Continuing our discussion of crop yields this past growing season vs. key variables, in this post we look at the percent that 2017 yields as measured by the November crop production report for the top 18 corn growing states deviated from their respective 30 year averages.
We also show these yields deviations vs. the end of September crop rating as a percent of each states 1990-2017 end of Sep crop rating and this year's July-September rainfall as a percent of the 1960-2017 July-Sep precipitation average.
For crop ratings, we use our usual system where we weight the crop based on the percent in each category and assign that category a factor of 2 for very poor, 4 for poor, 6 for fair, 8 for good, and 10 for excellent and then sum the results.
All of these are plotted on the left hand axis while the end of September Palmer Drought Severity Index (PDSI) is plotted on the right hand axis.
The U.S. set a new national corn yield record at 175.4 bushels per acre (bpa) and as expected most of the top states did quite well with yields in KY, MO, NC, PA, TN and TX all at least 13% above trend.
In fact, the only states that did not have above trend yields were those in the drought stricken Plains including NE, ND and SD.
There has been a lot of talk about the disconnect between crop conditions and yields this year for in a season in which a record U.S. corn yield was established this should have come when national crop ratings were especially high but that was not the case in 2017.
Still, only seven of the states had end of September crop ratings below their 1990-2017 average with Iowa 4.3% below average though yields this year were 2.6% above trend, Michigan with ratings 6.7% below average yet a yield 3.3% above trend and South Dakota crop ratings 13.7% below average yet this year's corn yield just 1.6% below trend.
The drier states included IL with July-Sep 2017 rainfall 27.1% below average yet yields were 9.7% above trend, Iowa with rainfall 24.6% below average and Michigan with precipitation 22.3% below average.
Of all the states, perhaps the biggest surprise was Illinois where an outstanding yield of 198 bpa, the second highest ever, was achieved with average crop ratings, the just mentioned 27.1% shortfall vs. normal summer rainfall and a Palmer Drought Severity Index of -1.37; one of only two states that showed drought conditions as of the end of September with North Dakota's even more severe -2.32 end of Sep PDSI rating the other.