problem with using short term for trend is one or two outlier years can skew results.  Be that as it may trend yield certainly hasn't  increase at a straight, predefined, slope. Part of the flattening of trend is pure weather.  Part of the flattening of trend is the increase in acres, especially at the fringes. With a decrease in acres, especially at the fringes, what would be the most likely result on yield? 

In the end weather is still king in a particular year. 2014 could be well above or well below trend. Weather could make corn $7+ or $3 by years end.