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Issue No. 19, Article 6/August 1, 2008

Will This Corn Crop Mature?

The latest NASS report says that 79% of the Illinois corn crop had silked as of July 27, compared to a 5-year average of 95%. As a result of the late planting that was common in the southern half of Illinois this year, only about 65% of the corn had silked in the four crop reporting districts that make up the southern half of the state, compared to more than 85% in the northern half. While the weather has generally been good in June and July, development of both corn and soybean is far behind normal, and as we head into August, there is increasing concern about whether crops will mature before the season ends.

Growing degree days remain our best method to track both past and expected progress in corn development, though we recognize that this method is not always precise. Research done in Ohio and Indiana measured the effect of late planting on GDD requirements of three corn hybrids (R.L. Nielsen et al., 2002, "Delayed Planting Effects on Flowering and Grain Maturation of Dent Corn." Agron. J. 94:549-558). The work showed that for each day of delay in planting past May 1, about 6.5 fewer GDD were required from planting to maturity. This means that a hybrid that requires 2,700 GDD if planted early would need only about 2,500 GDD if planted at the end of May, about 2,400 if planted by June 15, and about 2,300 GDD if planted by the end of June.

The researchers further divided the decreased need for GDD with late planting into GDD from planting to silking and GDD from silking to physiological maturity, and they found that about a fourth of the decrease is before silking and three-fourths after silking. For example, corn planted in early May required 1,417 GDD to silk and 1,199 from silk to maturity, while corn planted on June 11 required 1,355 to silk and only 1,001 GDD after silking. It is likely that at least some of the reduced requirement after silking is due to deteriorating crop conditions in some years, and perhaps to early death of plants. It is also likely that the reduction in GDD after silking is closely linked with the amount of yield loss that late-planted corn often suffers.

Table 1 shows GDD accumulation through July 29, starting on different "planting" dates at Carbondale, Urbana, and Rockford. If we make the simplification that corn planted in June needs 1,350 GDD to reach silking, then it makes sense that corn that is just silking at the end of July was planted about June 8, June 3, and May 28 in southern, central, and northern Illinois, respectively. We generally accumulate about 25 GDD per day at this time of year, so we can expect that corn planted on June 10 in southern and central Illinois might reach silking by about August 2 and August 6, respectively, while that planted on June 20 might reach silking on about August 11-12 in southern Illinois and August 15-16 in central Illinois. High temperatures might move this up by a few days, but probably at the expense of success of pollination. Corn planted at the end of June will probably not start silking until after August 20 if temperatures stay average.

The difficulty that corn faces in reaching maturity when planted late is due mostly to the decline in air temperatures that takes place starting in late August and accelerates into October. Figure 1 shows this decline, calculated using 30-year average temperatures (1971-2000) for three Illinois locations. Dates of 10% and 50% likelihood of frost, based on incidence over these years, are also indicated. Note that early frost (1 year in 10) occurs at the point where GDD accumulations have slowed to 10 to 12 GDD per day, while the chance of 50% frost is at the point where GDD accumulations reach about 7 to 8 GDD per day. These two dates are about two weeks apart, and about 150 GDD apart in southern Illinois to about 120 GDD apart in northern Illinois.


Figure 1. Daily GDD in September and October, based on 30-year average temperatures (1971-2000). Calculated from temperature data from the Midwest Regional Climate Center. The solid triangle symbol shows the date of 50% frost probability for that location; the open triangle symbol shows the date of 10% frost probability.

Table 2 shows the expected GDD accumulations by the date of 50% likelihood of frost, using actual 2008 temperatures through July 29 and 30-year average temperatures after that. There is an anomaly at Carbondale, in that the 10% and 50% frost dates there are earlier than those at Urbana. This is not the case in other locations in southern Illinois, so I chose to use the frost dates for Benton instead. The 10% and 50% frost dates are October 4 and 19 at Benton, October 2 and 16 at Urbana, and September 21 and October 6 at Rockford.

Based on this, if we assume that planting a midseason hybrid in early June reduces the GDD requirement by 200 GDD and that planting in late June reduces it by 300 GDD, and if we assume average temperatures for the rest of the season and an average frost date, we can estimate that in southern Illinois, corn planted in late June should have about a 50% chance of reaching maturity before frost. In central Illinois, corn needs to have been planted by about June 15 to have the same 50% chance to reach maturity. Corn planted in late June in central Illinois is unlikely to accumulate enough GDD to reach maturity, as is true for corn planted after June 1 in northern Illinois. Fortunately, few fields in the northern part of the state were planted that late in 2008.

Might weather conditions be better than average, so that even the late-planted crop could finish with a flourish, with near-normal yields? Yes, it's possible, though the chances are certainly not high. In 2007, GDD accumulations in September and the first half of October in southern Illinois were 200 to 250 GDD above normal, and frost was later than the 50% likelihood date. If that happened this year, there would be plenty of GDD to finish out the late-planted crop. If the crop's water supply remained favorable as well, the cooler night temperatures in September could help boost yields, though the decline in the daily amount of sunlight as days shorten will work against this.--Emerson Nafziger

Author:
Emerson Nafziger

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