Spirulina, microalgae, is also a very good choice - with even more benefits!
It does require some extra adjustment for example high temperature (over 20 celsius is good, 35 celsius is optimum) and artificial lights (in winter) but in summer, you can grow it outdoors in open ponds.
Here is couple studies of giving it to rabbit feed (just scroll them as they have also other animals):
I live on the old family farm and have my breeders hanging inside my open chicken coop to make winter months easier for my chores but i built a rabbit tractor and put the litter into the tractor shortly after moving them from the mother and move them every morning before work and evening before dark sometimes ill move them when i get home or mid day on the weekends. They chew the grass and weeds right down to the dead grass and this help cut back on the feed i give them as well. The breeders stay in cages full time but the young i grow out in the tractor its 6x2x1 how ever the next one i will make is going to be taller for the fact the rabbits were crawling out the wire roof since i used a wider wire for part of the top that or a solid top for shade. I fill a small feeder 3/4 full a day for them they eat it and the grass and hay. I also just planted comfery along the house and while it is having a rough grow from the chickens and the cat getting into it, it grow great big leaves that you can harvest and is basically super food. I feed a leaf to my breeders now and again and plan to cut and dry the rest before the cold hits too hard for winter snacks. Dont forget garden scraps look into what they can eat, i just feed a large amount of fresh carrot tops to them even had some left over i dried in the sun for a day and stuffed in the hay rack for them the next.
My rabbits were on forage and grazing only for 8 months of the year. They had a big variety of many of the things listed on this thread and kitchen/ garden scraps, plus grow outs were in grazing tractors. Adults were in colony. Pumpkin, squash, tree clippings, comfrey, dandelion, grass, on and on.
"The world is changed by your example, not your opinion." ~ Paulo Coelho
A book that talks about forage and pellets Raising Rabbits by Ann Kanable. Not only just se talk a bit about forage she also mentions that rabbits raised for generations on pellets may not do as well on forage as you hope but you can select offspring that do the best and within a few generations your rabbits will do better on forage then pellets.
In the south when the wind gets to 75 mph they give it a name and call it a hurricane. Here we call it a mite windy...
I’ve raised them without any “bought” feed, just a mineral block which I make sure each of them have and fresh water daily. I make sure they get something high in protein, especially for growing kits, or pregnant or nursing does. Over winter is a little harder because I don’t have much fresh weeds for them, but they get hay free choice daily in their feedback and whatever greens I can find. Clover in moderation, with other foods, or Greater ragweed, purslane, as a source of protein. Not only do they survive on this, they thrive. Secret is in getting a well balanced diet for them. Look up forages and you’ll be surprised at the high protein items that are not alfalfa or clover. Some are even higher than alfalfa. Dry some for winter feed for them. I give them greens from the garden too, the occasional carrot, they love radishes and their tops, collards, kale, but not too much, it can cause indigestion.
With appropriate microbes, minerals and organic matter, there is no need for pesticides or herbicides.
...To speed up the process, savvy producers are now learning how to make "Vinegar Hay". It is accomplished using high-quality, unpasteurized, and whole-apple-derived Apple Cider Vinegar (ACV). The sooner it is sprayed on the cut forage, the better it works. Many hay producers simply drive up and down the windrow with an ATV and spray about 3 gallons per ton of hay right on the center of the window.
It starts working immediately with the living enzymes and microbes doing all the work. Sure, another name for vinegar is acetic acid, the natural chemical that preserves silage or forage, but that acid is only a small fraction of a living (unpasteurized) product, maximally only 6% of the total liquid. The next best method is to add the pure raw ACV to a tank on the front of the baler and roll it up right into the bale...
There is a lot more detail in the article. I remember reading on Permies that fermenting chicken feed can reduce he amount of feed the chickens eat. Would this be the same thing?