Calculations put together by Dr Marty Adams PAS
The National Research Council (NRC) has estimated the minimum selenium requirement for horses at 0.1 milligram (mg) per kilogram (kg) of feed consumed. Using this requirement as an example, a horse eating 25 pounds (lb) or 11.3 kilograms (kg) of feed per day (25 lb ¸ 2.205 = 11.3 kg) would require at least 1.13 mg of selenium daily (11.3 kg x 0.1 mg/kg of feed = 1.13 mg).
The NRC estimated the maximum tolerable level of selenium for horses at 2.0 mg/kg of diet. For a horse consuming 25 lb, or 11.3 kg of total feed daily, (25 lb ¸ 2.205 = 11.3 kg), this would be a maximum limit of 22.6 mg of selenium per day (11.3 kg x 2 mg/kg = 22.6 mg).
The NRC has also calculated the LD50 for selenium, or the amount that would be toxic enough to kill 50% of horses consuming this amount, at 3.3 mg/kg of body weight. For a 1,100-lb (500 kg) horse, this would be 1,650 mg of selenium per day.
The Food and Drug Administration (FDA) has approved a maximal dietary selenium supplementation of 0.30 ppm for cattle, sheep and poultry, and this regulation has been generally adopted for horses. However, the FDA has never made a specific regulation for horse feeds and selenium supplementation is restricted only by nutritional recommendations and industry practices.
Let’s figure the amount of dietary selenium for a horse eating 6 pounds of XX horse feed (0.6 ppm selenium content), 15 pounds of Grass Hay (0.1 ppm selenium content) and 2 ounces XX Horse Mineral (16 ppm selenium content) per day.
XX feed: 6 lb or 2.7 kg (6 lb ¸ 2.205 = 2.7 kg) x 0.6 ppm or 0.6 mg per kg of selenium (2.7 kg XX x .6 mg/kg selenium) = 1.62 mg.
Grass Hay: 15 lb or 6.8 kg (15 lb ¸ 2.205 = 6.8 kg) x 0.1 ppm or 0.1 mg per kg of selenium (6.8 kg hay x 0.1 mg/kg of selenium) = .68 mg. XX Horse Mineral: 2 ounces or 56.7 g (2 ounces x 28.349 grams/ounce) or .0567 kg XX Horse Mineral (56.7 g ¸ 1,000 g/kg) x 16 ppm selenium = .91 mg.
Daily Selenium Intake = 1.62 mg + .68 mg + .91 mg = 3.21 mg.
Dietary Selenium Level = 4.37 mg ¸ (2.7 XX Feed + 6.8 kg Grass Hay + .0567 kg XX Horse Mineral) = 3.21 mg ¸ 9.56 kg = 0.34 ppm.
This amount of selenium is above the minimal requirement of 0.1 ppm of the diet, slightly over the FDA’s 0.3 ppm recommended dietary level for other livestock species, but is in the optimal range of 3 mg per day recommended by most equine nutritionists, but well below the maximum tolerable and toxic levels.