Death comes to us all. Life's but a walking shadow | Why Iowa can be -16 bpa under last year and the national average is only 4.7 bpa under last year.
I've been a little curious as to how the USDA arrived at their most recent corn yield estimate of 169.8 bpa especially in the light of their state yield estimates for Iowa, South Dakota, North Dakota, Illinois and even Minnesota of -16, -16, -34, -8 and -11 bpa compared to last year. They seem like too substantial declines to justify just the 4.7 bpa drop. But when I used USDA's estimated state yields, sure enough I arrived at very comparable numbers, -4.9 vs their -4.7.
Then the question arises, where would the extra corn come from to offset these quite substantial state declines listed above. Could good yields in Pennsylvania and Alabama really offset the declines? If you plot the declines in production state by state shown below it doesn't seem possible.
But if we dig a little deeper we see a different message. The question is, where does the nation's corn crop come from? Well, I don't have available to me the detailed field by field, acre by acre yield information that is available to the USDA. But we do have 2016 county yield and acres harvested for grain information for some 1700 counties in the US. Now if you tabulate that county information, rank it by yield and plot harvested acres vs yield you see the scatter plot below. It shows pretty much what you'd expect. Counties that have low yields don't grow much corn and corn acres increases in counties where yields are higher until you reach a certain point where there are fewer very high yielding counties. Then the question becomes how does all that build to the "national corn field" and the "national corn crop". If you "accumulate" either harvested acres or production (yield x acres) and again plot them against yield you see the following two plots. You add acres and you see a classic sigmoidal curve ramping up to an almost straight line rise in acres only to level out at last year's 86 million odd harvested acres. Why does it level out? Simply because there are fewer and fewer counties with the very highest county yields. Likewise with production only more so. higher yielding counties really add production but in the end the fewer very high yield counties don't actually count that much. But if you look closer you can see something interesting. The national average is just the total national production divided by total harvested acres which last year came out to 174.6, close to 175. What you see is this, the 38 million acres below that 175 bpa only contributed 5 billion bushel, a third of the production. It was the other 48 million acres that averaged better than 175 bpa that contributed the remaining 10 billion bushel.
The point is this, even the poor crops in North & South Dakota can't bring down the average that much simply because there are just so many counties where the yield is so much higher than the 175. And finally, just to see what would happen, I derated all the county yields either 5 bu or 10 bu. and you see it doesn't make all that much difference to the final outcome simply because there are just so many high yielding counties.
Edited by 1234 9/20/2017 13:33
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 production declines by state-page-001.jpg (72KB - 59 downloads)
Acres vs yield by county-page-001.jpg (81KB - 58 downloads)
accumulated acres vs county yield -page-001.jpg (70KB - 63 downloads)
accumulated production vs yld-page-001.jpg (71KB - 59 downloads)
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Why Iowa can be -16 bpa under last year and ...
