Selenium is an essential mineral nutrient that is required for the best health, vitality and performances of all horses. Selenium is present in all grains and forages at levels that are dependent on the selenium content of the soil in which they were grown. Low selenium soils deliver low selenium grains and hay, Unfortunately a large portion of the soils in the U.S. and Canada are selenium poor. The most deficient areas are the great lakes region, the entire east and eastern seaboard, the pacific northwest and almost all of Florida. A large portion of the south central united states, the Midwest and the north central western states are marginal at best in sore levels of selenium and may not yield grain or hay with adequate selenium levels. In areas that are considered adequate in soil selenium, selenium requirements may not be met with diet alone depending on the work and stress level of the horse. Marginal selenium deficiencies seem to be widespread especially in unsupplemented diets.
Selenium along with vitamin E is an essential part of the enzyme glutathione Peroxidase (GSH-PX). This enzyme, which is present in all tissues of the body aids in the detoxification of toxic free radicals that can damage cell membranes, As such selenium is classified as an anti-oxidant. Anti-oxidants protect the cells from these highly reactive, damaging radicals that can negatively affect any tissue in the body. The antioxidant capabilities of selenium along with vitamin E help the body eliminate toxic free radicals. Free radicals can form from radiation exposure, drugs, chemicals, environmental poisons, air, water and other pollutants, toxic metals and a number of normal metabolic activities such as immune function and the breakdown of proteins, carbohydrates and fats.
Low levels of selenium can increase free radical based injuries and damage resulting in inflammation and an imbalanced immune system.
The liver, kidney and muscles including the heart contain the highest levels of selenium, but selenium deficiencies can affect a wide number of body tissues. Selenium deficiencies can cause decreased fertility red blood cell damage, muscle weakness and degeneration including the heart muscle, slow wounds healing, decreased tolerance to stress, decreased ability to detoxify drugs and chemicals, excessive inflammation, weakened immune function, an increased generation of toxins from body fat, so called “Dummy Foals” with problems swallowing, nursing and moving, A possible decrease in thyroid function and a possible connection to muscle cramping or tying up. Also in foals low selenium levels can cause poor muscle development and weakness (white Muscle Disease). Sudden cardiac death can occur if the heart muscle is affected. Some hoof problems can also be related to low selenium levels.
The NRC requirements for selenium are .1mg/kg of diet consumed with an accepted maximum tolerance of 2mg/kg of diet consumed for toxicity. For a 500-kg horse (1100 pounds) a selenium intake of about 1-3 mg/day may be best. Since feed manufacturers are required by law to limit the amount of selenium added to foodstuff at the .1mg/kg feed, Additional selenium supplementation may be helpful for optimal selenium intake.
A number of stress factors can affect an individual horse’s selenium need.
The amount of general stress the horse is exposed to such as temperature extremes,travel, abrupt dietary changes, boarding and injury can increase selenium need.
The horse’s workload. Hard working and performance horses have higher requirements for selenium. Exercise increases the amount of free radical stress and more selenium may be needed to remove the toxins generated from exercise.
Fever, infections, immune problems and other illnesses can increase selenium need.
Brood mares have generally higher nutritional requirements especially for the trace minerals including selenium.
Young horses foals, and yearlings, during rapid growth periods will require optimal levels of selenium for normal development.
Selenium is relatively well absorbed from natural selenium containing feeds or as sodium selenite, selenium absorption is greater in horses that are selenium deficient and less selenium is excreted until the tissue levels of selenium are stabilized. Selenium itself is stored in the body tissue mainly as selenomethionine or selenocystive. Both methionine and cystine are sulfur containing amino acids important for selenium utilization.
Balance is essential for optimal selenium bioavailability and use. Adequate levels of vitamin E are needed for selenium utilization and vice-versa. Vitamin C and the nutrient mineral copper are also important for optimal selenium status and use. Interestingly when copper levels are well balanced the dose level for selenium to become toxic increases, this is yet another example of how the intricate balance and interrelation ships between nutrients can protect the tissues from toxicity and improve nutrient function.
