Fundamentally Speaking
Joel Karlin DTN Contributing Analyst

Friday 09/28/12

% IL Corn Yields Deviation From Trend vs. July Temps

Corn harvest starting to wrap up in Illinois and not a moment too soon as that states’ farmers are quite eager to forget one of their worst growing seasons in memory.

The USDA in its September crop production report projected the Illinois 2012 corn yield at 110 bushels per acre (bpa) vs. the 157 bpa yield recorded in both 2010 and 2011.

This is the lowest yield since 107 bpa back in 1991 and is 33.9% below the 40 year trend.

This is the lowest negative deviation from the 40 year trend since the 1988 yield of 73 bpa was 41.3% below the 40 year trend.

As we have stated in prior pieces, it is quite clear that corn yields are not helped by hot weather as evidenced by the dreary U.S. and Illinois corn yields seen over the past three years which have been hotter than average.

The accompanying graphic shows the relation between Illinois July temperatures in degrees Fahrenheit vs. the percent deviation from the 40 year trend for corn yields.

We use final yield data except for 2012 where the USDA September projection is used.

The 40 year temperatures data showed an average of 75.6 degrees with a standard deviation of 2.3 degrees.

Hence, we classified cool years as the average less one standard deviation so years with July temperatures 73.3 degrees or cooler and these are the blue columns.

Warm years, designated as the yellow columns are those July’s where readings were 77.9 or higher, which is the average plus one standard deviation.

Clearly, there is a relation with the correlation coefficient being 0.63 showing rising temperatures results in yields below trend and vice-versa.

Since 1970, there were eight years classified as cool and the Illinois corn yield for those seasons averaged 7.9% above trend.

For the warm years, seven having occurred since 1970 showed an average corn yield 16.2% below trend.

No surprise then that this year’s Illinois July temperature of 81.7 degrees, which is the second highest ever next to the 83.1 degrees in 1936, had a very adverse impact on that states corn production.


Posted at 8:37AM CDT 09/28/12
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