Expected 2010 Corn and Soybean Yields

In a recent Illinois Farm Economics: Facts and Opinions article, actual 2009 corn and soybean yields for Illinois counties were compared to trend yields.  Overall, corn yields were slightly above average and soybean yields were slightly below average, with maps in the article illustrating the variability across counties.

Not included in that article were maps showing expected 2010 yields.  Figure 1 shows expected corn yields based on linear regression models fitted through data from 1972 to 2009.  As expected, the central part of the state has the highest expected corn yields, with Macon County having the highest expected yield of 186 bushels per acre.  There are twelve counties with expected yields over 180 bushels.  Of those twelve counties, all but Whiteside County is located in central Illinois.  The lowest expected yields are in southern Illinois, with Perry County having the lowest expected yield of 108 bushels per acre.

The same geographical pattern exists for expected soybean yields (see Figure 2).  Piatt county has the highest expected yield of 55 bushels per acre.  Central and northern Illinois have the highest expected yields while southern Illinois have the lowest expected yields.  Thirty-six countiies have expected yields above 50 bushels per acre.  Six counties have expected yields below 30 bushels per acre.

Expected yields provide a good estimate of yield for the coming year.  If 2010 could be repeated 20 times, the average of the 20 yields would be close to the expected 2010 yield.  

Yields tend to be skewed.  There are more observations above the trend than below the trend.  This may be why some believe that trend yields understate the true yields.  Given that more observations are above the trend, observation below the trend tend to be further away from the trend yield than those yields above the trend yield.