This is not necessarily true, there are many varieties of alfalfa grass, some are poisonous, some are not.

White-flowered alfalfa grass is slightly poisonous, cattle, sheep, rabbits eat too much will be fatal, usually planted as an ornamental plant for flower beds.

Alfalfa grass is not poisonous, and because it is high in protein and calcium, it is generally grown as pasture.

The sky blue alfalfa grass is not poisonous, can also be used to feed herbivores, a very common weed, young leaves blanched and cooled people can also eat.