This graphic is a scatterplot showing the week 30 (around the 7/30-8/1 period) crop ratings for U.S. corn vs the percent that the USDA's August corn yield forecast deviated from the 25-year trend of final yields.
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.
The 2021 U.S corn rating as of week 30 is 726 which is slightly above the 20-year average of 723. Such a rating would project corn yields to be 0.5% below trend.
This year's calculated 25-year trend is 176.4 bushels per acre (bpa) so this year's rating would project the August 2021 yield forecast 0.7% below trend or 175.5 bpa.
The USDA weather adjusted corn yield projection this year is 179.5 so that would equate to a yield of 178.6 bpa.
In the chart we also include a table that shows the change in the percent deviation from trend yield from the August to final yield projection and the week 30 crop rating.
As an example, last year's August 2020 yield of 181.8 bpa was 4.2% above trend but the final yield of 172.0 bpa was 1.4% below trend, a net move of down 5.6%.
This shows that August and September weather can make a difference as an August yield forecast that is ether above or below trend can change in both direction and magnitude by the time the final crop year projections are released in January.
The bottom line is that as of the beginning of August, it appears the 2021 U.S. corn crop is on track to attain trend or slightly below trend yields, so question is what happens to the crop over the next two months and what one's calculation of a trend yield is.
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