Anti-oxidants such as selenium and vitamin E are your horse’s first line of defense against cellular damage by free radicals, and are essential for the proper functioning of the immune and muscular systems. Symptoms of deficiency can include muscular atrophy, lameness, sweating, and overall muscle pain. This deficiency can make many activities, including riding and even grooming uncomfortable for your horse.
Essentially, organic selenium is attached or associated with a compound that contains carbon, the most common example being selenium-enriched yeast. The inorganic form contains no carbon and is often chemically formulated, with the most common form being sodium selenite. It has been proven in research that not only is a higher percentage of the organic form of selenium (selenium-enriched yeast) absorbed than the inorganic form, sodium selenite, but selenium-enriched yeast seems to be more effectively stored and utilized within the selenium-dependent processes in the body. There is also evidence that the organic form tends to be safer than the inorganic forms.
The selenium content of feeds varies depending on where the plants were grown across North America, the soil content of selenium fluctuates significantly. Some areas, such as Central Oregon and parts of Washington, are so selenium deficient that crops grown there are considered to contain no selenium at all, so animals fed on them can develop deficiencies and require supplementation.