Lucerne cubes are generally used in the horse world as a convenient way to feed. In the small animal world lucerne cubes are generally used as a "treat" or "toy".
Lucerne cubes are compressed chaff (finely chopped hay) to form a solid block of hay. Small animal owners use these as a chew toy with the bonus of nutrition. For older guinea pigs and rabbits use these as treat no more then once a week as an alternative to normal chew activities.