Kohl rabi is a cabbage variety grown for its roots which are used as a vegetable. Said to taste similar to broccoli stalk but slightly sweeter, young kohlrabi have a similar texture to a crisp, juicy apple.
Kohl rabi contains 108mg calcium and 73mg phosphorous per 100 gm giving it a fairly good Ca:P ratio. In addition it also provides 20.2mg vitamin C per 100mg, 3-4 times the level in carrots.
The advantage of growing your own kohlrabi is that you would obviously also be able to feed the leaves to your rabbits or guinea pigs. The leaves would have a similar nutritional value to cabbage leaves.