Diane Larkin's answer is correct but I wanted to add a couple of details.
When formulating a ration, I will add in a small amount of alfalfa cubes (or if I can get it, alfalfa chaff or alfalfa hay) to horses to increase chewing time and to act as an acid neutralizer. This will reduce the risk of non-glandular gastric ulcers.
Alfalfa cubes can be of different sizes. If they are small, you can add a handful directly into the cereal or feed ration. If they are large, then they can be soaked and given as part of a mash. Or the horse can just pick them apart.
So basically, the role of the alfalfa in any form is to increase the protein and calcium content of the diet, it has a few absorbable minerals, it is a good fibre source and it acts as an acid neutralizer.
I do not use alfalfa hay as the only fibre source for the horse because of the high protein content and increased risk of formation of enteroliths. I also wouldn't recommend alfalfa cubes (or any hay cube) because the horse will eat it faster and end up with nothing to do all day long. These bored horses end up with vices like weaving or crib biting. Get a good grass hay and mix in 2-3 kg of alfalfa hay or chaff.
If you want to use cubes, mix them into a mash or mix them with the feed to keep him chewing. If you do decide to do a mash, make sure that it doesn't sit around too long and spoil. If it ferments, you are looking at another colic.