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Expect decent harvest weather

Posted on September 23, 2014 at 8:00 AM by Iowa Corn

Jean Caspers-Simmet,

Expect decent harvest weather — not sopping wet and not bone dry, Extension climatologist Elwyn Taylor said during the recent field day at Iowa State University's Northern Research Farm.

"The length of the harvest season should be about average," Taylor said.

With north central Iowa behind about 300 growing degree days, he said farmers can expect to spend some money drying corn.

With an El Nino weather pattern establishing itself, Taylor said farmers should see a close to normal frost date, which is Oct. 10 for Kanawha. El Nino means a more average winter will replace the extremes seen the past few years.

Taylor is forecasting a 170 bushel national average corn yield.

He summarized how growing degree days, precipitation and heat stress work together to affect yield. Corn and soybeans develop fastest at 86 degrees. If water is ample, that number may increase to 92 degrees. Corn does best if GDDs get 100 or so ahead of normal between planting and silking, but 300 ahead becomes a strain. After silking yields increase with diminished GDDs.
"About 150 GDD behind from silking to dent is ideal," Taylor said.

Comparing GDD accumulation and changes as the season progresses to past years can give farmers a meaningful basis to estimate yield, Taylor said. Using weather information available at (choose ag weather), farmers can compare the current year with good-, average- and poor-crop years. Taylor selected 2004, 2008 and 2012 as an example.

He said 2014 is much like 2004 and 2008 and much different than 2012, a drought year.

Precipitation in 2014 has been similar to the high yielding 2004 season and so-so 2008.

"The 2014 heat stress was even less than 2004 and 2008 and less than one-fourth of the normal stress," Taylor said. "2012 experienced more than double normal stress degree days. To date, 2014 will be expected to yield much like 2004 or at least as good as 2008 with respect to trend for the field, assuming the pattern continues to maturity."

Paul Kassel, Extension field agronomist, discussed corn and soybean development. Soybeans at Kanawha were well into R6 on Sept. 4, with seed filling the pod cavity at the top 4 nodes and about two weeks from maturity. At R6.5, when soybeans turn light green, they are nine days from maturity. The field was planted May 19.

He estimated the field will be ready for harvest the first week in October. An early frost shouldn't cause too many problems. Kassel recently looked at a field of replanted soybeans in Palo Alto County that were at R4, full size pods. These beans need a good 30 days to reach maturity.

"If we could get a normal frost date, I think they could make a crop," he said.

Corn at Kanahwa was at R5.5, one-half milk line, with 40 percent kernel moisture and 90 percent dry matter accumulation — still a few weeks from black layer or maturity.

"It takes 625 growing degree days from silking to dent stage and 575 GDDs from early dent to maturity," Kassel said. "We're looking at early October maturity."

He said it takes 30 GDDs to lower grain moisture each point from 30 percent to 25 percent. Field drying corn from 25 percent to 20 percent requires about 45 GDDs per point of moisture with sun, wind and rain affecting final dry down rates.

The problem is GDDs decline steadily in fall. Some years, such as 2009, there were just 9 GDDs accumulated from Oct. 8-15 because of cool, wet weather. Normal is 62.

"It's good to spend some time in your fields this time of year," Kassel said. "We may spend $6 per point per acre to dry down a 200-bushel corn crop. You need to weigh that against your harvest schedule and field losses. You might have to spend a little more on drying this year. Once you get to mid-October field dry down is not real good unless you get real warm weather. Hopefully you have your propane needs lined up."

Kassel predicts farmers will see tremendous yields on fields where the water ran off, but there will be some 140 bushel yields in fields with drowned out spots.

For more information on crop maturity and frost risk, see ISU Extension's "How Much Risk of Frost Do You Have?" at

Tagged As: El Nino, Elwyn Taylor

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