Although the guinea pig mix and hay will provide the guinea pig with a good basic diet it is necessary also to feed fruit, vegetables or plants as a source of Vitamin C as guinea pigs lack the enzyme to produce this.
An adult guinea pig needs 2.3 mg of Vitamin C per pound of their body weight and pregnant guinea pigs require 13.4 mg of Vitamin C per pound of body weight. The guinea pig will appreciate some variety and so it is best to feed a mixture of fruit, vegetables and plants.
If too much fruit or vegetables is fed and left in the cage this should be removed and replaced the next day to avoid it being left and becoming mouldy.
Some Fruit, vegetables and herbs that are safe to feed a guinea pig are listed below with the mg Vitamin C per 100 grams marked in brackets where known (according to the USDA Nutrient Database).
Some vegetables and herbs that should not be fed are: