Guinea pigs need fresh and dry food every day, along with an unlimited supply of hay. As a rule of thumb, each GP needs around one cup of fresh veg each day. Fruit should be looked upon as a treat, not a staple. It’s acidic and can cause sores around a guinea pig’s mouth.
Guinea pigs require one quarter to one third of a cup of dry food per pet per day - a little more if it's particularly cold outside, if they're recovering from illness, if they are young, or if they are pregnant or suckling. Some owners advise keeping the dry food bowl constantly full so that your pets can graze throughout the day - although you need to make sure that old, mashed up dry food does not stay in the bowls and become moldy .
A guinea pig nibbles its way through lots of dry food each day
If your guinea pigs become overweight, reduce their dry food supply. If you've been giving each one a third of a cup, cut it to one quarter of a cup for a few months. If the problem persists, try reducing it a little bit more, and make sure your GPs are getting plenty of exercise. Their fresh hay and veg supply should be maintained as normal, but cut out all treats.