When introducing veg to youngsters it needs to be done slowly so as to not upset their tummies, a little of one veg at a time. Once they are used to veg you can feed a variety equating to one cup of veg per pig per day.
The guide below will help you with how much to feed and what foods are suitable.
The second guide gives details on what veg is safe to fed
I would urge caution over feeding sprouts too often though - they are fine to feed but not daily as such foods (cabbage and broccoli included) can cause bloating. Baby corn are fine as a treat along with some carrot and tomatoes - these foods are too sugary of acidic to be fed too often. Kale, parsley and spinach are too high in calcium to be fed daily - these are fine to feed one item once a week though.
Daily, my piggies get some lettuce, coriander, cucumber and peppers. I may then add in other things if I have them in the fridge but if not, then they just get the basics.
It is all a good idea to filter their drinking water if you live in a hard water area. Pellets and water contribute a lot of excess calcium into the diet, so filtering and ensuring pellets are kept to the limited amount will help ensure they don’t get too much calcium (which can lead to bladder stones)
Their food bowl should be empty. Hay needs to be the only food they have constant access to.
Long Term Balanced General And Special Needs Guinea Pig Diets
Edible And Forbidden Veg And Fruit List With Vitamin C Grading