Bocking #4 comfrey is excellent for fodder and can be used medicinally.  Will not spread through self-seeding.  Use fresh or dried to supplement feed.  Large leaves can be pruned or snapped off as needed throughout the growing season.  Remove any flowering stalks to keep comfrey producing large leaves during the growing season.  Does best when watered regularly for production.  
 
Excellent as a "chop and drop" mulch for fertilization, or make a compost tea for plants.  Easy to plant; just lay the fragmented rootlet horizontally in a trench approximately 2-3 inches deep and water regularly.  Vigorous grower, should be divided every few years to maintain the health of the plant and expand your comfrey plantings... or just share with a friend!