Korean Lespedeza is the one I am most familiar with.  Fescue would be another one, but I have not even attempted to monitor it.  Basically anything which is yielding in the area of 500 lbs per acre or less I have not been able to make work, including poor soybeans. 

I wasn't aware there was an optional impact sensor.  That may well make them work....I assume the flow rates we encounter are not sufficient to meet the minimum force to activate the load cell in the standard sensor.