If you really wish to do things right, the answer would be to get your hay tested and find out where its lacking and fill in the holes of the diet. You end up not only providing adequate nutrients where they are needed, but also balancing them in the process, which is very important. For example, if you have sufficient selenium in your hay and you add more, you could be getting toxic. If iron is high, it will be competing metabolically for copper and zinc and will win, making copper and zinc deficient even though your feeding it. If the calcium is high, it will make phosphorus and magnesium deficient in the face of it. Balance is everything and this is how you tailor it to each horse's needs, depending on age, weight and workload. A perfectly balanced diet for that horse....things couldn't be better.
A hay test is not expensive and will tell you the situation with every aspect of your horse's nutrition....major minerals, trace minerals, Se, salt, and more importantly, sugar and starch.....the whole nine yards. The hay test will also tell you its quality and how much your horse is really getting out of it.
A hay test is like a unique thumbprint. Doesn't matter which country it was grown in, what the growing conditions were, or when it was harvested. It will tell you exactly what the majority of your horse's diet is, right there and give you a plan for a perfectly balanced diet.