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Issue No. 7, Article 8/May 7, 2004

Waiting for Warmth

More than 80% of the state's corn crop was planted during April this year, and there is a running start to soybean planting in Illinois. Temperatures for the week ending May 4 haven't risen to the point where growing degree-days (GDDs) are accumulating as fast as we would like, however. With the 50°F cutoff that we use for "modified" GDD calculations, daily high temperatures less than 60 mean accumulation of less than 5 GDDs per day. Expect corn and soybean plant development to proceed very slowly until daytime highs reach into the 70s.

A more immediate concern with low temperature in the past week has been frost. Nighttime lows reached into the upper 20s and lower 30s in many northern Illinois locations on May 2 and 3. In lower parts of the topography, where colder air can drain on a still night, night temperatures were probably several degrees lower than recorded temperatures. It is likely that some emerged corn in parts of fields lost leaf area as a result, though most of the planted crop in the area with the lowest temperatures did not yet have much leaf area to be killed or injured.

As I pointed out last week, losing leaf tissue on very small plants shouldn't have much effect, as long as belowground parts of the plant do not freeze. Plants that were just starting to spike through will likely show some discoloration on the tip of their first leaf or two. Any rain that fell before the low night temperatures was helpful, since wet soils hold heat better. Soils that are warmer than the air radiate to the leaves at night, helping to protect them from their own loss of heat by radiation.

There were few soybean fields emerged and susceptible to the cold nights in the past few days, so this isn't much of an issue for soybean at this point. It is possible that cotyledons of soybean can freeze just before they emerge from the soil, but that would require temperatures below 30 for a number of hours, especially if the soil is moist. Emerging soybean plants, however, can be even more susceptible to plant injury and death from frost than corn is. Their susceptibility lasts only 2 days or so, from emergence of the cotyledons to after the start of growth of the growing point, which is visible as the tiny leaf cluster that first unfolds from the cotyledons. This growing tissue toughens up quickly, but temperature of 32°F can kill the young plant if it's in just this vulnerable stage. We seldom see such injury, both because the soybean crop is (properly) planted later than corn and because the window of vulnerability is so small. But we need to keep in mind that such injury is possible.

For either corn or soybean, the only sure signal that the plant has survived freezing temperatures is the appearance of new green leaf tissue. As long as daytime high temperatures stay in the 50s, recovery will be slow, and we may have to wait 3 or 4 days to see much new tissue. If temperatures return to the 70s, we should see a fairly rapid green-up, though the soils have now cooled to the point where low soil temperature might slow growth to some extent.

Now that we have had enough GDDs to result in corn emergence in most areas, concerns are developing about nonuniform stands in some fields. This does not seem to be across the board, as stands in many fields are uniform and stand counts are good. Several factors can result in uneven stands in early-planted corn, though no single factor stands out as the cause of widespread and serious stand problems, at least so far this year. The responsible factors include insect injury; planting into soils with variable moisture, including some too dry; rodent damage; mechanically damaged seed; and possible effects of some seed treatments (though these have not really been documented). Dr. Bob Nielsen at Purdue has also discussed the "corkscrew" shape that some seedlings can exhibit, which may result in poor or uneven emergence. This type of injury may also result in plants of differing ability to grow and compete early in the season. We know from earlier work that uneven competition between adjoining plants is almost certain to cost yield, with extra yield from the better competitors seldom compensating for the lower yields from the plants that lose the competitive battle early in the season.

Does any of this suggest that we might have rushed planting a bit this year? Not at all; such ups and downs are normal at the start of the growing season. Serious frost damage comes from low temperatures between mid-May and mid-June, when the plant is more susceptible and replanting is less of an option. With cool, cloudy weather, the color of the crop is not great. It's good to know that this problem is self-correcting; once we have temperatures warm enough to support growth, leaf color will quickly return to a healthy green, and we'll be off and running. GDD accumulation in May, which averages about 420 at Urbana but has varied from 300 to more than 600 over the past 15 years, represents less that 15% of the seasonal GDDs, and generally shows little correlation with final corn yield. If we manage to get crops up uniformly and can keep them intact for the next few weeks, we'll have done what we could to get the crop started.

The wheat crop continues to benefit from the cool weather as heading progresses up through the wheat-growing area of southwestern Illinois. Dry weather during flowering should largely prevent Fusarium head blight (scab) infection, and few other diseases have been reported. The only concern seems to be the "tied-in" heads in some fields, caused by the tip-awns failing to pull free from the flag leaf sheath as heads emerge. Twisting of leaves has also been reported in some fields. Cool weather during head emergence often results in "crooked" heads, and some varieties tend to show this more, probably because of differences in head morphology. Growth-regulator herbicides can also contribute, especially if they were applied a little late. There can be some loss in kernel number from failure of flowering on the inside of the crook in the head, but this is partly compensated for by larger kernels on the rest of the head. Wheat curl mite and growth-regulator herbicides can contribute to leaf curling, but we have often seen this phenomenon when there was no apparent cause. Some varieties tend to show more leaf twisting than others. We don't know whether the twisting of leaves causes yield loss, but it probably does if light interception after heading is never complete as a result.

The rule of thumb that it takes 6 weeks from heading to harvest needs to be used with caution; the cool temperatures during June last year extended the filling period by several days and contributed greatly to the outstanding yields in Illinois. Doublecrop soybean yields were also very good in most areas, reflecting the favorable rainfall that came late in the season. So, while most double-croppers hope for early wheat harvest, it often comes at the expense of lower wheat yield.--Emerson D. Nafziger

Emerson Nafziger

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