No. 4 Article 10/April 29, 2010

Most Corn Is Planted--Now What?

According to the official report, 73% of the state's corn crop was planted by April 25, up from 34% on April 18 and 1% on April 11. With soils dry enough to plant almost everywhere in Illinois and little rain for the first three weeks of April, planting progress is on track to be one of the fastest ever in Illinois. Rapid planting progress has been unusually uniform, with percentages planted by crop reporting district ranging from 61 in the West to 83 in the Southwest.

To put early planting into perspective, progress in both 2004 and 2005 was almost as fast as in 2010, with 67% and 64% planted by April 25 in those two years. The two years turned out very differently, with 2004 producing the highest corn yield ever in Illinois at 180 bushels per acre, followed in 2005 by a state average yield of only 143. The main difference between the years was the good rainfall in 2004 and the very dry weather in part of Illinois in 2005. A less-noticed difference was the persistently warm May weather in 2004 and the cool temperatures during part of May in 2005, including a frost event the first week of May that killed some of the earliest-planted fields in part of the state. I believe that cool temperatures that occur after the corn crop is up and growing--probably at about the V3-V4 stage--may negatively affect yield potential, but how this happens is not clear.

While we could hardly have hoped for a better spring this year to help us forget the troubles of last spring and fall, planting conditions were less than ideal in many fields. Many people tilled fields, especially corn stalks, before they were dry enough to till well. With rain after tillage this might not have been a problem, but as soils continued to dry, clods formed. Secondary tillage equipment used by most people today stirs soil more than it compresses it to break up clods. So even though most planters do a very good job of placing seed and firming soils around it, some seed planted into cloddy soils ended up with less than normal contact with soil particles. This, along with cooling soil temperatures after planting, has meant slower water uptake and slow emergence. In some cases, some seed might have germinated while other seed still hasn't taken up enough water to germinate.

Rainfall this past weekend will bring most of the planted crop up, but we expect that stands may be uneven in size, if not in percentages, across fields. The cooler temperatures along with sunshine (which can heat the soil proportionately more than it does the air) may help speed emergence of late-germinated seeds and so help to even out plant size. Whether differences in emergence times down the row will be a problem depends on the pattern of unevenness and the degree of delay. The best way to measure the degree of delay is not in number of days, but in growth stage differences, which tracks differences in growing degree-days. If emergence within a field occurs over a period during which less than 70 to 80 GDD accumulate, it is unlikely that the unevenness in plant size and growth stage will have a large enough effect on yield to justify any remedial action, or even to cause much concern. We expect plants that emerge over such a range to all end up being within one growth stage of one another during vegetative development.

GDD accumulations have been above normal in April, but with an upside-down pattern: accumulations at Urbana totaled 92 GDD for the first week, 82 for the second, and 67 for the third, and we expect a total of only about 60 GDD for the fourth week. This cooling trend was capped off by temperatures at or near freezing on the morning of April 28 in some parts of Illinois. Corn planted on April 5 that was at V1-V2 stage did not appear to have suffered from this frost event, though some leaf damage may appear within a day or two. We would normally expect visible damage the morning of light frost, but in this case it appears that temperatures spiked down and back up quickly, so radiational cooling may have been limited. Liquid water gives off heat as it freezes, so the dew on the leaves would have provided some protection against frost injury.

Emergence of planted seed has followed reasonably well the 110 to 120 GDD accumulation that is considered normal to bring about emergence. This means that corn planted the first week of April took only about 8 to 10 days to emerge, and corn planted the third week of April may need as much 15 to 18 days to emerge. According to the guideline given above for unevenness in emergence, at the current rate of GDD accumulation we need not worry too much if emergence within a field takes place over as much as a week. Our typical guideline--of wanting emergence to take place over no more than 48 hours--may be appropriate when it's warm, but it is probably unrealistic under cool conditions when less than 10 GDD accumulate per day.

Some parts of the state have had more rainfall than is ideal, and it is possible that some soil crust formation may take place this week as soils dry under good sunlight. Soil crusting tends to be more of a problem when temperatures during drying are high, though, so we don't anticipate an unusual amount of crusting. Still, with slow GDD accumulation and slow emergence, soil crusting can be a concern, especially in fields where seed-soil contact is not what we'd like. In a few fields standing water may also be a problem.

While we have no reason to expect much replanting at this point, I'll remind readers that we have revised the replanting guidelines, which are available in the new (24th) edition of the Illinois Agronomy Handbook. Unfortunately, the website that existed for the older version is no longer up and running, and the site for the new version is not yet in place. So the handy calculator we had to assess expected returns to replanting is not available this year, even though its URL is given in the new handbook. You can order the handbook online at

The table that underpins the replant decision (and the calculator) is provided in Table 2. Changes from the previous version include the use of higher plant populations--40,000 now versus 30,000 in the older version. But other than having higher optimum populations, the replant decision making process remains the same:

  1. Find the number (expected yield in percent of maximum) on the chart that represents the first planting date and the stand that resulted from it.
  2. Find the expected yield, assuming a replant date and plant population.
  3. Subtract the original number (step 1) from the replant number (step 2) and multiply this difference (which is in percent) by the yield you expect normally from this field to convert the replant benefit to bushels.
  4. If the extra bushels you expect from replanting multiplied by a price exceeds the cost of replanting, then you can expect replanting to pay.

Let me give an example. If a field planted on April 10 ends up with a stand of 25,000, it is expected to yield 91% of maximum. If it can be replanted to 35,000 plants on May 9, expected yield will be 93% of maximum, only 2 percentage points more. We would not normally expect 2 percentage points--3 or 4 bushels--to pay replanting costs. Later-planted corn tends to be wetter at harvest as well, so it can cost more to dry. We saw in both 2008 and 2009 that later-planted corn yielded very well, but that does not improve our prediction very much; we still need to go with averages, which the table contains. These are numbers from northern Illinois; in southern Illinois, we think the planting date response is likely to be similar to that shown here, but in upland soils, it would be appropriate to subtract 5,000 plants per acre from each column heading (for those now with 20 to 40) to reflect the fact that optimum plant populations in southern Illinois tend to be about 5,000 less than in northern Illinois.--Emerson Nafziger

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