Hay alone - even good alfalfa hay - is not enough. If you watch the wild rabbits, you will be amazed at the variety of plants they eat: grasses, broad-leafed weeds, seed heads off grasses, grain, windfall apples and other fruits etc. etc. They also self-medicate as necessary by selecting the kinds of plants they need.
Hay makes a good start for a natural diet for rabbits and it is an excellent safeguard against digestive disorders for all rabbits... but if you want to feed your rabbits naturally you will need to add a variety of other things to their diet.