And then a drought hits and your region puts its last 100 bushels on the market? It needs to be "above X threshold".
I think it's reasonable to require Lords to suddenly revisit their trade arrangements when a drought (or looting) occurs. The problem with the current system is in big part that you need to take care of it constantly even when nothing particular is occurring, which becomes tedious because there is really no judgment involved. Create a system where player input is required only when something has changed and you will ensure that players will need to think about their actions before pressing the buttons, and no one will complain that that's tedious.
