The main ingredient in rabbit pellets is alfalfa, because it is high in protein, however it is low in fibre, usually wheat middlings are added to increase the roughage in the feed, so if your rabbits are on pellets then a grass hay like orchard or timothy is best to aid in digestion.
If your rabbits are just pets then I definitely don't recommend alfalfa as it is too nutrient dense and can cause issues later in life like bladder stones and kidney disease.