Ask any exotic vet what the single most important food for a rabbit or guinea pig is, and the answer is almost always the same: hay. Rabbits and guinea pigs have delicate digestive systems that rely on a constant supply of fibre to keep everything moving — a diet low in hay can quickly lead to serious tummy trouble. That’s why grass hay should make up around 80% of their diet. Small pets’ teeth also grow continuously, and the natural chewing action of eating hay wears teeth down evenly, helping prevent painful dental problems.