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Corn closing in on maturity

This Fayette County corn has been progressing rapidly. Limited rainfall and the heat have pushed the crop in many areas and tip die-back is also showing up in many fields.
By Peter Thomison, Ohio State University Extension
Ohio’s corn crop continues to develop rapidly as a result of this season’s early planting and above average temperatures. According to the NASS (www.nass.usda.gov), as of Aug. 15, 82% of corn was in dough, compared to 43% last year and 59% for the five-year average. Thirty-four percent of corn was dented, compared to 4% last year and 10% for the five-year average. In many fields, corn in full dent has achieved the half-milk line stage (also referred to as the “starch line”). Thermal time from half-milkline to physiological maturity (“black layer”) is approximately 280 GDDs (http://www.kingcorn.org/news/timeless/GrainFill.html), which corresponds to about 10 days if we accumulate at least 28 GDD daily. Based on conditions as of Aug. 1, the NASS has forecast Ohio’s corn average yield at 176 bushels per acre, up 2 bushels from last year’s record yield of 174 bushels per acre. If these estimates for maturity and yield come to pass, we may be looking at a very large, early maturing crop.
In the Aug. 12, 2010 issue of the Illinois Pest Management Bulletin, Dr. Emerson Nafziger provides an interesting perspective on effects of growing conditions, especially temperature and rainfall, on development of the Illinois corn crop this year. The following is an excerpt. The complete article is at http://bulletin.ipm.illinois.edu/article.php?id=1402
“Corn to the Finish Line: Racing or Collapsing?”
The 2009 and 2010 growing seasons represent two extremes in terms of temperatures and growing degree-day accumulations since planting. Planting was a full month earlier in 2010 than in 2009, and GDD accumulations since planting are running as much as four to five weeks ahead of accumulations after planting in 2009….
Soil moisture ratings currently show 74% of fields with adequate-to-surplus topsoil moisture. This number was 80% in 2004 and only 43% in 2007. As is usually the case, some parts of the state, especially parts of central, eastern and southeastern Illinois, are dry or very dry, while other parts, mostly in the west and northwest, have had too much rain. Rainfall patterns have been close to ideal in some places. The effects of rainfall during the remainder of the season this year will be less than we would normally estimate during mid-August, only because the crop is so far along that water isn’t going to be a serious limitation if it hasn’t been limiting up until now. Exceptions to this are in replanted or late-planted fields, where more rainfall will be needed to finish out the crop.

Many people worry that the continued (and continuing) high temperatures have been hard on the crop and that we cannot expect it to yield as much as it might have with average or below-average temperatures in August. I would concur with that concern, not so much because the crop is filling grain poorly this year, but because it tends to continue filling for a longer period when temperatures are lower, in some cases producing the “bonus fill” that results in larger-than-normal kernels in years like 2004 and 2009. We can’t accurately forecast final kernel size, but it might be closer to the 85,000 to 90,000 kernels per bushel that we consider normal, and not the 70,000 to 80,000 that we can see under unusually favorable conditions during late grainfill.
Another widely reported issue this year is “tip-back,” which refers to abortion of some kernels on the tip of the ear. Conditions during and after pollination were generally favorable this year, and it’s not clear that kernel number per ear (or per acre) is lower than normal. But we have a tendency to view aborted tip kernels as lost yield potential. In some cases, when conditions after pollination are unfavorable and we end up with only 300 to 400 kernels per ear, this view may be accurate; the crop can usually fill more kernels than this unless filling conditions deteriorate. But if tip-back takes kernel counts down from 700 to 600 per ear (in fields with around 30,000 ears per acre), it’s quite possible that kernel size will increase a little as a result, and there may be little or no loss in yield.
Rapid development and early maturity will be among the more memorable aspects of the 2010 corn crop. In our planting date trial at Urbana, corn planted on April 5, April 21, May 10, and May 28 had by August 10 accumulated about 2,575, 2,400, 2,200, and 1,920 GDD, respectively. At current accumulation rates of about 28 GDD per day, a hybrid that needs 2,750 to mature needs only about 6 more days to reach black layer, and the same hybrid planted on May 10 needs about 20 more days to reach maturity. These projections generally seem to track with what we’re seeing in the field.
Many people feel that the high temperatures, which we commonly hear reported as the “heat index” (which dramatizes temperature by combining it with humidity into a “human misery” measurement) have been harmful. In reality, while high night temperatures are a negative factor, daytime temperatures in the 90s have not done much harm in areas where soil moisture has stayed adequate. Corn plants do not suffer from high humidity like we do, except indirectly: high humidity means higher night temperatures, and leaves may stay wet longer in the morning, which can increase disease development. Some insects also like higher humidity…”
Other references:
Nielsen, R.L. 2008. Grain Fill Stages in Corn. Corny News Network, Purdue Univ. [On-Line]. Available at (http://www.kingcorn.org/news/timeless/GrainFill.html ) (URL verified 8-16-10).

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