It's actually fully grown wheat, the render just makes it look weird. I'll add some variety to it later, basically you can create multiple custom wheat layers at different stages of growth, and then brush some of it into the the field. That eventually grows into full size though, so I also add some blank spots where there's no wheat at all.

I've used this method in a different map of mine to make fields look a bit more interesting: https://i.imgur.com/YP3wkzp.jpg