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Crunch Time for Corn

June 29, 2001
The latest crop condition report indicates that the Illinois corn crop is almost as tall as the 2000 crop was at this date, with 96% of the crop rated as being in fair-to-excellent condition. The crop is well ahead of average development for this date, similar to last year's. Tassels are starting to appear in some early-planted fields, especially those with early hybrids. There are differences between the two crop years, though, and we might consider what if anything threatens the current crop, using last year's crop for comparison.

Pollination was excellent in 2000, resulting in the highest kernel numbers per acre we have ever had. June rainfall was above average in most parts of Illinois in 2000; in contrast, June has been relatively dry this year in many areas. While we normally think that dry June weather tends to favor root development, recent days have also brought some midafternoon leaf curling in fields in eastern Illinois, indicating that water is inadequate to maintain leaf function. Curled leaves do not photosynthesize well because the area exposed to sunlight is reduced and the stomata, tiny holes in the leaf that allow carbon dioxide to enter, close when leaves are stressed by inadequate water. Typically, leaves relax at night when the atmospheric demand decreases under higher relative humidity. Stomata open in the morning, and the leaves function well until demand for water by the dry air (low relative humidity) around the leaves exceeds the rate at which the roots can take water up.

The severity of the drought effect as soils dry can be estimated by noting when the leaves start to curl; the earlier in the day this happens, the more deficient the soil water is. The key question is how much effect drying soils and plant water deficits have on yield potential. Clearly, the beneficial effect of moderate dryness on root development starts to disappear when plants start to show drought symptoms, with likely limitations to yields accelerating quickly the closer to pollination that leaf curling (think: loss of photosynthesis) persists. Contrasting with the drought symptoms is the fact that dry weather usually brings a lot of sunshine, so when the leaves are functioning, photosynthetic rates tend to be high. Leaf color is also good in most fields, which also means high photosynthetic capacity when water is adequate.

The actual effect of dryness is likely seen mostly through its effect on sugar level in the plant, which is difficult to measure. We can, however, probably expect kernel number to start to drop, probably by one or two percentage points for each day on which early-afternoon drought symptoms occur, starting about 10 days before pollination. More severe water shortages will limit kernel number even more. This is probably an oversimplification, in that return to adequate soil moisture can return plant status to normal quickly, especially early in the prepollination period. But a drop in leaf function due to water shortage is a serious problem when the crop approaches pollen shed.

Under normal conditions, tassels will emerge and start to shed pollen soon after they have cleared the last leaf sheath. First silks should appear at about the same time as the first pollen is shed. Pollen shed usually occurs in the morning after the dew starts to dry off and is usually completed for the day by mid- to late morning. There are a few clues to tell you that dryness is affecting growth, and probably yield potential, during this period. Plants stressed by dryness tend to grow more slowly, and tassels may take an extra day or two to emerge fully. They might also start to shed pollen (indicated by the exserted anthers) before they have fully emerged. A more serious indicator of dryness effects is delayed appearance of silks. Silks grow at night when plant water status is highest, and under ideal conditions new silks should have appeared to receive pollen as it is shed in the morning. If pollen seems to be shedding from several places on the tassel and no silks have yet emerged, pollination might not be completed before pollen shed ends.

Growing degree-day accumulations since planting have been near normal in many parts of the state, and where corn was planted early, it will pollinate relatively early. As indicated above, pollination has already begun in some fields. We expect the peak pollination period to be the first and second weeks of July. Even though we need sunlight to drive photosynthesis during this period, the crop needs water even more. Thus the success of pollination will be closely tied to soil water levels over the next 2 weeks.--Emerson Nafziger

Author: Emerson Nafziger


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