Ugh! That's a tough one. To calculate biomass, you really need a bare earth map to compare to. And then you have the same type of problem that photo analysts have when they try to do 'change detection', making sure that you are comparing the same spot in each photo. If you got a good elevation map early in the spring (say after the field is planted, but before the plants broke the surface) using dead on GCPs, and used the same baseline info and references, you might be able to compare subsequent elevation maps (most of the elevation mapping apps will produce elevation or dem maps). But again,same point comparisons would be difficult. I would try filtering the map data to smooth out the profiles and actually decrease the resolution then try differencing them (just subtracting one from the other). Easiest way might be to generate cvs files that you could input into! For example, Excel. The difference times the area would approximate the biomass. Interesting problem, let us know how you progress.