Hay and grass is an important ingredient in the guinea pig's diet and should be fed in unlimited quantities. Any hay given to guinea pigs must be dry, clean and free from dust and mould and any grass a guinea pig has access to should be free from soiling by other animals and chemical treatments.

Grass hay such as Timothy or Oat hay is best given although alfalfa hay can be beneficial to young growing guinea pigs and also pregnant and nursing mothers. However, alfalfa hay is considered too high in calcium and protein for continued use throughout adulthood although can be given as an occasional treat.