Most rabbits love to eat kale but it is very high in calcium and can contribute to urinary problems. It may also cause gas and digestive discomfort if too much is eaten. For these reasons you should limit how much kale your rabbit eats.
The rabbits digestive system is designed to process hay and eating too many vegetables can upset it. The fiber in the hay that a rabbit eats takes time for good bacteria ferment. They keep the digestive system balanced and healthy. They also produce many nutrients including high quality protein and many water soluble vitamins. Feeding rabbits kale can cause a shift in bacterial populations so that bad bacteria grow and cause a change in pH and eventually diarrhea.
Adult pet rabbits will be healthiest if they are exclusively fed a low energy grain-free and soy-free pet rabbit food that is fortified with chelated minerals and balanced with natural vitamins. This style of diet will improve their health and it will be reflected in the quality of their fur coat and overall health and playfulness. Baby rabbits need a high fat diet like "mamma's milk" and quality protein to support their rapid growth. Feeding them anything else will dilute the important nutrients they get in their balanced and pelleted diet.
Following these dietary recommendations will ensure long-term health for your pet rabbit. So can rabbits eat kale? Yes, but only a small amount as an occasional snack.