Farm Table says:
This article provides the following example about measuring grain in stubble:
A quick estimate of the amount of grain in a stubble can be gained by counting the number of seeds in a 0.1 m (squared) square (ie. the old cropcheck square or your Akubra hat). The number of grains that equate to 100 kg/ha are: 28 for wheat/oats; 25 for barley; 8 for lupins; 5 for field peas & chickpeas; 2 for faba beans.