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Issue No. 12, Article 10/June 12, 2009

Taking Stock of the 2009 Corn Crop

The official estimate is that the Illinois corn crop was 93% planted by June 7. The northern 60% of the state is more than 95% planted, while less than 90% is planted in the four southern districts, especially in the southeastern part of the state. This may not count all of the replanting that still needs to be done.

One positive note is that corn now planted will soon look better as it grows quickly following the late start. In the planting date study we have here at Urbana, corn planted on April 9 is at V8 and about 28 inches tall; that planted on April 26 is at V7 and 23 inches; that planted on May 11 is at V5 and 14 inches; and the last planting, on May 30, is at V2 and about 4 inches. Temperatures have been close to normal most of the past two months, though with the usual ups and downs, including temperatures cooler than normal last week.

The need to replant corn in Illinois has generally been less this year than in 2008, but replanted acreage will still be much above normal. Corn planted within a certain time period--when planting was followed by heavy rain within 1 or 2 days--has in many fields failed to produce adequate stands. We remember the great yields from most replanted fields in 2008, and while we can't realistically hope for such yields when planting in June, we know that good weather during the rest of the season can result in good yields.

Late planting and replanting again bring up the issues of hybrid maturity and growing degree-days (GDD) we can expect from planting date to frost as well as the enduring question of when it's "too late" to plant corn. Insurance coverage complicates this question, and I do not have the ability to sort that out, especially when there seem to be last-minute adjustments in some insurance policies.

To update the numbers I provided here 2 weeks ago, modified GDD from June 15 to the 50% chance of first frost are about 2,300 2,500, 2,700, and 2,850 for northern, central, south-central, and far southern Illinois, respectively. These totals are for average temperatures; if temperatures are much below average, GDD totals will be 100 to 150 less.

A corn hybrid planted as late as mid-June typically needs perhaps 200 fewer GDD to reach maturity than it does if planted early. So hybrids with relative maturity (RM) ratings of 110 to 112 days planted in mid-June in southern Illinois should still mature before frost, unless frost is early or the summer is cool. Cool weather during the summer will improve yield prospects greatly if moisture is adequate, but it will both diminish the late-planting reduction in GDD requirement and delay maturity directly, so will considerably increase the chance for frost damage before maturity. Chances for that are still, however, less than chances of yield loss due to dry soils during the season.

So what is the "latest date" corn can be planted? Corn grain has been harvested from plantings in early July in Illinois, and as we saw, corn planted in the last half of June in 2008 produced good yields. On the other hand, corn planted on June 5, 2007, at Brownstown yielded less than 10 bushels per acre. So we have some idea of the range of possibilities. Based on accumulated data, corn yield approaches 50% of its maximum yield when planting is delayed to June 15-20. But past data are so variable that such predictions are likely to be inaccurate for any given year. In a recent article on farmdoc.com, ag economists here at the U of I suggested that this year's planting delays might reduce yields by about 14 bushels off the trend yield of 166. They also note, as I have said here several times, that better-than-average weather for the next three months could largely overcome this handicap. Let's hope that happens.

Even stands that are now up are not great in many fields, but most fields with stands above 20,000 to 25,000 should probably be kept now, given the severe penalty for planting so late. Many are surprised by the fact that so many fields are missing gaps in rows and in parts of fields, often after planting conditions may, while not ideal, have seemed uniform. In most cases we think this is simply due to "drowning" of seeds, or lack of enough oxygen in saturated soils to allow germination to proceed. Some such seeds have short sprouts; others have no sprouts at all but seem to have just died suddenly. Eventually such seeds will turn mushy as microbes invade, but they will usually still be firm and appear sound right after they have died.

One of the unusual aspects of such stand reduction in 2009 has been what seems to be a random pattern of losses. In 2008 most death of seeds and seedlings took place in the low spots with standing water. In 2009, stand loss is often more randomly spread across the field, with missing segments often not well related to things we can see. In most cases the patterns are likely related to subtle differences in seedbed conditions, such as tillage, tire tracks, residue, and the like, which are not visible at or after planting. As an example, in our strip-till comparison here at Urbana, tilled plots have very good stands while strip-tilled plots have stand reductions of at least 30% in some places in the field. The difference was likely a combination of slightly wetter soils in the strips and the slightly greater amount of residue near the surface after planting in the strips.

Damage from birds or rodents is also considerable in some fields this year. The pattern of missing row segments might be similar to that described above, but animals often leave the plant or seedcoat behind when they eat the starchy part of the seed. Such damage also tends to be worse near field edges, though in one of our fields here it is more spread across the field and seems to be related to slight differences in seed placement. Rodents are very good at selecting "choice" plants, probably by smell, and those planted slightly shallower might be eaten preferentially. Birds can pull up seeds or plants anywhere in the field. Once plants reach V2 to V4, damage from birds and rodents should stop.--Emerson Nafziger

Emerson Nafziger

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