Hi! It may be better to gradually switch over to pellets - that has the advantage that you won't end up chucking half the mix away due to selective feeding and you will also have an instant food source at home in case you need to suddenly hand feed a very sick piggy in an emergency (you can always mush up pellets for that purpose, you know!). Up to 80% of the daily food intake should be hay.
Piggies that have never had veg in their lives before don't always take to them quickly. It is a case of be patient and persist. Don't introduce more than one new veg at a time.
You may find this thread helpful; it lists what veg you can give; about 1 cupful per piggy per day in as many servings as you like. If you vary and feed everything in moderation, you can't go much wrong. What you feed is ultimately down to practicability, piggy preferences and your bank balance! Everybody needs to find out for themselves what works.
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Generally, we recommend a mix of several chopped veg per day, including one high vitamin C veg, like a slice of pepper of any colour, a bit of broccoli (any part) or some fresh parsley; the latter two should not be fed every day.
You can feed cabbage (not just kale) without problems if it is part of a mix and provided that you don't have a piggy with a tendency for bloating/tender digestion. Fresh coriander/cilantro, cucumber and chopped celery can also be fed daily.
Be very careful with exotic fruit (diarrhea) and don't feed fruit and tomatoes more than 2-3 times a week - too much can cause sores on mouth and lips. Spinach is very high in calcium, like kale - too much on a regular basis can further the build up of bladder stones in guinea pigs with a disposition for them; lettuce and carrot are high in vitamin A (which builds up in the liver). None of these foods should be fed on a daily basis, but you can rotate between them!
The things you really need to stay clear of are: any bulbs from plants of the onion family, potatoes (too starchy), mushrooms, pulses (but you can feed green beans and mange tout), some varieties of avocado (highly poisonous to guinea pigs), nuts, seeds (too fatty and can get stuck in the teeth), dried fruit, any dairy product (including yoghurt) and sugar/honey (i.e. pet shop snacks are junk food for pigges!).