If you’re looking for new and different foods to give your rabbits that’s safe to do so then fennel is a good choice. Fennel is a herb that has a decent amount of fiber. Rabbits require a high fiber diet to keep their digestive system working properly and avoid some of the common health issues that result as part of an unbalanced diet.
A typical diet of a rabbit consists of plenty of hay daily, Timothy hay is widely regarded as the best type as it’s high in fiber and nutrition. This is supplemented with pellets and a cup of vegetables or leafy greens each day.
If you think your bunnies need extra fiber then adding some fennel as part of their leafy greens allowance (which shouldn’t be more than 15% of their total food) is a good idea. In my experience they love the taste and won’t hesitate to eat it up.