... and vit B and C are two hydrosoluble vitamins, that means that the body cannot store up them (as it does with vit A and D for example). It is necessary a little constant amount daily and the unused part is simply thrown away. Symptoms are visible only when the disaster has started. Also in humans, a lack of vit C can only reduce the absorbtion of iron, this does not mean you immediately get an anemia; you may live well and have a lower immunitary system and nothing else... but this is dangerous as well for the future. People who start eating a lot of fruit for increasing vit C, end up in boosting also the level of insuline, which make different (and worst) damages. The same is with the piggies; you can give them more kale, parsley and spinach, you piggie will never develop scurvy but will end up with stones...
Grass. If you don't want to use supplements, cut a lot of fresh grass daily and serve them immediately (just after some hours from the harvest the grass and the veg have lost part of vit C). And hay of course... because from the fibres and good bacteria of the gut kept alive with the fibres the piggies are able to produce other vitamins, also the ones written in the label of (useless) pellet food.