Pasture breeding is the most natural form, as it consists of turning a stallion out in a field with the mares you'd like him to impregnate. The horses breed when they feel like it, developing a herd structure along the way. The regular mating promotes the chances of a mare getting in foal. However, it's rare that high-value stallions are used for breeding in this manner, as the risk of injury increases considerably. Mares who aren't interested in breeding can severely injure a stallion.