No. 5 Article 4/May 6, 2011

Concerns with Planted Corn

The wet weather continued through the end of April, and there was hardly any planting progress during the second half of the month in Illinois; we remain at 10% planted as of May 1. While there is starting to be some progress in parts of western and northwestern Illinois, much of the state remains wet. Low temperatures mean slow drying, and in many places this means further delays.

About half of that 10% was planted the first week of April, and the other half the second week. But only 3% had emerged by May 1, and there have been concerns about the slow emergence of what was planted in week 2, especially that planted just before rain halted progress mid-month. There are also concerns about the crop that did emerge and whether its yield prospects have been diminished by the weather over the past three weeks.

April 2011 was not only a wet month, with a statewide average of more than 7 inches of rain, but a cool one. The second half was cooler than the first: growing degree-day accumulations at Urbana in each of weeks 1 through 4 were 50, 85, 35, and 61. If it takes about 115 GDD to reach emergence, the last planted date for which we would expect emergence by May 3 was April 15. We would not expect any corn planted after April 15 to have emerged yet, but if we grant a small GDD cushion, we would expect any corn planted before April 13 or so to have emerged, or to be emerging now.

If fields planted before April 13 are not emerging, we need to find out why, and to find out soon. Andrew Bowman sent a photo from Knox County on April 28 showing seedlings with the symptoms we usually associate with "chilling injury," a rather rare phenomenon we see when seeds take up water with temperatures in the 30s or low 40s. This causes physiological damage, with symptoms often including corkscrewing and proliferation of roots, including adventitious roots that arise from the mesocotyl very near the seed. Such seedlings often fail to emerge, either leafing out underground or using up reserves and dying before they find their way to the surface. In severe cases, chilling injury may necessitate replanting.

Others have noticed unevenness of emergence, with some plants emerging while others remain up to an inch below the surface. If this has a pattern, with slower emergence in lower areas, it could be due to differences in soil temperature and aeration. There could also be differences in seedling diseases, and perhaps some insect injury, though seed treatments should reduce the incidence of these.

If the unevenness seems to be random down the row and seedlings appear to be healthy, the only explanation I can offer is that this results from small differences in temperature and perhaps in planting depth. The situation might be related to the presence of residue or to small differences in soil conditions, though planting conditions were very good in most fields and we would not expect a lot of that. Any such differences, though, would be greatly magnified this year, with all of the cool GDD days we have had. (There have been only three days since April 15 with GDD in double digits.) Air temperatures translate imperfectly into soil temperatures, and when GDD numbers are low, soil temperatures may lag GDD more than usual.

What about the crop that was planted the first week of April and is now up but growing very slowly? The 200 to 250 GDD we have accumulated since planting during the first week of April should translate to a crop at stage V1 or V2. This is about where we find the crop; the March 31 planting here at Urbana is just reaching V2 on May 4. Cool temperatures--the daily low temperature at Urbana has averaged 43 degrees since mid-April--and a lack of sunshine have the plants looking pale, and leaves are rather narrow as a result of slow growth. But the stand is good, and there are no obvious deformities.

It is not possible to say with any confidence whether the early-planted crop has suffered physiological damage that will limit its yield potential. When we have seen lower yields following early planting, it has often been when warm temperatures though V2 or V3 were followed by a stretch of cool weather at V3 or V4. This year, there has been little warm weather since these plants emerged. While that's not exactly comforting, I remain optimistic that a return to temperatures in the 70s (and nights in the 50s) would do a lot to return these plants to normal, with little lingering effect on yield potential. The start of ear formation remains some weeks away, and if conditions are good then we can hope that the plants won't remember what they've been through up to now.

The low temperatures in April also mean that the growth advantage of early-planted corn will be less than normal; that is, corn planted the first half of May will not be as far behind corn planted the first half of April as would normally be true. For perspective, average GDD accumulations for the earliest-planted corn will be only about 250 GDD more than corn planted now. While 250 GDD is more than we've accumulated during the past month, it is only about two weeks' worth by late May, and only about 10 days' worth when we get to midsummer.--Emerson Nafziger

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