Cat grass is a combination of barley, oat and rye grasses. When you purchase cat grass try to avoid buying just one type of the three varieties of grass. Your cat will be happiest with a combination of grasses. Think of it as a perfect side salad for your cat. Just as you would be happier with a mixed type of leafy greens so will your cat. Eating grass is a natural behavior for your cat. Cats enjoy the crunching and chewing of grass. The smell of fresh grass is also enjoying for them. Often you will see your cat rubbing and smelling the grass. This is all a natural part of outdoor behavior that indoor cats seek. Cat grass is also considered a healthy natural supplement for cats. The high fiber content helps eliminate hairballs and improves overall digestion.