No. 7 Article 13/May 6, 2005

Some Corn Stands Struggle

The warmer temperatures of the past four or five days (through May 8) have helped the corn crop recover from the cold temperatures of the first few days of May. I checked the corn planted on March 30 in our planting date study at Urbana this morning (May 9), and the percentage of plants that failed to grow back from the low temperatures of May 3 and 4 was higher than I had expected. At least 30 percent of the plants are dead in the plot area that we planted on that date. While the growing point may still look like it is alive, the absence of any new leaf tissue by now (about 70 growing degree days post-frost) means that these plants will not grow back.

There was some effect of position in the field on survival, with plants next to grass borders and in slightly lower places in the field less likely to survive, due either to slightly less growth when the frost occurred or slight differences in wind speed during the lowest temperatures. There is less unevenness than I expected, mostly because those plants that were more badly damaged died and will not grow back to experience competition. Still, at that level of loss, replanting would be called for. We will make the third planting in the study this week, so will have a direct comparison between keeping and replacing the damaged stand.

As I indicated previously, most farm fields seemed to have survived the low temperatures better than the corn planted in late March in our trials. Even so, if early-planted fields were checked just after the frost last week and the growing point was alive but most or all of the leaf tissue was dead, the assumption that these plants would grow back might not be accurate. Such fields need to be revisited immediately and a count taken of plants that are adding new, green leaf area. This count will likely approximate the final stand if the field is not replanted, and so can be used in making a replant decision.

Why would plants with living, protected growing points not grow back? One reason is that such plants had very little reserve from which to regrow, especially after they were frosted for the second time in a week. Most of the remaining seed contents were lost to microbes that invaded the seeds after the plant stopped drawing on the reserves stored there. The regrowth of green leaf area after the first frost thus came mostly at the expense of stored reserves in the crown of the plant. The second frost event occurred before the new leaf area had produced much, and plants simply did not have reserves to grow new leaf area for the second time, even if the growing point was still intact.

Another important reason why some plants failed to grow back was infection by seedling diseases, which invade plants more easily after they have been weakened by leaf loss and tissue death.

While most seeds planted the last two weeks of April are still intact and trying to emerge, soil crusting has turned into a real problem in some fields, and emergence is slow and uneven as a result. Rotary hoeing is helpful but is often not a cure-all, especially when soils are hard enough to resist penetration by the rotary hoe itself. A second rotary hoeing might help emergence, but it will also do some damage to emerged and emerging seedlings, so check carefully to see if the net effect is positive. A good shower would help, but heavy rain could reduce emergence of more recently planted corn and soybean.

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