USDA Nutrient Database
Pros: THE source for detailed nutritional data. Includes most foods commonly eaten by humans. See also the Oxalic Acid Content of Selected Vegetables. This information was originally printed in Agriculture Handbook No. 8-11, Vegetables and Vegetable Products, 1984.
Cons: must look up each vegetable and fruit individually to find nutritional data.
Dr. Duke's Phytochemical and Ethnobotanical Databases
Pros: searchable nutritional database. A single search can list highs and lows of a single nutrient in multiple foods. Includes some plants ("weeds") like chickweed not found in the USDA Nutrient Database.
Cons: must look up each vegetable and fruit individually to find nutritional data. High/low searches are of whole database (including foods you may not be interested in).
Nutrient Requirements of Laboratory Animals, Fourth Revised Edition, 1995.
The National Academy of Sciences lists the Nutrient Requirements of the Guinea Pig .
Pros: a reference bringing together what is known about the nutritional requirements of cavies.
Cons: some data is over 50 years old, other data is extrapolated from rat studies (the authors of this article encourage more research). Some of the specific diets recommended are only suitable for laboratory animals. The authors note the high protein, low fiber diet typical of a laboratory animal and mention that a cavy's diet in the wild consists of much more fiber and green vegetation.
A complete list of requirements is printed on page 104. Keep in mind, these are not daily requirements but instead the amounts of vitamins and minerals they recommend adding to a kilogram of food to provide proper nutrition. Your pig will only eat a fraction of a kilo per day.
Pros:includes protein, calcium, phosphorus, magnesium, ascorbic acid, and oxalates. Gives some information on hays.
Cons: Hard to read. Includes foods generally thought to be inappropriate for guinea pigs like seeds and cereals.