There are supplements that will compliment your forage.

Grass hay is low in calcium and Alfalfa hay is low in phosphorus.

Please keep in mind that protein does not make a horse "hot", carbohydrates are the culprit. Protein is needed to build muscle and bone. Most of they hay that is available is not made under perfect conditions and will not provide your horses with the minerals and vitamins they need.

I hope this helps!