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Issue No. 19, Article 4/August 13, 2010

Corn to the Finish Line: Racing or Collapsing?

The development of the Illinois corn crop continues on its very rapid pace, with 78% in dough stage by August 8, about the same percentage as by this date in 2004 and 2007. In 2010, 29% of the crop was in the dent stage by August 8, compared with 31% in 2004 and only 14% in 2007.

The acreage of the corn crop rated as good or excellent (G-E) is currently at 64%, about where it's been for most of this season. This contrasts with 74% rated G-E by this date in 2007 and 83% in 2004. A major contrast between those two earlier years was temperature: in 2004, the first week of August had temperatures 4 to 5°F below normal, while in 2007, the same week had temperatures 8 to 10°F above normal. This year, the first week of August was 3 to 4°F above normal.

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. As an example, corn planted this year on April 15 (close to the median planting date) at Urbana has already accumulated more than 2,400 GDD. In 2009, corn planted here on May 20 (about the median planting date in 2009) had not yet reached 1,800 GDD by August 10, and it was September 12 before it reached 2,400 GDD.

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 till 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.

When the crop reaches maturity as early as it will this year, warm temperatures help it to dry-down rapidly; under normal temperatures in early September, we can expect field drying rates to approach 1 point per day if the weather is sunny and there is some breeze. Grain moisture at black layer (maturity) tends to be a little lower when maturity is reached during warm weather, and so should be in the low 30s this year. It's thus no stretch to guess that we will have harvest well underway in some areas by the end of August or early September.

Remember that as grain dries below 20% moisture, harvest loss (shelling at the corn head) increases. As rapidly as we expect dry-down to take place, it may be difficult to get most of the crop harvested before moistures drop below the 18% to 22% that usually represents the best balance between low drying costs and increasing harvest losses. Drying with unheated air typically works much better in early September than later, and some may want to consider starting to harvest in the mid-20s in order to decrease the number of acres harvested at moistures below 16%. If that's the plan, be sure to start monitoring the crop as soon as it reaches black layer, which is approximately when the husks lose all green color. It's easy to underestimate drying rates and to be surprised at how dry the grain is if you wait the normal few weeks after maturity before checking it.

Though early planting and rapid development of the Illinois corn crop have helped it in general this year, 2010 has not been without its challenges. Loss of nitrogen and damage to root systems are both common in the large part of the state that received too much rain in May and June (and, in places, July). Those who have flown over fields generally report lots of problems, including places without corn plants. In general, how late into grain-filling the canopy stayed healthy and green will be very closely tied to final yields in 2010.

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.

The first "objective" estimate of yield is being released by the USDA this week (on August 12). Compared to last year, it should have been much easier this year to count actual kernels, since the crop is so much farther along. One issue will be the many areas in some fields with missing or damaged plants, where yields would be estimated at or close to zero. To make estimates in your own fields, you can either include low-yielding areas or just estimate how much of a field is badly damaged and estimate yields only from the better areas, reducing the harvested area. (This is for "home use" only--all acres in harvested fields get counted for official purposes.)

Estimate kernel number in 1/1,000 of an acre by counting ears in that distance (17 feet, 5 inches for 30-inch rows) and multiplying ear count by the average number of kernels per ear, counted on three or four ears. Divide that number by 90 if you expect kernel size to be only average, or maybe by 80 if you expect kernel sizes to be a little larger. As an example, if there are 29 ears that average 580 kernels per ear, that's 16,820 kernels, which at 90 (thousand) kernels per bushel would be 187 bushels per acre. Of course, this year many of us can wait a few more weeks and harvest the field, which will give us real, not estimated, yields. We hope most of the surprises will be on the positive side.--Emerson Nafziger

Author:
Emerson Nafziger

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