Guinea pigs can, and should, eat fresh grass daily. Guinea pigs graze, nibbling grass and plants both day and night, which keeps them full and helps keep their teeth from becoming too long.
While hay and fresh grass are recommended as the main staples of a guinea pig's diet, lawn clippings should not be given to the creatures as they can cause digestive problems. Citrus fruits should also be avoided, but guinea pigs can eat leafy greens, such as kale and broccoli, daily. Apples and root vegetables, such as carrots, can be given to guinea pigs as treats. Additionally, guinea pig food pellets provide essential vitamin C to keep the small creatures healthy.