Does Cat Grass Have To Be Cat Grass?

I need to plant some cat grass inside, but am wondering if I can take the "grass" out of pots outside and feed that in the meantime. The grass is actually bird seed growing like grass, namely millet and milo. I'm not sure which is growing or both. It just looks like grass. I know most cat grass is wheat or rye or something similar. The bird seed grass is clean and in pots, as I said. I had empty pots at the end of the summer and just threw some bird seed in them to see if anything would germinate.