It’s true that some rabbits are very sensitive to things, so just trying a little at a time is a good way to test out what agrees with your bun. I remember some member’s bun would have issues if he ate a single browned mint leaf! If I remember correctly, your bun had more of an issue with fruit and treats?
If your bun is prone to gas, then I would start slow with kale, as it does fall in the cabbage family. Collards, cabbage, broccoli, cauliflower, kale, arugula, and bok choy are actually all in the same Genus, so they are very closely related! Of those, bok choy and arugula seem very well tolerated by buns. I love giving my buns bok choy as a staple veg. Kale usually is too, while collards, cabbage, broccoli, and cauliflower can tend to cause more issues with gas.
Endive and chicory are awesome (and have a good shelf life in the fridge), so def worth trying!
If you find your bun is having issues with many veggies, you can try feeding dried greens. Many buns with sensitive tummy do better with dried greens, so you can either purchase dried veg (such as dandelion), or dry things yourself so you can give your bun more variety.
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The answers provided in this discussion are for general guideline purposes only. The information is not intended to diagnose or treat your pet. Seek the advice of your veterinarian or a qualified behaviorist.