If you want something really high in C, I´d go for bell peppers, especially the red ones. Nursing mothers and babies can also benefit from additional calcium, so I would grab parsley too (actually not bad in C, either) and alfalfa.
You are feeding them grass hay, too, right? Alfalfa is a supplement only. It´s too high in calcium to be part of the regular diet and can lead to bladderstones. It´s a good supplement for pregnant or nursing mothers and for young pigs.
If you have any more suggestions on good vegies that are high in c that you´ve had luck with let me know. i don´t know about that list anymore... thanks
off to the store i go.