I have had good luck with cubes, except on one horse... He decided they were very tasty and tried to eat them as fast as possible and had a minor choke episode. Since then, I will only feed them soaked. I spray a moderate amount of water over the cubes, to wet them down thoroughly, then let them set for 10-20 minutes until they start to break up. Depending on how "hard" you cubes are, you may need more water.
You should feed by weight. Feed about 1 lb a day for a few days, then you can increase as needed. Generally you should feed between 3-5 lbs of Alfalfa daily when supplementing for poor quality hay or for weight gain, but you may need less if you notice horses gaining too much weight or getting a little frisky
My TB x Arab gelding can only have 1 lb of alfalfa a day. He gets 2 lbs of Timothy hay pellets on top of that to maintain his weight. With light work, he maintains his weight well with just that, no feed, plus 1/2 cup of flax meal, a vit/min supplement, and free choice grass hay. Before going the "all natural" route for feeding, he had to have 6-7 lbs of grain a day plus 2 cups of fat (tried oil, rice bran, and flax) and a probiotic just to maintain his weight with moderate training. We tried all kinds of feeds and he was high as a kite on all of them... Since taking him off all of that he has calmed WAY down, holds his weight on a lot less feed, and has better feet