Hay is an important part of a guinea pig’s diet, and should be available to it at all times.
The best type of hay for guinea pigs is timothy. You can buy timothy hay at pet supply stores, feed stores or even from local horse stables. Make sure the hay is fresh, has a sweet smell and not much dust. Mold can be harmful to guinea pigs, so make certain the hay is dry and free of it. Store the hay in a cool, dry spot.
Give guinea pigs the hay in a hay rack, or by placing it in a small litter box inside the cage. Remove old, uneaten hay before you add fresh hay.