Additionally, Se is a component of an enzyme that helps produce the active form of the thyroid hormone that regulates metabolism.
Dietary Requirements of Se
Too little or too much Se can lead to serious health consequences, but there’s still some debate about the horse’s actual dietary requirement of Se. The National Research Council’s Nutrient Requirements of Horses (NRC 2007) recommends a minimum of 1-1.25 milligrams (mg) of Se per day for a 500-kilogram (kg) (1,100-pound) horse. To put that in perspective, a paper clip or a dollar bill each weigh approximately 1 gram, and 1 mg is 1/1000 of a gram!
Research suggests slightly more, about 2.5 mg of Se per day, might be beneficial for optimum immune function and prevention of deficiency.
Too Little or Too Much?
As we’ve ascertained, too much or too little Se can cause problems for horses.
Signs of Se deficiency include:
- Impaired movement;
- Respiratory distress;
- Impaired heart function, and
- Difficulty swallowing or suckling.
Conversely, horses are extremely sensitive to excess Se. Signs of toxicity include:
- Apparent blindness;
- Head pressing;
- Abdominal pain/colic;
- Increased heart and respiration rates; and
The minimum lethal dose of Se reported was 3.3 mg Se per kg of body weight, or 1,650 mg for a 500 kg (1,100-pound) horse. Signs of chronic, sub-lethal Se toxicity can include hair loss (especially around the mane and tail) and poor hoof quality, including sloughing of the hoof. Horses are much more susceptible to Se toxicity compared to other species, such as cattle and pigs.
Selenium concentration in common feedstuffs ranges from 0.001 to 0.3 mg per kg, and is largely dependent on what’s in the soils in which those products are grown. Soils in the eastern half of North America and the Pacific Northwest (including Canada) tend to be deficient in Se, whereas portions of the Midwest and Southwest have more Se-rich soil. Horses consuming a forage-only diet grown in Se-deficient areas might benefit from a ration balancer to ensure Se needs are met.
Despite the small requirement for dietary Se, its role in equine health is hugely important. A qualified nutritionist can help you determine proper Se balance in your horse’s diet.