Concentration is examined for both corn and soybeans. The following describes methods for corn. The same procedures were used for soybeans.

Total corn production by county came from the National Agricultural Statistical Service (NASS) for 2010, 2011 and 2012. These three years were averaged to arrive at average production for each county in the U.S.

Based on the 2010-2012 average, counties were ranked ordered from highest to lowest production. Then, production totals were calculated for successively more counties. For example, production from the first- and second-largest counties was added together to arrive at production from the two largest counties. Then, production from the three largest counties was added together to arrive at production for the three largest producing counties. This process was repeated until the total from the largest counties just exceeded 50% of total U.S. production from 2010 to 2012. These largest counties then accounted for 50% of total U.S. production. The resulting 220 counties for corn are denoted as dark green on the charts on the previous pages.

The above process was repeated to find the largest counties accounting for 75% of production. The resulting 456 counties include the dark green counties representing 50% of total production plus the light green counties. The process was repeated for 90% of total U.S. production. The resulting 753 counties are denoted by dark green, light green and yellow.

The procedure results in the lowest number of counties that account for the specified level of U.S. total production. It does not attempt to control for size of county. The procedure may have a bias for counties with more acres. A procedure holding acreage constant could result in a different map to the one shown in Figure 1. However, large differences from that shown in Figure 1 are not likely.

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