The question of the week is how much, if any, USDA will lower corn yield in the September yield estimate compared to August. This has become a tale of two summers in one – a wet and cool first half and a dry and warm second half. That makes it harder to know how corn yields will react.

Typically, cool and wet seasons favor higher yields. Past studies have shown that USDA's national average yield estimates tend to go up from August to final yield in cool, wet years. That appeared to be what traders were thinking when corn rose only a few cents per bushel after the August report, even though the yield was less than expected, notes Chris Hurt, a Purdue University Extension ag economist.

Shift in pattern: The shift in weather over the past month may have some negative effect on what appeared to be an excellent corn crop.

Since then, however, what was a cool, wet summer has turned more into a warm, dry summer in many areas. August rainfall is below normal in many locations. There have still been some cool stretches, but the number of hot days has increased.

In hot, dry summers, yield estimates from USDA tend to go down from the August estimate to September, and then through the final estimate. September is the first estimate in most years, including this year, when field enumerators checking fields for USDA had ears and kernels to count.

The bottom line is that it may be more difficult to determine whether the crop is going forward or backwards compared to the August estimate. What most agronomists are saying is that while yield should still be good, especially compared to last year, test weights may be off compared to what they would have been had the cool, wet trend held the rest of the summer. That will tend to bring overall yields down.