Veterinarians recommend that grass hay make up 70-80% of a small animal's diet (in this case, we are talking about rabbits, guinea pigs, and chinchillas).
Providing quality grass hay is extremely important for two primary reasons:
1) Your Pet Requires A Diet That Is High In Fiber
It's important to understand these animals have unique digestive systems that have evolved around a diet made up primarily of grasses and other fibrous material that they naturally find in the wild. Fiber provided by grass is absolutely essential for your pet to remain in good health. And the best way to do this for your pet is by providing high quality grass hay (after all, hay is just a dried grass).
2) Your Pet Needs to Be Chewing Constantly
Rabbits, guinea pigs and chinchillas have teeth that constantly grow throughout their lifetime. To survive in the wild, these animals have to constantly munch their way through plenty of tough food to get the nutrition they need and to maintain good digestive health. At the same time, the chewing of all this plant material causes friction and that friction wears down those ever-growing teeth. Despite domestication, rabbits, guinea pigs, and chinchilla teeth grow at a pretty ferocious rate. As a result, it’s essential to provide our pets with high quality hay they can chow down on whenever they want.