LPT, this is from this article on HRS – of course, I recommend you read the whole article since sometimes it can be confusing or misleading just reading one paragraph out of the whole article so it’s not taken out of context
:
http://www.rabbit.org/journal/3-4/pellets.html
Kale, mustard greens and spinach contain high levels of oxalates, the salts of oxalic acid, which can accumulate in the system and cause toxicity over time. Rather than eliminating these veggies from your list (because they are highly nutritious and loved by most rabbits), limit your use of them to 1-3 meals a week. One method is to feed kale (with other veggies) for several days until 1-2 bunches are gone, then avoid buying it again for a week or so. The same precautions can be taken with the veggies that are high in calcium if your rabbit is prone to urinary tract stones. Particularly for rabbits who eat no or few pellets, try to include at least one veggie daily that is high in vitamin A (such as carrots, collard or mustard greens, endive, or parsley). If all these instructions seem complicated, simplify things by making sure you vary your rabbit’s diet as much as possible to avoid giving too much or too little of any one food. This makes good sense for all of us!