I have a rabbit that can no longer have any pellets as they cause digestive problems for him, that can lead to stasis. I've tried many different kinds, and as I would increase the amount, he would start to show signs of pain, possibly due to gas. The one common thing in all the pellets that I tried was carbohydrates, so that's what I believe my rabbit is sensitive to. So now all he gets is some sort of non grain grass hay(right now it's timothy), as well as some alfalfa
hay pellets(no other added ingredients), to keep his weight up, and select non cruciferous leafy veggies. He's done well on this diet, has stayed at a good weight, and has been stabile with no stasis issues since eliminating the pellets.
Generally alfalfa hay isn't often given to adult rabbits because the high protein can lead to obesity. Alfalfa is also high in calcium so isn't recommended except for young and nursing rabbits, but can sometimes be helpful to give to rabbits that have difficulty keeping weight on, such as older rabbits. Though one problem that alfalfa hay can present, is that rabbits that have kidney issues or are genetically prone to bladder sludge, shouldn't have alfalfa hay because of the high calcium content. As long as your rabbit doesn't have these issues, adding in some alfalfa hay shouldn't be a problem. If you are concerned about using alfalfa hay, a good green early cut grass hay gradually added into the diet, may provide enough protein for good weight gain, because of the higher protein in young growth grass. Alfalfa hay should also be gradually introduced if used.
If you are wanting to try a different type of pellet, Supreme Vetcare Plus Digestive Health pellets, might be a possibility. Though if your bun is sensitive to carbs, they may not work as there is some grain in it as well as peas. Their Weight Management pellet doesn't have added grain or sugar, so that might be a possibility as well, but it is a little lower in protein.
http://www.supremepetfoods.com/supreme-products/supreme-veterinary-exclusive-range/
If you do decide to try a different type of pellets, just take it really slow, so that if it does cause any digestive upset, it should be minimal and you will be able to catch the signs early before it develops into anything serious. For my rabbit I literally only increased by one pellet a day and was very observant for any changes in behavior that would be a sign the pellets were causing digestive discomfort, and it was a few weeks before I started to see those first signs of them making him feel uncomfortable, of just some eye squinting and belly pressing after eating them.
