Selenium (Se) is a nutrient that is important to your horse’s overall health. This mineral's main functions are that it is used in part to detoxify substances that are toxic to cell membranes, as well as playing a role in the control of some thyroid hormone metabolism.
In 1993 veterinary and laboratory surveys done throughout the United States by a group of researches found that there were cases of selenium toxicity in only 7 states, while there were reported problems of deficiency in 46 states, with deficiency being an “important livestock problem” in 37 states. ¹ This finding suggests that there is a much larger problem with deficiencies than toxicosis.
So what are the signs of a deficiency or toxicosis?
Deficiency creates a myopathy, or muscle disease that causes muscle weakness.
This myopathy can present in many different forms, including general weakness, trouble moving about, difficulty swallowing or suckling, respiratory distress, or impaired heart function.
On the opposite end of the spectrum, selenium toxicity can be either acute or chronic. The acute version of toxicosis creates the condition known as blind staggers. Blind staggers is a disease that is characterized by apparent blindness, head presing, sweating, colic and other digestive tract upsets, lethargy, and increased heart and breathing rates. Chronic toxicity creates the condition known as alkali disease. This disease is characterized by alopecia, or hair loss, and cracking of the hoof around the coronary band. Hair loss caused by chronic toxicity is most often observed around the mane and tail.
Horses who take in large levels of selenium over time could possibly see selenium begin to replace sulfur in certain tissues, such as keratin, an important part of the hoof and your horse’s hair. This replacement could potentially lead to weak hooves and unhealthy hair coats.
So where does your horse get selenium from in his diet? This very important mineral is found in many plants in varying degrees. The biggest factors that play a role in the amount of this mineral in any given plant is the amount found in the soil, as well as the pH of the soil. Soils that are more alkaline tend to encourage plants to absorb more selenium than plants that are grown in more acidic soil. D
Drought also plays a role in selenium uptake by plants. During hot, dry weather, plants naturally grow roots deeper, and since higher concentrations are found at deeper levels, plants naturally accumulate more when they grow deeper roots. During droughts your horse’s pasture also may not have as many plants as it typically does, so plants that accumulate high amounts and would typically be avoided may be eaten, further contributing to the problem.
A good number of mineral blocks currently on the market also contain this important mineral.
¹ Edmondson, A. J., B. B. Norman, and D. Suther. 1993. survey of state veterinarians and state veterinary diagnostic laboratories for selenium deficienty and toxicosis in animals. J. Am. Vet. Med. Assoc. 202: 865-872.