The best greens to feed are turnip, mustard, and collard. If you read the stats on them, they are naturally high in calcium. Kale is perfectly fine also..

Anything that is high in oxalates should not be given. Oxalates are what blocks the bodies ability to absorb calcium..

http://whfoods.org/genpage.php?dbid=48&tname=george

" Foods that contain oxalates:

The following are some examples of the most common sources of oxalates, arranged by food group. It is important to note that the leaves of a plant almost always contain higher oxalate levels than the roots, stems, and stalks.

Fruits:
blackberries, blueberries, raspberries, strawberries, currants, kiwifruit, concord (purple) grapes, figs, tangerines, and plums.

Vegetables:
spinach, Swiss chard, beet greens, collards, okra, parsley, leeks and quinoa are among the most oxalate-dense vegetables.
celery, green beans, rutabagas, and summer squash would be considered moderately dense in oxalates."