Do you HAVE to start your baby on rice cereal as their first food, can't you pick another grain?

I understand the importance of it just being one grain (so no mixes of whole grains or whatever), but what if I wanted to start her on a different grain like amaranth for instance. I can still prepare it the exact same way as you would rice cereal (I am making my own, all you need is a food processor). I have my reasons for wanting to do this, but I am not going to do anything stupid. So, I guess my question is, is there any good reason why they say rice cereal and not some other grain? Or is rice easier to digest or something?