Guinea pigs are native to the grasslands of the Andes. They feed on grass and other low-growing plants. To have a healthy domestic guinea pig, their diet should ressemble this as much as possible.
Guinea pigs do NOT naturally eat grains so the 'muesli' style foods are completely unnatural. They encourage selective feeding where the guinea pig chooses all the tasty bits while leaving the more healthy pieces behind. This is much like humans having a diet of junk food with no vegetables.
To simulate the diet of the wild cavy, guinea pigs should have the majority of their diet made up with grass and hay. These need a lot of chewing so help to keep the guinea pigs teeth down as well (all of a guinea pig's teeth grow continuously).
A run on a lawn is a perfect source of growing grass - do not use weedkiller on the grass however as this may cause severe problems to the guinea pig. The grass should be supplemented with good quality hay. Timothy hay is one of the best. It is very tasty and keeps the teeth down well.
A lot of the guinea pig treats available are very high in sugars and are not good for your guinea pig. The best treats that you can give them are fresh vegetables. Fresh green vegetables are high in vitamin C which guinea pigs need, just like humans. Guinea pigs love lots of different fresh vegetables and herbs. Fruit should be a very occasional treat as this is also full of sugar.
Not really! If provided with a lot of hay and grass, supplemented with fresh vegetables, guinea pigs really shouldn't need to eat much else. It is a good idea to give them a small amount of a pelleted diet, however, to top up any minerals and vitamins that they are not receiving in the diet. Most guinea pig food says that it is supplemented with vitamin C but this vitamin goes off quite quickly so only the fresh food will have a decent amount.
The best food to feed is the Burgess Supa Guinea Excel. This is a completely pelleted diet which prevents selective feeding. Only a small amount (about 1 teaspoon per guinea pig) should be fed daily.
On no account should guinea pigs be fed on rabbit food. This does not have added vitamin C and may contain a medication which is toxic to guinea pigs.