I have been feeding high quality alfalfa hay for the past 18 years. In Arizona, it's what we feed.
Really, I know alfalfa gets a bad wrap, but it's awesome hay, high in protein and calcium and easy-keeper type horses stay fat on two flakes a day.
It's not something you feed free choice because it's rich. But boy, the horses love it and they always look very healthy.
Now with my yearling I am also giving him some bermuda grass hay as filler so he always has something to much on. And it DOES have a better calcium to phosphorous ratio than alfalfa. So I think in an ideal world, about a 50/50 mix is perfect.
But I and everyone I know has fed straight alfalfa to adult horses (in limited portions such as two flakes a day) and they seem to thrive on it.
It is low in sugar too.
PS. Someone worried about the calcium to phosphorous ratio could also feed a bit of straight oats to help balance the alfalfa out. That is what my vet told me to do when my mare was pregnant to make sure she had some phosphorous.