Legumes are higher than grasses in potassium. Only toxicity is seen in HYPP disease in quarter horses. Magnesium and Sulfur - not a problem in normal diets of horses. Early grass is high in potassium and low in magnesium and may rarely cause a grass tetany problem in lactating mares. Magnesium oxide is often added to grains as a safeguard.
Sulfur is adequate in diets of horses.
Micro minerals - four are a problem in the North Central States. Iodine, selenium, copper, and zinc. Forages and grains produced in this area are usually deficient. Can handle with the use of trace mineralized salt. May want to use high level horse trace mineralized salt for pregnant broodmares and fast growing animals.