Food and Water: Good-quality food and fresh, clean water must be readily available at all times. Commercially available pelleted chows provide all of the essential nutrients, as long as the pellets are fresh and wholesome when offered. Some guinea pig owners are tempted to feed rabbit pellets, assuming that they are roughly equivalent to guinea pig pellets, but this is not so. Unlike most mammals (including rabbits), guinea pigs require a high level of the vitamin, folic acid. Unlike rabbits, guinea pigs cannot manufacture their own vitamin C and must, therefore, receive it from an outside source. Interestingly, people and our primate relatives share this dependence on vitamin C from the food we consume. Pellets milled for guinea pigs take these special requirements into consideration and are appropriately fortified with these 2 nutrients, among many other essential ones.
Guinea pig chows generally contain 18-20% protein, 16% fiber and about 1 gram of vitamin C per kilogram of ration. Even when the fresh pellets are properly stored in a cool, dry place, about half of the vitamin C content is degraded and lost within 6 weeks of manufacture. Therefore, the diet should be supplemented with vitamin C as follows: 200 milligrams of ascorbic acid (vitamin C) should be added to about 1 qt of drinking water, made up fresh every 12 hours, or a single guinea pig should be offered one handful of kale or cabbage or one-quarter of an orange daily. Researchers are not in agreement on the advisability of adding other items to the balanced ration (pelleted chows). We recommend that fresh greens, hay and small amounts of fruit be offered daily with several precautions: These items should not exceed 10-15% of the daily diet Furthermore, the fresh items must be thoroughly washed to avoid pesticide residues and possible bacterial contamination. All foods should be provided in heavy ceramic crocks that resist tipping over. The sides of the crocks should be high enough to keep bedding and fecal pellets out of the food, or the crocks should be elevated slightly above the bedding.
Water is most easily made available and kept free from contamination by providing it in one or more water bottles equipped with "sipper" tubes. Guinea pigs tend to contaminate and clog their water bottles more than other pet rodents by chewing on the end of the sipper tube and "backwashing" food particles into it. For this reason, all food and water containers should be cleaned and disinfected daily. Guinea pigs tend to be creatures of habit and do not tolerate changes in the presentation, taste, odor, texture or form of their food and water. Pet owners should avoid making radical changes in the food and water containers. Any changes in the food itself should be made gradually. Failure to do so usually results in the guinea pigs' refusing food and water, which can lead to disease.
According to Dr. Curt Nakamura, an exotic vet specialist at Adobe Animal Hospital in Los Altos, California, grass hay is an important part of a cavy's diet:
"One of the most important items in the guinea pig diet is grass hay, which should be fed in unlimited quantities to both adults and baby guinea pigs. It is important to provide an unlimited source of hay because pellets do not provide enough long fiber to keep their intestines in good working order. The long fibers stimulate muscle contraction of the intestines to improve and maintain gut motility (to prevent gastrointestinal obstruction). 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. Alfalfa hay is rich in protein and calcium, but when combined with pellets it doesn’t have the proper ratio of calcium and phosphorus. This can lead to improper gastrointestinal motility, such as diarrhea. It also may predispose certain guinea pigs to calcium oxalate bladder or kidney stones. Timothy hay is a better choice and is becoming more readily available. It is important to keep your guinea pig sleek, so cut down on the amount of protein and calorie-rich pellets while feeding timothy hay."