You're vet would be a wonderful place to get information that is specific to your area. Ask him if he has recommendations or knows of deficiencies in your area. For example, I know that our are is very selenium deficient, so I provide a selenium lick for my mare.
Here is an informational website about horse vitamins: http://www.healthy-horse.com/products/ref/vitamin.htm
It outlines the vitamins that horses need (minerals are a completely different subject). It doesn't specifically say how much to provide for your horse, but it will help make the owner more conscious of what they give their horses because it outlines the harms of deficiency and toxicity.