She means that Safe Choice is cheaper than the LD senior.
I was suggesting the senior because it worked for my mare. Not a fan of the Safe Choice...sorry
Alfalfa pellets: start slow, half a pound dry weight per feeding, then work your way up. I mix it in with the feed, soaked, so they don't fuss about supplements. Mine enjoy it a bit warm when it's cold out.
How much to feed depends a bit on how much he will eat. If he leaves some, he's obviously full.
As long as it doesn't freeze in the feeder he can finish it later( unless he is with another horse, of course).