USDA/NASS has now released the 2015 county corn and soybean yields which can be seen in the maps below. Corn yields ranged from 142 bu/acre in Davis County to 210 bu/acre in Cherokee County. Soybean yields ranged from 41 bu/acre in Clarke County to 64 bu/acre in Sioux County. The Iowa average corn yield in 2015 was 192 bu/acre and the state average for soybeans was 56.5 bu/acre.
Note that the individual corn yields for four counties including Mills, Taylor, Union, and Monroe were not disclosed. For soybeans, the two counties not disclosed were Mills and Taylor.