Death comes to us all. Life's but a walking shadow | Clay SEIA pointed out an interesting chart showing Iowa corn yields and the bulletin from which it came. I took a look at that bulletin and as is often the case there is more to learn. First Clay's chart with trendline. My chart looks a little different because I changed the scale from 0-200 to 120-190 but it is the same numbers. Even if you take out 2012 it looks like Iowa has been losing yield. I suspect that Clay's chart reflects the trend of planting corn on less productive soils. We thought that only applied to the "fringe" states.
Since the Clay's chart uses section averages instead of county averages I went back and recalculated state wide average by county. The problem with using section averages is that the sections have different numbers of counties which can skew the results. It is also the case that different counties may be different sizes and have more or less corn to say nothing of irrigation patterns soil types etc. I included probability limits to help estimate the reliability of any conclusions. As you can see we are still well within the range of expected values but clearly the trend is down.
Oh, I deleted 2012 from my calculations because 2012 was so poor due to drought that it doesn't seem fair to count it toward ordinary yield decline.
 (iowa yield by section-page-001.jpg)
 (iowa corn yields-page-001.jpg)
Attachments
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 iowa yield by section-page-001.jpg (47KB - 28 downloads)
iowa corn yields-page-001.jpg (48KB - 30 downloads)
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Peak Iowa continued
