No. 3/April 11, 1997
Cold Temperatures and Corn
Planting got under way in March in some areas in Illinois, and some 1% of the crop has been planted by April 7. More thousands of acres have been planted in the past 2 days. With temperatures reaching 20°F or lower on April 9, many who have planted corn are wondering if they made a mistake. We covered the pros and cons of early planting last week; but it might be good to remind ourselves that, even if a decision on management doesn't turn out well in a given year, it does not mean that the decision was a bad one.
Whether or not corn has been injured by low temperatures this week depends on when it was planted. Dry corn seed--before it has had the chance to take on water and start the germination process--is in little danger, though its progress toward germination and emergence has been slowed. The corn most in danger is that planted more than a week ago, and that has had time and temperatures resulting in its having taken up water. Dry soils have lower heat-storing capacity than wetter soils, so their temperatures fluctuate readily and may have reached the freezing point at seed depth by dawn, April 9. If so, these seeds may be killed outright. If seeds have had enough time to germinate, the chance of freeze injury increases. If corn was planted 4 or 5 days before the freeze and has taken up only a little water, it might survive a brief time at near-freezing temperatures.
As you might guess, the only way to know whether corn seeds have survived is to dig them up. If they've started to soften, and if any emerged root is limp and discolored, the seed is probably dead. If seeds have taken on water but look normal, bringing them into a warm place and wrapping them in moist paper towels for 3 or 4 days should show whether or not they're still alive.
If you dig up seeds, be sure that you take samples from a number of different areas in the field. Cold air settles in lower parts of the field, but higher parts of the field may be drier, and thus subject to faster soil cooling. There is just no way to predict this, so samples should be taken in all parts of the field.
If a survey shows that most seeds have been killed, then replanting will be necessary. One consideration will be whether or not to try destroying any of the first-planted seeds that might emerge. There are several considerations. First, it may be difficult to destroy viable seeds and seedlings without extra tillage, which most operators would prefer not to do. It is not possible to kill seedlings with herbicide before they've emerged, and not very practical to do so if they're emerged but very small. Neither tillage nor planting operations will destroy all viable seeds. We do know that a significant percent of plants emerging in the same rows but a week or more earlier than the rest of the plants might reduce yields; and, in the areas of the field where many of the early planted seeds emerge alongside the replanted ones, final plant populations could get to be higher than the optimal range.
Thus, while our preference would be to have none of the early planted seeds emerge after replanting, this will be difficult to do if fields are replanted early. Remember that the risk of too-low plant populations is generally greater than that of having populations too high, so regardless of what we do to try to destroy early planted seeds and seedlings, it is probably better to assume little or no emergence of early planted seeds than to replant with the idea that a certain percent of early plants will "fill in" replanted stands.
Emerson D. Nafziger, Department of Crop Sciences (217)333-4424