Few other species of animals can survive solely on eucalyptus leaves-certainly the horse could not By the same token the koala can not survive on grass hay. Before animals were domesticated and farming became commonplace, the horse evolved in such a way that it could exist on the prairie grasses where it roamed in the wild. We assume today that alfalfa hay, and more specifically grass hay, supplies the same elements, in the same form, as the prairie grass of old. This is not a valid assumption. Each animal requires just so much of each element-no less, no more.
Problems arise when the narrow range of dietary tolerance is violated.
Selenium deficiency and toxicity is a good example. Horses are very sensitive to the proper level of this element. Evolution is still taking place, but at a very slow rate compared to the change in our environment. We see only slight physiological differences among the various breeds of horses. The time it takes to develop a really different breed of horse shows how slowly evolution works.