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Don’t rely on hay and grass alone. Feed your guinea pigs bell peppers daily. Add leafy greens to your guinea pig’s diet. Feed fruits and other vegetables to your guinea pig as a treat. Limit certain vegetables to 1-2 times week.
Although timothy hay, other hays, grasses, and alfalfa are the foundation of a guinea pig’s diet, they are low in Vitamin C.  Supplementation of some kind is necessary.  Provide unlimited access to hay for your guinea pig no matter what supplements you decide to use.  You may add lucerne hay to a pregnant guinea pig’s diet to provide increased protein and calcium. Red bell peppers, otherwise known as capsicums, are high in vitamin C and are one of the only vegetables that can be fed every day. Yellow and green bell peppers can also be fed, but contain less vitamin C to red varieties. Dark, leafy greens such as kale, parsley, cabbage, spinach, chicory, lambsquarter, and dandelion greens provide a good source of Vitamin C. If you feed dandelion greens or lambsquarter, be careful not to harvest ones from lawns that have been treated with pesticides, fertilizers, or herbicides.  Leafy greens should be the main vegetable supplement for your guinea pig. Provide 2-3 leaves of various leafy greens each day. Cucumbers, guava, broccoli, cauliflower, strawberry, peas, tomatoes, and kiwi are good sources of Vitamin C for guinea pigs. Vegetables and fruits can be offered a few times a week. Since fruits are high in sugar, these should be given more sparingly. Various vegetables contain different amounts of vitamins and nutrients. Some vegetables, while healthy for guinea pigs, can cause dietary issues if fed in high amounts. For example, high-calcium vegetables that are fed too often can cause bladder stones. Nutrient-deficient vegetables, such as lettuce, can cause diarrhoea and bloating if fed in excess amounts. For a reliable and updated chart of vegetables types, visit https://www.guinealynx.info/diet_vegs.html  Lettuces such as cos, romaine, and butterhead should be limited to 1-2 times a week, as they contain little nutrient value, and can cause bloating, diarrhoea and an upset stomach if fed in excess amounts. Iceberg lettuce should not be fed at all. Broccoli, cauliflower and brussel sprouts should be limited to 1-2 times a week, as they can cause bloating. High calcium vegetables, such as kale and spinach, should be limited to a few leaves every 2-3 weeks. Large amounts of calcium in their diet can lead to bladder stones.