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Too Early or Too Late?--Cold Injury and Planting Date

April 27, 2001
The below-freezing temperatures on April 18 had minor effects on Illinois crops but only because the crops--especially soybean--were not emerged on very many acres. My suggestions in last week's Bulletin that soybean might escape serious injury turned out to be too optimistic. The first planting of a planting-date study went in here at Urbana on April 5 and started to emerge on April 14. By the morning of April 18, some plants had their cotyledons opened, and the first leaves starting to expand, while others were just emerging. Those that were just emerging were most badly damaged by the freeze; the hypocotyl hook tissue was killed on these plants, and the plants died rapidly. The most advanced plants had the tissue above the cotyledons killed, and many of those will die due to lack of buds from which to grow back. Other plants in this stage retained one or both cotyledonary buds, and these will grow back but not always normally.

Emergence of the April 5­planted soybeans was good--close to 80%. But I estimate that less than 50% of the emerged plants from this planting will survive to produce seed. We have the next planting date scheduled for this week and so should get some interesting data on the effect of "replanting." One lesson we can certainly take away is that while soybeans may be good survivors of early-season cold weather, young plants can still be killed if they happen to be at a vulnerable stage when the cold hits.

While some corn was planted in late March and early April, the only corn I have seen that was emerged a week ago is some volunteer corn. Leaves that were above the soil surface froze off in many cases, and those leaves might be gone now or the ends of the leaf might be crinkled from freeze injury if the leaf wasn't fully emerged. In this area, it does not appear that corn plants were killed by the freeze--appearance of new leaf tissue has resumed. But low-lying fields and those that were emerged a week or more before the frost should still be checked to see if the cold might have penetrated the soil enough to cause injury 3/4 to 1 inch deep.

Despite concerns a week ago about freeze injury to wheat, the crop seems to have come through the low temperatures with little or no effect. The dry weather in the southern half of the state has been positive for the crop, and it is entering boot stage in good conditions in most areas. Development of the crop is about average for this time of year; heading usually starts in the last week of April in the southern tip of the state and moves north at about 20 miles per day with average temperatures, reaching Urbana around May 10. Tiller numbers and leaf color are good, and if we get through the next 3 weeks without a lot of warm, wet weather, crop potential should be very good.

Corn planting has once again made rapid progress in April this year; the official estimate is that 19% of the state's crop had been planted by Sunday, April 22. The most rapid progress has been in central and southern Illinois, and showers have missed much of this area this week, so it is likely that many areas are more than 75% planted by now. Progress north of I-80 is still limited by wetness, but it appears that we will have more than half of the state's corn crop planted by the end of April, meaning that this will be another early planting year. By most accounts, soils are working up very wellfrom the looks of some "powdery" fields I have seen, perhaps a little too well. That should not be a problem if we don't get heavy rain and crusting before emergence. Current weather patterns suggest that we might get lucky on that score.

Because planting has been early the past few years, many people start to get nervous if their whole crop isn't planted by April 30, if not earlier. While it is true that planting much after May 1 in 2000 reduced yields considerably in many areas due to the weather pattern, on average we expect corn yield decreases due to late planting to be very modest up to May 10 or so, after which they begin to accelerate. The cold-hot-cold weather we have had in April this year has not allowed corn to grow as quickly as it did last year, and thus it is likely that early planting will not be quite as beneficial this year since corn planted in mid-April may not have much more growth a month from now than corn planted the last week of April, or even the first week of May.

With soybean, there is even less reason to worry if planting is delayed into May. We are doing more research to define the planting date response in different parts of Illinois, but for now we have little reason to think that soybeans planted by mid-May will have lower yield potential than those planted in early May. April planting might in fact be negative for soybean, depending on whether it freezes after emergence and on whether the conditions are favorable for development of diseases such as SDS.--Emerson Nafziger

Author: Emerson Nafziger

The Pest Management and Crop Development Bulletin
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