Ok, let's assume for a second that only yield matters and yield got us to the lows in price last fall.  Now, let's overlay another element, the US Currency.  Using same dollars to the same price, the price would be lower since our currency has rallied by 19% since October 1st.  So using your logic, with demand, exports, weather, acres, all being identical,  we need a crop reduction of at least 19% to maintain the price of corn at the same level as this fall.  That 3.20 corn last fall is priced at 3.80 today