The most important fact to know about guinea pigs is that, like us humans, they need a daily intake of Vitamin C. This can be provided by providing a balanced diet with plenty of fruit and vegetables. Most good guinea pig dry mixes now also contain vitamin C. Carrots and Broccoli are great sources of vitamin C, and a carrot a day keeps the vet away!
The dry food
Guinea pigs like a bowl of dry food a day. Make sure you get a brand made specifically for guinea pigs, as it will have vitamin C included in it. Only buy a quantity that they are likely to eat in 3-4wks, as if the dry food is kept longer and allowed to go stale the vitamin content will decrease.
Guinea pigs will eat virtually anything! As well as grass in the summer, they can be given a variety of wild plants such as dandelions, plantains, chickweed and milk thistle. When wild plants are not available they can be given vegetables, herbs and fruit. The key is to introduce as many different fresh foods when they are young, as they may be reluctant to try something new as they get older. The only no-nos from the kitchen are potato peelings.
During the summer, and when they are in their eglu, grass will provide the main component of their diet. They will graze (and fertilise!) your grass for hours. They make excellent lawnmowers. In the spring, when the first rapid growth of grass occurs, they should only spend 1-2 hours outside, as this grass can be very rich and cause digestive upsets. They can then build up their grass-eating hours, so that by the summertime they can be outside all day.
Hay - not straw
Hay is the other most important daily component of their diet. Only the best quality hay should be fed, and it should not be either dusty or mouldy. If you have somewhere to store it, it is often worthwhile to buy a bale from a farm, of a quality that would be fed to horses. As well as eating it, they will snuggle under it for warmth. Straw should not be used; it has no nutritional value, and the sharpness of its stalks often causes eye injuries as the guinea pigs burrow around in it.
Water is very important, and their water bottle should be refilled daily. If they are eating a lot of grass and fresh food, which will be full of moisture, they may drink very little from their bottle, but it is still very important to provide water at all times. If the weather is cold the water may freeze, and need to be thawed and refilled more frequently. Over the winter soluble vitamin C can be added to the drinking water to make sure they are getting enough of this important vitamin.
There are lots of different treats available from pet shops, many based on carbohydrates, or sweets like yogurt drops. These are all expensive gimmicks, and your guinea pig will be far happier (and healthier) with an extra carrot!
Don't worry, your guinea pig will prepare and eat these by himself, you don't need to become involved! Caecotrophs are the soft (and smelly) droppings that the guinea pig produces, usually overnight, and are full of protein and vitamins. There are formed during the process of digestion, and are designed to be eaten as soon as they are passed. So don't be disgusted if you catch him eating his own poo he is doing it to stay healthy.