Pellets
Guinea pig pellets should be fed daily. Most guinea pigs will not overeat (usually eat about 1/8 cup per day) but pellets may need to be restricted if a guinea pig becomes obese.
Choose a good quality pellet designed guinea pigs.
Typically, domesticated Guinea pigs eat processed pellets made from timothy hay and vitamins and minerals that Guinea pigs need to be healthy. They also love vegetables and fruits such as kale, mustard greens, bell peppers, carrots, bananas, apples and blueberries.
Since the vitamin C in pellets also loses its potency over time, look for pellets that use a stabilized form of vitamin C, or at least one with a "use before" date ensure freshness, and also buy in small quantities and store in a cool dark place.
For growing guinea pigs, alfalfa based pellets are fine, but for adults, it may be better to search out a Timothy hay-based pellets.
When choosing a guinea pig diet or pellet, avoid those with nuts or seeds, dried fruits, corn products, animal by-products, or beet pulp or other fillers.
Also watch for high sugar content (sucrose, fructose, corn syrup, etc) and foods with lots of preservative or added chemicals. I am not a huge fan of Kaytee so I choose Oxbow for my Cavys' base diet then mix it up with Sunburst sometimes.
Guinea pigs should eat more than hay and pellets. You can also feed your guinea pigs romaine lettuce and bell peppers on a weekly basis. we create treats our of very basic nutritious foods.