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Issue No. 7, Article 10/May 6, 2005

The Good, the Bad, and the Unsightly

Dr. Bob Nielsen at Purdue used the phrase "refrigerated conditions" to describe the crop growing conditions over the past 2 weeks. I think it's an appropriate term. While we raised a number of issues about cool conditions in the Bulletin last week, we did not then anticipate that the cold weather would stick around as long as it has, nor that we would get some of the lowest temperatures during the first week of May. To best describe how "upside down" the past month has been, we accumulated 257 growing degree-days from April 1 to 20 at Urbana, and 52 GDDs from April 21 through May 3. April GDDs actually ended up above average, but the setbacks to the emerged crop probably eliminated most of the anticipated advantage to early planting this year.

There are positives and negatives to this situation, though the negatives outweigh the positives. Let's look at what we can expect for the 2005 corn crop.

The Good

  • Most emerged stands I have seen are good, reflecting the very good soil conditions under which the crop was planted. It took less than 2 weeks for the crop to emerge in most areas if it was planted before April 17 or so, but corn planted since then is just beginning to emerge in some areas.
  • Most of the crop in emerged fields is showing relatively uniform growth. This indicates that care was taken to avoid tillage-related or fertilizer or seed placement problems, and also that seed quality was good.
  • We had 82% of the corn crop in Illinois planted by May 1, which is even ahead of the rapid pace in 2004. No other Corn Belt state (unless we count Missouri) is even close to that figure, and the U.S. crop is only 52% planted. With limited rainfall this week, corn planting should be nearly finished by this weekend.
  • In the northwestern part of Illinois, where temperatures were lowest, only 11% of the corn had emerged by May 1, compared to 30% statewide. Emerged corn is more vulnerable to cold injury.
  • Most reports I have been getting indicate that plants survived and are likely to grow back once warm weather returns.
  • Because of currently low soil temperatures and setbacks to early-planted corn, corn replanted within the next 4 or 5 days will not be very far behind early-planted corn in its development or yield potential.
  • Nitrogen should be staying in place, with cold soils meaning slow conversion from ammonium to nitrate.

The Bad

  • The earliest-planted corn has experienced considerable loss of leaf area, which has set back its development and negated much of the early-planting advantage we had hoped for. In most cases, it's probably better to have a root system and a regrowing plant compared to seed just planted, but we have had to reinforce that early planting does carry a risk and that this risk is real.
  • Some fields and parts of some fields may have plants that were killed by low temperatures or so badly injured that they will not grow back to produce normal plants. [See the note on replanting near the end of this article.] The March 30 planting here at Urbana looked better on the morning of May 3 than it does this morning (May 4); the calm winds and temperatures in the upper 20s on May 4 seem to have added more injury to plants that had lost leaf area but that otherwise seemed ready to grow back.
  • The possibility that cold temperatures during the growth of plants before stage V3 (3 leaf collars) might reduce yield potential of plants remains unclear, but we do not foresee rapid development of the high May temperatures credited with getting the crop off to such a great start in 2004. Is the other side of this a "slow start means lower yield potential" problem? We simply don't know, though some contend that "cold corn early" may translate into shorter plants and even lower final leaf number. A compromise in canopy cover usually means lower yield potential.
  • While I do not anticipate the need for extensive replanting, replant "baggage" such as hybrid choice and availability, insecticide use, weed control, and killing off the old stand will be issues, sometimes without clear or low-cost solutions.
  • Nitrogen mineralization rates are slowed by low soil temperatures, and along with reduced crop growth and uptake rates, this could mean development of N deficiency symptoms over the next 2 to 3 weeks.

The Unsightly

  • Almost without exception, the corn that has emerged has ghastly color for a crop, ranging from very pale yellow to yellow-green to "dead leaf brown." While unsightly, pale yellow tissue is at least alive, and it may even restore to "photosynthetic green" once we get warmer temperatures for 2 days or so.
  • Plants that have lost much of their leaf area to cold temperatures will continue to look bad until new leaf tissue can grow back to replace the dead tissue.
  • As we discussed last week, uneven effects of temperature down the row may compromise crop uniformity. As with other unsightly consequences of low temperatures, a week of warm weather will appear to "cure" some of this, though we have to be concerned that such a cure may, in the case of uneven plant size, be more cosmetic than real.

It should be possible by the end of this week (May 7-8) to tell whether or not plants survived the low temperatures and are starting to grow back. Prospects of survival are lower for plants that had about two leaves exposed (V1 to V2) than for older or younger plants and are lower for plants that showed some regrowth after the frost of April 23-24 before being frosted again on May 3-4.

If you think that a replant decision is necessary, I suggest that you use the Replant Decision Aid at the Web site http://www.aces.uiuc.edu/iah/index.php?ch=ch2/replant.html.--Emerson Nafziger

Emerson Nafziger

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