Hotter than normal weather the last half of August added heat units to Iowa's late-planted, slow-growing 2013 corn crop. The crop had been lagging in development all summer. The recent blast of heat helped speed up crop maturity somewhat. But many Iowa fields still need more time to develop the crop. Iowa needs a later-than-normal frost this fall.

In a nutshell, that's what Iowa State University Extension corn agronomist Roger Elmore told the crowd attending a field day August 30 at the ISU Agronomy Farm near Boone, just west of Ames. It was 104 degrees in central Iowa, a good day to talk about the effects of hot, dry weather on growth and development of this year's crop.

TRACKING CORN MATURITY: Barb Halbur, a Carroll County farmer, looks at the milk line on corn kernels developing on an ear. The milk line moves from the top of the kernel toward the cob as kernels mature. When the milk line reaches the kernel tip, a black layer forms in the tip. The black layer is the first visible sign that physiological maturity has occurred and the corn is safe from frost.

Elmore also has recently spoken at other ISU field days, fielding questions from farmers wondering: Will my corn reach maturity before first killing frost hits this fall? Are we headed for a late harvest this year or not? We had a cool growing season, and then it turned hot. What are the effects of the continued dryness in this 2013 growing season—what kind of yield can I expect?

Is Iowa headed for a late harvest this year or not?

Will Iowa have a later than normal harvest? Yes, says Elmore. Harvest is likely to be a stop-and-go affair for a number of farmers this fall due to a wide range of planting dates and variable growing conditions. "Despite the blast of heat units that came during the last two weeks of August, which sped up crop development somewhat, many corn fields in Iowa were planted so late that they need a later-than-normal frost this fall to be safe from damage," he explains.

Because of the wet spring only about half of Iowa's 2013 corn was planted before May 16-17. So half of the crop was planted in the last half of May or sometime in June—a very late start. Some corn was planted in mid-June. Then it quit raining. But the weather was cooler than normal so that helped conserve moisture. On the other hand, with the lack of heat units the crop continued to lag in development.~~~PAGE_BREAK_HERE~~~

So July and the first half of August were cooler and drier than normal. It turned very hot the last half of August, with little or no rain. The National Weather Service weekly drought monitor map at the end of August showed Iowa had returned to drought conditions. The entire state on August 30 was rated abnormally dry and one-fourth of the state was in severe drought.

Iowa has wide range of planting dates and variable growing conditions in 2013

"Corn is a heat-unit based crop, that's the way it grows and develops," Elmore told folks at the Aug. 30 field day at Ames. "The extreme heat we've had the past two weeks is speeding up maturity. The corn crop is going through its development stages now faster than it normally does because of the heat we've received the last half of August."

But corn planting this year stretched out for almost two months starting in late April and continuing through June. As a result the pollination period was also spread out. July 28 was a key date for this year's corn crop. That's when 50% of Iowa's corn crop silked this year. Yet, some fields in Iowa were just starting to tassel in mid-August.

Corn needs about 60 days from silking date to when it becomes mature, says Elmore. When it reaches maturity corn is safe from frost. Corn will form a black layer in the tip of the kernel soon after it's reached physiological maturity. Looking at this year's situation, fields that silked and pollinated in mid-to-late August won't be safe until mid-to-late October.

Watch the milk line forming on corn kernels, to give you an idea of where your crop is now in terms of maturity

At the end of his presentation at the August 30 field day, Elmore gave people a printed handout. It had a handy table to help tell where your corn is in terms of maturity, how far away it is from reaching black layer stage and being safe from the first killing frost. That table accompanies this article, so you can use it to help judge the maturity of your own fields.

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Elmore also handed out six bags of corn ears he picked that morning from a nearby field where he has a date of planting study. The bags were labeled Group Number 1,2,3,4,5 and 6. People sitting in the front row were handed the "Group 1" bag of ears. I was sitting there next to Barb and Jim Halbur who farm at Carroll in western Iowa. Everyone opened their bag, took out an ear and noted the stage of corn growth. Ours was "early dent" stage. Dent is the R5 growth stage of a corn plant (see the table).

Next, Elmore had us break an ear in half, and instructed us to "Look and see how far the milk line has developed on the kernels. The milk line separates the milky white portion nearest the cob and the starchy solid portion at the top of the kernel. The milk line moves down from the top of the kernel toward the cob as the kernel matures."

Ears from our Group 1 bag on August 30 were still 24 days from maturity

The ears from our Group 1 bag had the milk line formed about halfway down the kernels. We looked at the chart and saw that at the "half milk" stage, our ear of corn still had 24 days left before it would reach physiological maturity. That's the R6 stage of growth, which precedes what is commonly known as "black layer" stage. The black layer is the first visible indication that physiological maturity has occurred and the corn is safe from frost.

Will that ear of corn reach maturity before first killing frost? The answer was "yes"—if the date of the first killing frost comes no earlier than normal. Corn in the Group 1 bag was planted April 29, said Elmore. The average date of the first killing frost in central Iowa is around October 10. So if frost occurs around that date this year, the field where this ear of corn came from will be safe. Adding 24 days to August 30 you figure the corn will reach maturity on September 22 or so.~~~PAGE_BREAK_HERE~~~

Elmore ran this "audience participation" exercise with the remaining five groups of people gathered at the field day—with bags of ear corn labeled Groups 2, 3, 4, 5 and 6. These were all the same hybrid, but planted at different dates. Group 1 was planted April 29, Group 2 planted May 8, Group 3 was May 13, Group 4 was May 23. Group 5 was planted in early June and Group 6 was June 18.

You can use this system to figure the odds of the corn in your fields reaching maturity before first killing frost

THE BOTTOM LINE: You can figure the odds of the corn in your fields making it safely to first frost. In the example exercise Elmore conducted with us at the ISU field day, our Group 1 corn should make it to frost OK. Same goes for the Group 2, Group 3 and Group 4 corn—assuming the average date of the first killing frost in 2013 comes no earlier than normal. 

However, the ears in Group 5 and Group 6 aren't going to make it—unless frost occurs later than normal, Elmore noted.

Physiological maturity (R6 stage of growth) is the point when maximum kernel dry matter occurs—normally around 35% grain moisture, says Elmore. That's when corn is safe from frost; corn doesn't put any more dry matter or "yield" into those kernels once the corn reaches maturity. And contrary to what some people think, says Elmore, kernels do not lose dry matter after they've reached physiological maturity.

The critical issue for this whole 2013 growing season will be the timing of the first killing frost (28 degrees F) this fall. "A later-than-normal frost encourages a longer kernel-fill period and higher yields," notes Elmore. "And what happens if there's an early frost? Well, let's hope it doesn't happen!"