Ag Weather Forum

The final full week of August saw 60% of the U.S. corn crop move into the dent stage, according to the USDA Crop Progress report for the week ended Sept. 1. For much of the ag industry, the dent stage is the beginning of the home stretch for the corn crop; there are only 30 days on average between the dent stage (R5 in agronomy speak) and full maturity (R6).

But, that last 30 days is important.

Purdue Agronomist Emeritus Bob Nielsen noted in August 2021 that "kernel dry matter content at the beginning of R5 is only about 45% of the eventual final accumulation and there remains approximately 30 more days before physiological maturity occurs. This is sobering considering that farmers and agronomists alike often breathe a sigh of relief when the crop reaches R5 because of a mistaken and, frankly, emotional belief that the "crop is made" by this grain fill stage."

Corn does need more than just a garden-variety shower's worth of moisture in the 30 days between denting and full maturity. A 2018 Pioneer Seeds Crop Insights article by agronomists Stephen D. Strachan and Mark Jeschke placed the total water usage of corn between R5 (dent) and R6 (full maturity) at an average of 0.14 inches per day in Iowa. That equates to 4.2 inches over the 30-day period between R5 and R6.

DTN Ag Meteorologist John Baranick pointed out that this moisture total is not 4-plus inches of additional rainfall, as fields with ample moisture may be "done" with their precipitation needs for the season. Also, the drying of the corn plant in these final weeks consumes much of that final-stage moisture. The actual grain needs could possibly be just over half the daily crop usage.

"Using an average of 0.08 inches/day for the crop itself as an average over the R5 timeframe might make more sense," Baranick said. That is still more than 2 inches of moisture needed for the corn kernel to maintain its yield potential.

This final-stage moisture requirement shows up at a time when a key indicator of El Nino or La Nina in the Pacific Ocean, the Southern Oscillation Index (SOI), appears to be hitting high gear in moving to the La Nina phase. The SOI -- a comparison between the barometer readings on the island of Tahiti in the central Pacific and Darwin in far northern Australia -- is used to track the atmospheric component of the El Nino Southern Oscillation (ENSO) phenomenon. In less than two weeks, going back to Saturday, Aug. 24, the 30-day SOI average has gone from an ENSO-neutral reading of -0.02 to a reading of +8.15 -- above the La Nina threshold of +7.0.

La Nina tends to support the development of dry and warm conditions in the interior U.S. during the fall season. And indeed, forecasts through mid-September call for above-normal temperatures and below normal precipitation in most primary U.S. row-crop areas.

This combination calls into question whether corn areas now in the dent stage and in recent moisture-short areas will receive enough moisture to hold yield potential. There is also 40% of the corn crop that had not yet reached the dent stage and needs even more moisture for a longer period of time. We may also be seeing the crop get pushed toward maturity and early drying; 19% of the U.S. corn crop was placed at full maturity in this week's report, which is 25% ahead of the 15% mature a year ago and more than 40% ahead of the five-year average of 13% mature.

The full Purdue University article on "Grain Fill Stages in Corn" by Bob Nielsen is available here: https://www.agry.purdue.edu/…

The Pioneer Seeds article on "Water and Nutrient Uptake During the Corn Growing Season" by Stephen D. Strachan and Mark Jeschke is available here: https://www.agry.purdue.edu/…

Bryce Anderson can be reached at Bryce.Anderson@dtn.com

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