You can certainly feed oats, barley or wheat as straights. Whether in the long run they would provide all the necessary nutrients you'd have to work out against what else they get.
The strange thing is that sometimes "human" food really is cheaper than dedicated animal feed. I make my own wild bird feed from lard, porridge oats and some mixed bird feed... That's cheaper and they like it better and waste less than if I fed just the bird seed. (And I have less wheat growing in my garden!)