Dry alfalfa will grind through a grinder/mixer hammermill just fine if you have a way to feed it in.     Grass hay is much tougher (if not nearly impossible) to grind through grinder mixer.


My dad used to have a Farmhand grinder mixer that the intake side of the mill folded down into somewhat of a hay table.    We'd just set about half a square bale of hay on there at a time and slowly push it in using a scoop or a slice of hay to push with.

Its been a while but I think that the hay was ground through the same 1/4" screen that the corn was ground through. This was mainly mixed into the ration for gestating sows.