The latest model N-Tester will give you a variety specific nitrogen recommendation in the field. Yara recommend using the N-Tester on winter wheat and winter barley at GS 37-39 to help fine tune the final nitrogen application.
All the latest variety calibrations are built in and can be accessed via the menus. If needed the latest calibrations can be downloaded and installed from the Yara website.
Find more information on the Yara N-Tester