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Issue No. 21, Article 11/August 15, 2008

Short Corn Ears

We continue to see slower-than-normal progress in corn development, with only 12% of the Illinois crop in the dent stage by August 17, compared to the 5-year average of 45%. This is due to the crop's being behind normal since planting and to the average to below-average accumulation of growing degree days: GDD since May 1 range from less than 2,000 in northern Illinois to about 2,500 in southern Illinois, averaging about 130 behind normal.

The good news is that the crop is not crashing due to dry weather, so development remains tied to GDD totals to date. Much of the crop in the southern part of the state was planted late, so in many fields it has accumulated only 2,000 or fewer GDD since planting--similar to or less than accumulations since planting in northern Illinois. So the corn crop over much of the state needs 500 or more GDD to bring it to maturity, providing that it has an adequate water supply to maintain photosynthetic activity up to maturity.

Though the appearance of the corn crop is good to very good in most fields, and the pollination period was one of generally good water supply and temperatures, we have started to get reports that ears in some fields are shorter than expected. The phenomenon appears to be widespread, and it may be more common in fields that were planted and then pollinated relatively early.

From reports, it appears that there may be more than one cause of shortened ears. The most widespread problem is ears that are "nosed back," with unfilled cob tips. Cobs appear to be normal length. The outer end of the cob is without kernel remnants, indicating that fertilization of tip kernels failed. Between this area and the developing kernels, we often find remnants of aborted kernels, where fertilization took place and kernels started to accumulate dry matter, and then stopped. Aborted kernels range in size from small, empty seed coats to kernels perhaps one-fourth the size of normal kernels, and with some visible contents, often with yellow color (in yellow corn). The following photo shows ears with this symptom, gathered from a plot near Urbana on August 19.

"Nosed back" corn ears, of normal length but with unfilled cob tips.

The other symptom, which appears to be much more hybrid-specific, is "blunt ear syndrome," also known as "beer-can ear" and by other names. Like the nosed-back ears, these have reduced kernel numbers per row, but normal numbers of rows of kernels. The main difference is that both the length of the kernel rows (number of kernels per row) and the length of the cob are shorter than normal in these ears. I learned from a seed company agronomist that several hybrids in a strip trial north of here are showing this symptom, and I was able to locate one of those hybrids in the hybrid trial here at Urbana. The following photo shows ears from that hybrid.

Corn ears affected by blunt ear syndrome, with both length of kernel rows and length of cobs shorter than normal.

While the early part of the 2008 growing season was characterized by cool, wet conditions and slow early growth of the corn crop, the conditions during pollination were very good, and we did not anticipate that kernel numbers would be low. When we see problems like this, and when insects or diseases were minor problems during the pollination period, we usually resort to blaming the weather. That's not so easy to do this year: rainfall in July ranged from some 4 inches to more than 10, and average temperatures for the month were slightly below normal, making July seemingly an ideal month for pollination.

High rainfall amounts obviously mean considerable cloudiness, and many are thinking that lack of sunshine might be a primary cause of pollination problems. As we've often observed, anything that reduces photosynthetic rate is a problem for corn, and this is especially true around the time of pollination. We recognize that more rain means less sunshine, and so when temperatures are normal and there is a lot of rain, lack of sunshine is a logical choice.

Using monthly summary data provided by the Illinois Climate Network project of the Illinois State Water Survey, I compared July sunlight and rainfall for 2006, 2007, and 2008 at five Illinois locations (Table 1). The fact that pollination time is different for different years might influence the usefulness of this comparison, but the numbers show somewhat less variation among years, and perhaps less correlation with rainfall, than we might have expected. This does show that sunlight intensity was 5% to 7% less in 2008 than in 2007, except at DeKalb.

While sunlight doesn't appear to be a strong predictor of success of pollination, it might well have been a factor in the lower success of pollination in 2008, and so might explain some of the loss in kernel number that we see. We also saw some unfilled ear tips in 2006, when sunlight was similar to that in 2008. But our all-time favorite year--2004--had lower sunlight intensity in July than any of the years in the table.

Though it is not clear to what extent blunt ear syndrome (BES) and the unfilled ear tips represent different aspects of the same problem, the fact that BES includes shorter cobs suggests that this problem started earlier in the season than did the problem of unfilled tips. Corn with BES has been reported and discussed, by Dr. Bob Nielsen at Purdue and others, for more than a decade. It is a frustrating problem, in that it appears in some years but then is very difficult to find in other years with similar weather patterns.

We have never clearly identified a common cause for BES, though having a stretch of cool weather sometime during vegetative growth has often been linked to appearance of this problem. In 2007, we saw it on the outside rows of fields where corn had experienced some frost damage on about May 20 in central Illinois. Finding it in 2008 was unexpected. Even though it was quite cool throughout May, much of the corn was planted somewhat late, and it grew slowly, with the crop in most fields barely at the 4-leaf stage by the end of May. June was without cool weather--the lowest night temperature at Champaign was 57 degrees on June 1, and there were no night temperatures less than 60 after the first week of the month. While it is possible that cool temperatures in May (only 10 days had minimum temperatures above 50 degrees) might have contributed, we have normally thought that corn needs to have started to form reproductive structures for this damage to take place, and that the plant does this only at or after stage V6 or so.

While we tinker with our theories of why ears sometimes end up with fewer kernels per row than normal, we need to consider the consequence of this development in the 2008 crop. The ears in the photos were taken from stands of about 30,000 plants per acre. The unfilled-tip ears in the first photo averaged 509 kernels per ear, giving a kernel count of about 15.27 million per acre (MKA). The BES ears in the second photo averaged only 399 kernels per ear, which would be 11.97 MKA. Let's assume that these will fill out completely, and that fewer kernels per ear will mean slightly higher weights than those with more kernels, due to more sugars being available per kernel. So instead of the 90,000 per bushel (282 mg per kernel) that is a conservative estimate of normal kernel weight, we will use 80,000 kernels per bushel (318 mg per kernel) for these. At that size, the unfilled-ear-tip corn will yield 191 bushels per acre and the corn with BES ears will yield 150 bushels per acre. If we consider full-sized ears (with smaller kernels) to have 640 kernels (19.2 MKA), the yield would calculate to 213 bushels per acre.

From reports we've had, not all fields show this ear shortening, and I don't mean to raise a general alarm about an Illinois corn crop that is forecast to yield 172 bushels per acre, or the third-highest on record. The August yield estimate was based on actual kernel counts, so I have to believe that lowered counts were included. Most stands look very good, and it is possible that many producers turned up populations somewhat in 2008 in response to high prices. This would lessen the impact of lowered kernel numbers on yield.--Emerson Nafziger

Emerson Nafziger

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