No. 22 Article 5/September 4, 2009

Can This "Turtle" 2009 Corn Crop Get to the Finish Line?

I've learned that making predictions in novel crop situations is a very inexact "science." The 2009 Illinois corn crop definitely fits into the "novel" category, and we need to keep "inexact" in the forefront as well. The Ag Statistics Service report indicates that, as of August 30, the Illinois corn crop is only 78% in dough stage and 26% dent stage. This compares to 90% dough and 26% dent in 2008 (the most recent year with planting delays) and a 5-year average of 96% dough and 73% dent. A quick look at the records shows that these percentages in 2009 are lower than in any of the previous 25 years.

The fact that rainfall has been plentiful in most areas and that temperatures have been average or below has meant that the crop canopy has generally stayed in good condition in most fields. Exceptions include fields where N loss has occurred, leading to loss of leaf color, firing, and premature loss of photosynthetic capacity. This is especially notable in low-lying areas, but some whole fields seem to be deficient as well. In most cases, if N management was done well, we need to view the excessive loss conditions in 2009 as unusual and to not make many large changes in rate, form, or timing. This is the second unusually high N loss year in a row, however, and while we don't expect such conditions to occur next year, some may want to consider ways to manage N to reduce loss potential.

The NASS August 1 yield estimate of 175 bushels per acre for Illinois was based mostly on kernel counts. The extent to which kernels will fill is always a guess, especially when counts are made so early. Still, the estimate indicates that there are enough kernels developing to produce good yields. As usual, this is not the case in every field; in 2009, more than 20% of the crop was not yet pollinated by August 1, and while the weather remained cooperative, the August 1 yield estimate was made before kernels could be counted in these fields.

But the real story of 2009 has been the delayed planting and the slow rate of growing degree-day accumulation that has resulted in the crop's entering September needing, in many fields, more GDD than average September temperatures may provide. How far behind are we? The GDD accumulated since May 1 range from about 90 below normal in southern Illinois to 200 below normal in central Illinois to nearly 300 below normal in the northern part of the state. May and June temperatures were about normal, so these deficits resulted from below-normal temperatures in July and August. Losing 125 to 150 GDD in each month is the result of temperatures averaging 4 to 5 degrees below normal.

Most hybrids we grow in Illinois require 2,500 to 2,800 GDD to reach maturity. Adding the average GDD accumulations that we can expect from September 1 (based on average temperatures) to the 50% chance of first frost date (Table 2) to the GDD since May 1 gives total GDD accumulations ranging from about 2,600 in northern Illinois to some 3,700 GDD in southern Illinois. With the exception of northern Illinois, GDD accumulations should allow the early-planted crop to reach maturity well before frost. It will be close in northern Illinois, but most hybrids that were planted early should get close to maturity before frost, in which case yields won't suffer much.

The problem, of course, is that so much of the crop was planted after May 1. I believe the reduced GDD requirement that we usually apply to late-planted corn (6 or 7 GDD less for each day planting is delayed past May 1) may not apply this year, given the cool temperatures over most of the summer. That observation is supported by the fact that development stages of later-planted corn seem to be tracking predicted development for early-planted corn. Corn planted on June 1 in northern Illinois can be expected to accumulate only about 2,300 GDD by frost, while the crop planted at that time in central Illinois will accumulate about 2,600 GDD. These numbers are less than most hybrids will need, especially in northern Illinois. In the southern regions, GDD accumulation should be enough to mature most hybrids that were planted in early June.

Even when corn gets frosted within 100 to 200 GDD before black layer, some sugars will still migrate from the stalk to the ear, so yield losses are often small. This means that even later-planted corn, other than late-maturing hybrids planted very late, should have a good chance to get close to its yield potential, even in northern Illinois.

Planting delays to mid- or late June (much of this crop was yet to pollinate on August 1) were most common in south-central Illinois. If we subtract about 150 GDD for each week after June 1 that planting occurred, the crop planted on June 15 in south-central Illinois should still accumulate about 2,600 GDD by frost. In years with warm fall and delayed frost, GDD accumulations from September 1 in southern Illinois can be as high as 1,000. That would be enough to mature most of even the late-planted crop.

In central and northern Illinois, warm fall weather and delayed frost could add about 100 to 150 GDD to the numbers in the last column of Table 2. That would allow the late-planted crop in central Illinois to get close enough to maturity that yields would not be reduced much. In northern Illinois, late-maturing hybrids planted late will be challenged to reach maturity before frost kills the leaves and stops the filling process.

Overall prospects for 2009 corn continue to be good, with a fair chance that most of the crop will reach maturity before frost. We'll continue to hope for warm, sunny days in September; if we get those, we could even end up with yields above the August 1 estimate, like we did in 2008, when final yield (179) was 7 bushels per acre above the August 1 estimate (172).--Emerson Nafziger

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