Alaa A is basically chopped alfalfa isn't it? With some additives probably - might want to look on Dengie's website to check what else is in it, as over-supplying nutrients such as vits and minerals can be just as dangerous as undersupplying them.
Why do you want to feed it instead of hay? So you can get more nutrients in? So your oldie has less to chew?
Hay chop is also available much more cheaply, if you can find someone who makes it (not seen any branded bags, the people I know who get it source it from a local producer in re-usable sacks, if you understand what I'm trying to say)
Only thing I'd worry about is making sure the horse has little and often like they would when grazing or eating a haynet: remember their digestive system was built for trickle feeding rather than meals. Most horses I know would guzzle the Alfa-A and then stand bored for the next few hours
You are not advised to feed it as a total hay replacer as protein level too high and minerals can also be unbalanced when fed in large quantities. Can cause excessive urination in some horses. Dengie will advice maximum per day for your horse based on his size.
Sugar Beet makes good hay replacer as well. I would steer clear of Alfa-A because of all the added molasses. Speedi-Beet or Simple System PuraBeet can be fed in large quantities and is also a lot cheaper than A-A.