Selenium (Se) deficiency occurs in horses that don't receive enough selenium in their diet. A deficiency in selenium is more likely to occur in mares and foals living in regions with low selenium content in the soil. The amount of selenium that horse's require in their diet varies by age, weight, level of training/work load, and stage/type (growing, lactating, pregnant, etc).
Se Nutritional Requirements for an average, 500 kg horse

• Adult Pleasure horse: 1 mg/kg
• Performance horse: 1-1.25 mg/kg
• Stallion:
  1 mg/kg
• Pregnant Mare:
  1 mg/kg
• Lactating Mare: 1.25 mg/kg
• Young, growing horse: 0.42-1.07 mg/kg

European legislation recommends that horses should not be fed more than 0.5 mg of selenium per kg of total diet on a dry matter basis (equivalent to 5 mg for a 500 kg horse).