Alfalfa pellets are a great addition to a horse's diet to increase the calories. 

They can be fed in conjunction with almost any other feed, including beet pulp or alfalfa cubes. 

If fed with a long-stem fiber source (grass/pasture, beet pulp, alfalfa cubes) they can be used to replace calories lost when hay is scarce or unavailable, but they cannot be used alone to completely safely replace hay.