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Selenium is recommended by the Food and Drug Administration (FDA) at a daily limit intake in the average horse at 0.3 ppm (parts per million) or 3 mg per day. Over 5 mg per day should be avoided in all class of horses unless recommended by your veterinarian.
Manganese--One study confirmed that horses require the level of manganese recommended by the National Research Council (NRC). This level is 40 ppm, and Rich said that horses in regions known to be deficient should be supplemented. In the study, horses receiving 40 ppm gained more weight, had better feed efficiencies, higher serum copper, higher serum hydroxyproline (an indicator of bone resorption), and a higher total bone mineral content than those fed a diet deficient in manganese.
Vitamin A-- According to the latest edition of the NRC's Nutrient Requirements for Horses, the maintenance level of A for horses is 30 IU/kg (14 IU/lb) BW. For growth the requirement is 45 IU/kg (20 IU/lb) BW, while for breeding, gestation, and lactation it is 60 IU/kg (27 IU/lb) BW. The presumed upper safe limit for intakes of A is 16,000 IU per kg of dry matter (16,000 IU/kg DM) (7,272 IU/lb DM).
Vitamin E toxicity is not a concern in horses, even with large amounts of supplementation. The upper safe dietary limit, which has not been determined specifically in horses, is set at 1,000 IU per kg of dry matter (1000 IU/kg DM) (455 IU/lb DM) based on observations in other species.
Vitamin K toxicity from the overconsuming of the vitamin has not been reported in horses. It is estimated that the toxic level of K by ingestion in the horse is at least 1,000 times the daily recommended intake. That's the good news. The bad news is that in one study, a researcher managed to cause acute renal failure in every study horse when he administered a single dose of menadione (the synthetic form of K) by injection according to the manufacturer's recommendations. This, paired with the fact that phylloquinone (one of the two naturally occurring forms of K) injections appear safer in newborn human babies, has led to the suggestion that phylloquinone injections be used if a horse needs to have K administered by injection.
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