Yes most definitely, the biomass readings will give you a good indication of how much nitrogen has been taken up by the crop. In the instance of last year, the under performing field would have had a lower biomass reading due to less N response from the slurry. This would have given you a good basis for increasing the N applications. SAR should really help fine tune N applications in all situations.
Thanks for the reply it sounds really interesting. I will give you a call in the next few days.