Guinea pigs are herbivores and require a similar diet as rabbits with plenty of  grass hay and greens and limited   commercial pellets. They have continuously  growing incisors and molars which wear down, as in the rabbit, with the normal  action of eating. Guinea pigs also produce nutrient rich cecotropes in a similar  manner as the rabbit which they eat directly from the anal area. Guinea pigs  should have unlimited grass hay for the same reason as rabbits. While other  rodents, rabbits and ferrets can produce their own vitamin C inside their  bodies; guinea pigs cannot produce their own vitamin C and require an external  source.  Dark, leafy greens are very high in vitamin C, for instance a cup of  fresh kale contains approximately 250mg of vitamin C compared to a cup of  oranges (without the peel) which contains only 50mg of C. The minimum daily  requirement for vitamin   C in the guinea pig is 10-30 mg per day. Guinea pigs  can easily get this amount and more with the feeding of 1/2   to 1 cup of fresh  leafy greens daily. Some particularly high vitamin C foods are kale, dandelion  greens, parsley, collard, green peppers and mustard greens. Supplementing  vitamin C in the water is not very effective due the   rapid breakdown of the  vitamin when it is exposed to light and heat and the fact that some vitamin C  products have a very bitter taste. Feeding fruits and other vegetables in small  amounts is also acceptable.  Commercial guinea pig pellets can be fed in limited  quantities. Guinea pigs can become overweight on unlimited   pellet feeding,  particularly if they are not allowed to exercise daily. Feed 1/8 cup pellets per 2  lb guinea pig daily.  Although guinea pig pellets have vitamin C added, you  should not depend on them to provide the full daily amount.  Guinea pig pellets must be used within 90 days of the day they were produced to guarantee sufficient amounts of vitamin C.  In many situations, bags of pellets have been sitting on the shelf for longer   than 3 months and the vitamin C content is drastically reduced due to breakdown by contact with air, moisture and high environmental temperatures. Scurvy, a disease caused by vitamin C deficiency, is still one of the most common diseases seen in the pet guinea pig fed a strictly commercial pelleted diet. Rabbit pellets should not be substituted for guinea pig pellets because they may contain excessive levels of vitamin D which can be toxic to guinea pigs. Do not feed cereal grains or sugary foods to guinea pigs.  Guinea pigs are notorious for playing with their water bottles and spilling a great deal of water into the cage. The bottle may have to be refilled frequently and the bedding underneath it changed daily to prevent mold from growing.