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Will the crop mature? It depends

By Jean Caspers-Simmet

Date Modified: 09/23/2013 9:32 AM

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NASHUA — Will the corn crop mature before the first killing frost?

That was a question on everyone's mind during last week's field day at Iowa State University's Northeast Research Farm at Nashua.

The answer is that it depends — it's been that kind of year. In many parts of the state, wet has turned to dry, with the most recent U.S. Drought Monitor showing all of Iowa now abnormally dry or facing moderate to severe drought.

Much of Minnesota is abnormally dry or experiencing moderate drought.

Up until recent weeks concerns were that cooler-than-normal temperatures would keep late-planted crops from maturing. The last few weeks growing degree day accumulation is running 20 percent to 30 percent above normal.

Extension corn agronomist Roger Elmore said 50 percent of Iowa's corn crop was planted by May 18 with 90 percent planted by the month's end. That compares with 2010 when 50 percent of the state's corn was planted by April 18 and the entire crop was in the ground by May 15.

Corn planting went from 15 percent to 70 percent planted during the week starting May 12.

"That's 1.5 million acres of corn planted per day," Elmore said. "That is an incredible pace. I've never seen it that high."

For top yields the corn planting window is April 10 to May 10 in Iowa as a whole with northeast Iowa having about a week shorter window, Elmore said. The Aug. 12 USDA yield forecast reflected late planting in Iowa, he said. Iowa's USDA August forecast yield of 163 bushels per acre is almost 9percentbelow 30-year trend-line yields.

USDA's estimate of the crop in good to excellent condition dropped and this week's high temperatures and dry weather will make it fall more, Elmore said. Much of the state remains dry. Warmer temperatures with dry conditions will stress the crop even more.

"The heat is a mixed blessing," Elmore said. "It expedites maturity but there is also less time for kernel fill. Days over 94 degrees after silking reduce yield by 3 percent to 5 percent per day. If there's moisture stress, it's a double whammy."

Cool temperatures after silking also can increase yield potential given specific conditions, Elmore said. The record yields of 2009 resulted from slow GDD accumulation after silking coupled with a late frost. On the other hand, warm temperatures after silking in 2010 reduced corn yield potential.

Corn in early dent has about 60 percent grain moisture, accumulated about 45 percent of its dry matter, and needs another 33 days to mature, Elmore said. At three-quarter milk line, 97 percent of the dry matter is accumulated and it will take about two weeks to mature. Physiological maturity is the point when maximum kernel dry matter occurs, which is normally around 35 percent grain moisture.

Elmore said that there is a 50 percent probability of temperatures less than 29 degrees F on Oct. 14 for north central Iowa and Oct. 15 to 17 for northeast Iowa. There is a 10 percent probability of a killing frost Sept. 30 in north central Iowa and the first few days in October for northeast Iowa. There is also a 10 percent probability that killing frost will hold off until late October for north central Iowa and early November for northeast Iowa.

In Nashua's date of planting study, corn with a relative maturity of 100 days that was planted April 30 was at dent and should be mature by Sept. 27, Elmore said. Corn with a relative maturity of 100 days that was planted June 3 is at milk stage and has seven weeks to go. The 100-day hybrid planted on May 15 should reach maturity by Oct. 9.

"We could slip through and get that crop mature," Elmore said. "The bottom line of this whole season will be the timing of the first 28 degree F frost. A later than normal frost encourages longer seed-fill period and higher yields. An early frost, well let's hope it doesn't happen."

Wet turns to dry, cool to hot

Northeast Research Farm superintendent Ken Pecinovsky said rainfall this growing season was well above average for April through June at 6.41, 9.92 and 8.22 inches. July was 2.14 inches below normal at 2.65 inches and August is 0.95 of an inch below normal at 3.29 inches. It takes about 21 inches to grow a crop.

"We're starting to catch up on heat units this month after the cooler weather that we had," Pecinovsky said.

For 2013 temperatures were 7.79 degrees below average for April, 5.28 degrees below average for May, 1.05 degrees below average for June and 0.30 of a degree below average for July. So far in August, temperatures are running 0.81 of a degree above average.

Pecinovsky said typical frost dates in northeast Iowa are Oct. 5 to Oct. 11. In 2007 and 2011 frost came Sept. 15, and in 2008, Oct. 21. Corn harvest started Sept. 18 for corn and Sept. 11 for soybeans in 2012. In 2009 corn harvest started Oct. 26 and finished Dec. 7. Soybean harvest in 2009 started Sept. 29 and finished Nov. 7.