Hay, or grass hay is an extremely important part of a guinea pig's diet. Guinea pigs need plenty of fibre to maintain gut motility, and without hay, this may cause gastrointestinal problems such as diarrhoea.
Timothy Hay is generally recommended for guinea pigs. For young cavies under the age of 4 months and pregnant or lactating mothers, alfalfa hay (too high in calcium for regular adult guinea pigs) is also recommended to be mixed in with Timothy hay.
Chewing hay is also important. Like rabbits, the molars in guinea pigs are constantly growing and must be ground down by chewing. Constant chewing on hay promotes healthy and normal wear on their molars. Treats and chew sticks are not efficient at wearing the teeth.
Hay should be kept in a dry and well-ventilated place, not in plastic. A wooden or cardboard box is ideal.