Typically, toxic selenium levels only become a problem if the horse consumes an extremely large amoumt at one time or if the horse has consumed upwards of 20 mg/day for an extended period. In reality, a selenium deficiency is far more likely to occur due to the fact that many parts of the United States have inadequate selenium in the soil. Low selenium intake symptoms inclued poor fertility and myopathy problems. In addition, it causes a predisposition to inflammatory issues and decreased immunity.