Hmm Ive never heard of anyone feeding Swiss Chard. I think its a lot like spinach only more bitter, not sure if the buns would even like it.
Have you checked out the HRS list of recommended veggies? The recommend feeding at least one a day with Vitamin A (marked with an asterisk *):
Alfalfa, radish & clover sprouts
Basil
Beet greens (tops)*
Bok choy
Broccoli (mostly leaves/stems)*
Brussels sprouts
Carrot & carrot tops*
Celery
Cilantro
Clover
Collard greens*
Dandelion greens and flowers (no pesticides)*
Endive*
Escarole
Green peppers
Kale (!)*
Mint
Mustard greens*
Parsley*
Pea pods (the flat edible kind)*
Peppermint leaves
Raddichio
Radish tops
Raspberry leaves
Romaine lettuce (no iceberg or light colored leaf)*
Spinach (!)*
Watercress*
Wheat grass
(!)=Use sparingly. High in either oxalates or goitrogens and may be toxic in accumulated quantities over a period of time
I have this list printed out on my fridge. I sometimes mix in carrot tops, collard greens, endive, kale, mustard greens, and spinach to give variety.