Many animals require fiber in their diet to stay healthy. Without adequate amounts of fiber, illnesses such as gut stasis and enteritis can occur. Rabbits need more fiber than other animals because of their unique digestive systems. Their wild ancestors were sources of prey for many animals, so they had to be able to run quickly from predators. If there stomachs were full of food, they could not run as fast and survival depended on speed.
Rabbits are grazers, eating small meals continually throughout the day.
The digestible fiber contained in leafy portions of hay passes quickly through the stomach and intestines and is partially digested by the cecum, or hindgut. When food is ingested, the larger particles pass through the digestive system faster. This faster rate of passage keeps the other food particles moving and prevents blockages such as hairballs. The digestive system works best when there is a consistent rate of passage instead of it continually changing due to varying types of feedstuffs. There is no way to guarantee how hay will look or feel from one year to the next.