Horses can become seriously ill from overdosing on vitamins and minerals. When given free choice they take in what they need. When the vitamins and minerals are "hidden" in goodies that the horse wants to eat, it is easy to overdose them. How have you determined what she needs? Are you guessing or have you had blood work or advice from a vet?
Some vitamins are water soluble and excess is thrown off by the kidneys. Some vitamins are fat soluble and are stored in body fat, no excess being thrown off. Too much of a good thing can be bad.