How much Alfalfa cubes should you feed to supplement SR. Horses?

I have a older gelding that has to have some teeth pulled recently. Even though he is in very good condition now I would like to make sure he does not start to lose wieght now with the bitter cold winters we have up here. He is on 6lbs of Purina Sr. now along with 3/4 of a bale of hay per day. I would like to cut back a little on the quanity of grain he gets but still get the same if not more calories. From what I have expierenced with our sub zero winters up here is a horse fares better with more hay than grain. I was always under the impression that a horse gets more calories from hay than grain. We grow our own hay which is a timothy/treefoil mix which wiegh about 80 to 90 lbs.