day for a mostly resting 500 kg horse (National Research Council, 2007).

Selenium deficiency can result in poor muscle development and pale, weak muscles (White Muscle Disease) in foals on deficient diets, lower fertility in mares, poor performance, and may predispose to “tying up” in racing horses.

The maximum tolerable level of selenium in a horse’s total diet is only 20 mg per day. As little as 50 mg per day total intake can result in mild signs of toxicity. Toxicity is more common in dry regions and in these situations, horses can suffer from “alkali disease” in which excessive selenium causes a loss of hair on the mane and tail, lameness and bone lesions, including twisted legs in foals and sloughing of hooves (Frape., 1998). Animals consuming more than 1 g per day--or those that are inadvertently overdosed with selenium even once--can develop acute selenium poisoning, which will rapidly cause death. Clinical signs of acute selenium poisoning include gait abnormalities, garlic-breath odour, laboured breathing, and muscle tremors.

Selenium availability in pasture

There is a marked seasonal variation in the availability of selenium from pasture, with lowest levels occurring in spring and summer (Caple et al., 1980). Light soils and lush legume-dominant pastures are most often associated with selenium deficiency. There is also a variation in availability between years. In other species, White Muscle Disease is most prevalent in years when there is good autumn rainfall and abundant clover growth in spring (McDonald and Caple, 1977).

In one study, samples were collected during and after a spring flush of pasture growth in both “light” and “heavy” country in Western Australia. As the rainfall levels increased, the selenium levels were found to decrease as shown in the table below.

Table 1. Plant selenium levels in varying rainfall areas in Western Australia (Gardiner and Gorman, 1963).
Rainfall Belt Plant selenium levels
10 – 15 inch 0.260 ppm
15 – 20 inch 0.080 ppm
21 – 30 inch 0.056 ppm
30 + inch 0.036 ppm


Horses require about 0.1 mg available selenium per kg feed to meet their dietary requirements for this element. Based on the plant selenium levels tabulated above, in this study, the only pasture that would meet the horse’s requirement would be that found in the 10 – 15 inch rainfall belt.

Interestingly, pasture improvement programs also appear to aggravate the position. Heavy applications of superphosphate decrease the concentration of selenium in pastures and may also decrease the uptake of selenium by grazing livestock (Caple et al, 1980, Halpin et al., 1981). The age of the pasture also plays a role, with selenium levels decreasing as the plant grows older. For these reasons, selenium deficiency in livestock is often spasmodic, perhaps appearing in particularly good seasons. This may help to explain the puzzling situation in which poor animals (particularly livestock) are sometimes seen in the midst of abundant, high-quality pasture.

The selenium content of hay and cereal grains depends on the selenium content of the soil in which they were grown. Much work is still to be done to define selenium deficient areas in Australia and in marginal areas it will always be difficult to predict seasons in which problems are likely to arise. The following map outlines the identified areas of selenium deficiency in Australia (Judson and Reuter, 1999).

Selenium deficient areas in AustraliaFigure 1. Selenium deficient areas in Australia (Judson and Reuter, 1999).

When is supplementation required?

Selenium is an essential element which means it must be available in the horse’s diet for the horse to maintain health. Most importantly, it is essential that pregnant mares receive adequate selenium in their diet as their status affects that of their foals at birth, whereas their milk only provides modest amounts (Frape, 1998). Horses in heavy work also have an increased requirement for selenium with low blood selenium levels being associated with poor racing performance among thoroughbreds in the UK (Blackmore et al., 1979, 1982). If your horse’s diet is based on pasture or hay or “raw” ingredients such as grains, it is likely selenium supplementation is required.


  • Blackmore, D.J., Willett, K. and Agness, D. (1979). Selenium and gamma-glutamyl transferase activity in the serum of Thoroughbreds. Research in Veterinary Science, 26, 76 – 80.
  • Blackmore, D.J., Campbell, C., Dant, C., Holden, J.E., and Kent, J.E. (1982). Selenium status of thoroughbreds in the United Kingdom. Equine Veterinary Journal, 14, 139 – 43.
  • National Research Council (2007). Nutrient Requirements of Horses 6th Edition, The National Academies Press, Washington, D.C.
  • Caple, L.W., Andrewartha, K.A., Edwards, S.J.A. and Halpin, C.G. (1980). An examination of the selenium nutrition of sheep in Victoria. Australian Veterinary Journal, 56:160 – 167.
  • Frape, D. (1989). Equine Nutrition and Feeding, Second Edition, Blackwell Science Ltd.
  • Halpin, C., Caple, I., Schroder, P. and McKenzie, R. (1981). Intensive grazing practices and selenium and vitamin B12 nutrition of sheep. In “Trace Element Metabolism in Man and Animal. 4” pp 222 – 225. Australian Academy of Science, Canberra.
  • Judson, G.J. and Reuter, D. J. (1999). Selenium. In: Soil Analysis: An Interpretation Manual. pp 325 – 329. Melbourne: Australia: CSIRO Publishing.
  • McDonald, J.W. and Caple, I.W. (1977). The occurrence and distribution of selenium responsive disorders in Australasia – Victoria. In “Selenium in Australian Agriculture”. Workshop Papers, CSIRO Division of Plant Industry, Canberra.
  • Mendell, C. (2006) Selenium: A balancing act. The Horse, article 15982

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