Sorry Saddlebag, Delete is right. You are confusing lack of calcium with excess calcium. Here is an excellent article I found on the net. Living Legends - Horse Health - Nutrition
Alfalfa will actually prevent "big head" since "big head" is a sign of calcium deficiency. Feeding too much grain and bran without extra calcium can cause it. So the alfalfa actually prevents "big head."
I live in Arizona, and right or wrong, we have practically no other hay than alfalfa and bermuda. Right now I feed a mixture of both, but back in my boarding days the barn owners fed straight alfalfa, even in the 120F summers! No horse parished from straight alfalfa.
As a matter of fact, they seem to thrive on it.
The only negative I have heard about alfalfa is that it may increase the chance of entroliths. But I have head the kidney thing is false, although the horse may drink more water with alfalfa to clear out the excess protien and calcium.
But alfalfa is a great hay for growing horses. I have a yearling who is thriving on it. He does get some bermuda as well, but I would say at least 60% or more of his diet is alfalfa.
If you can get a mix of alfalfa/grass, that is ideal. But don't be afraid to feed straight alfalfa. Those of us in the southwest do it all the time.
Like all feeds though, I would try to make the switch gradual as possible by mixing a bale of alfalfa with the existing hay before you feed alfalfa straight. It IS a rich hay. But the horses LOVE it and are very sleek and shiny. It keeps horses fat, so you don't want to feed it free-choice.