Do you know whether your piggies have had veg before in their life or not? Most shop piggies haven't.
In that case, I would start slowly with just a not too big piece of cucumber, some fresh cilantro/coriander or so. Too much too soon can give them the runs. Add new veg one per mealtime. It is better going slowly at first. Often eating veg can be very hit and miss with piggies who aren't used to them at first. It is a matter of patience and persevere.
Please be aware that the same goes for fresh grass - too much too quickly can cause diarrhea.