Or you could run it all in the terrain editor. Make a really hi-res terrain, say 1024x1024 at least... Then spike it a bit, maybe paint-erode some areas for height differences. Then in the Material Lab, create the first channel for colors and the second color run a slope or hieght-based transparency, click it to fuzzy perhaps, but if you mess with this you can produce fairly convincing wheat, or any other plant for that matter...! Although it won't have the geomtric detail of the other technique, perhaps use both and I'll bet it will look awesome!