Feeding first cutting straight alfalfa can cause founder, but first cutting alfalfa is usually reserved for cows and goats. Feeding 2nd and 3rd cutting Alfalfa is fine for most horses, though it is high in digestible energy, so some horses can get "hot" or hyper if fed too much Alfalfa.
Timothy Alfalfa mix should be a good, safe mix and should sell well if you overproduce. If you want to feed the first cutting to your horses, I would let it cure in the barn for 30-60 days before feeding it, or let it cure in the field for a week if you cut during a dry spell. That will help take some of the "richness" out of the Alfalfa and make it safer for your horses.