Root depths of different grains

I'm looking for grains with the shallowest roots.

I'm looking for things with a depth of less than 2 feet, but a little more can work. Does anyone have a list of root depths? I'm in heavy clay.

I highly recommend Root Development of Field Crops by John E. Weaver.

The book gives valuable information on the size/structure of the root systems of most cereal crops.

EDITED to add: Free PDF download