Welcome to RabbitTalk, Hattie. :welcome:
Many people who feed their rabbits without commercial pellets use grain as part of their rabbits' more natural diet. I found grain was useful in small amounts, along with good hay free choice (alfalfa with some grass, here) and forage. Too much reliance on grain will make rabbits fat quickly and since much of that fat accumulates in the body cavity, it will interfere with the rabbit's health and reproduction. I've fed wheat, barley, and oats but usually only one at a time. It was most often wheat because I also fed it to the geese and they preferred it.
If you describe the full diet you feed your rabbits at present, we can help you arrive at a formula that works for you.