Dont drop the alfalfa! Alfalfa absorbs excess stomach acid that causes the ulcers to form, so keep that in the diet.
Like others have said, find a low starch, low sugar feed if the horse still needs extra feed on top of the alfalfa pellets. I would reccomend Dengie Alfa-A oil, or Dengie Alfa-A Molasses free. Niether of these feeds have molasses, so are low sugar, and both are low starch. These feeds aren't pellets, but I prefer the uncondensed feeds, more natural. GDA for a 500kg horse is 1kg for these feeds so it works out about £25 a month for me in winter, <£12 in summer.
I have tried SO many suplements for Alli, but non have worked as well as the pure alfalfa diet! Now trying nettle tea (LOLZ) as it helps repair the stomach lining. Liquorice root is something I am also looking into as it reduces acid secretion (a little like omeprozole and rantidine, just less severe and much less costly!)
We lose ourselves in the things we love, we find ourselves there too ~Kristen Martz