While the return to warmer temperatures has the existing corn crop looking much better this week, we have received some rather unexpected reports that frost damage resulted in extensive plant death in some areas and that replanting will be needed in some of these fields. The problem appears not to be direct freeze injury to the growing point but rather the loss of leaf area on early-planted corn (mostly planted during the mid-April warm spell) and the inability of the crop to grow back. We think that the unusually cool, wet weather and soils both slowed regrowth and stimulated attack on the remaining plant tissue by diseases. As a quick assessment, if the plant is not showing green regrowth by 3 or 4 days after the damage, it will probably not survive. If you have fields that were in the 2- or 3-leaf collar stage during the period of overnight temperatures in the low 30s, you will want to check those fields as soon as possible.|
For guidelines and more detailed information on replant decisions for corn, please check the Illinois Agronomy Handbook, either the paper version or the Web-based version at http://web.aces.uiuc.edu/aim/IAH/ch2/replant.html. The calculator in the Web-based version will help with the replanting decision. Replanting costs can be entered, along with existing stand and original planting date. As we all know, replanting this late means a serious yield penalty from late planting--existing stands become "more valuable" the later it gets. Using the calculator for 170-bushel yield potential fields and minimal replanting costs of $10.00 per acre indicates that existing stands of corn planted in late April would need to be less than 12,000 per acre to make June 1 replanting break even.
On the side favoring replanting, however, is the fact that predicted replant returns are based on healthy surviving plants, not on plants that have had their yield potential affected by disease or other ongoing damage. We don't have a good way to estimate this, but a plant that has lost its leaf area and is growing back very slowly will not yield up to its potential. Perhaps one approach would be to count such plants as "partial" plants (say, 3/4 or 1/2 a plant, depending on how "sick" the plant looks) when a damaged stand is being counted. To count stands, the measuring wheel method described in the Illinois Agronomy Handbook works well, or use the time-tested 1,000th acre (17'5" in 30-inch rows) counts, making sure to take enough of these to assess the stand adequately.
Wheat Crop Comment
While the extensive rainfall has hammered the wheat crop this spring, the crop seems to be holding its own, with diseases likely present but not spreading very rapidly due to cool temperatures. Heading date ranged from about normal in southern Illinois to later than normal in central and northern parts of the state. As a rule of thumb, we use 6 weeks from heading to predicted harvest date. If it's much less than that, it is usually because high temperatures cut short the filling period and usually the yield, as well. Warm temperatures will help diseases to spread rapidly as long as it stays wet. Standing water also has taken its toll on the crop. If we get some drier weather with some sunshine, the crop can fill quite rapidly and quite well. But the maximum filling period lasts only about 4 weeks under average temperatures, so every day of poor weather in the next 3 to 4 weeks will take a bite out of yield.--Emerson Nafziger