What's Ahead for the Corn Crop?

July 7, 2000
In most areas of Illinois, rainfall has been adequate to allow the corn crop to approach pollination in good shape, with enough stored soil moisture to carry the crop through pollination and into the start of grainfilling. Although moderate temperatures the past 2 weeks have slowed the race toward pollination a little bit, many fields in the central part of the state are fully into the silking stage, and some have probably completed the pollination process.

Most fields that tasseled by July 1 were either planted in the first half of April or were planted to hybrids earlier than average in maturity. Growing degree-day accumulations since early April have been about average for most parts of the state. At Urbana, the midseason hybrid we planted on March 10 and again on March 23 reached 50% silking about July 1, followed by the April 6 planting about July 3; the April 26 planting is close to full silking. An earlier hybrid planted March 23 silked about June 28. The latest official estimate indicates that 11% of the state's corn crop had silked by July 2, compared to an average of 2% on that date. This number will increase considerably in the next week, especially if temperatures reach into the 90s, which they have not done much up to this point. Pollination should end this year in many fields by about the time it would usually start.

We normally say that tassels appear and start to shed pollen, then silks appear about 2 days after pollen shedding begins. This year, due perhaps to the unusually early start to pollination, and maybe to the influence of day length on the flowering process (corn does respond some to day length but less than soybean), we are noticing that silking is beginning as early as pollen shedding or even a day or two earlier. In some fields I have seen, silks have emerged even before the tassel is fully emerged. In such cases, silks may reach 3 or 4 inches in length before pollen shedding gets under way.

Pollen grains can land and germinate anywhere along the exposed length of silk, so even where the silks predate pollen, the fertilization process should take place without a hitch. After fertilization, silks stop growing and eventually turn brown. The only concern when silks emerge before pollen shedding might be when insects are feeding heavily on silks, thus eating away the part of the silk with pollen grains attached before the pollen germinates and sends genetic material down the silk to the kernel. Pollen can land on a silk and initiate fertilization after the silk has been clipped, but constant eating off of silks can interrupt this process and result in unfertilized embryos, which means loss of kernels and yield. Because conditions are good for silk growth and pollen shed this year, it would likely take fairly high insect pressure to affect success of the pollination and fertilization sequence.

Because the crop needs to have high rates of photosynthesis during this critical period, general leaf health and color need to remain good. We can't do much about cloudiness, which reduces photosynthetic rates directly, but the rain, or at least the lower rates of water loss, that result from cloudiness is a positive that may outweigh the lack of bright sunshine in many cases. In other words, if it's raining in July, it's probably a good thing.

Several people have commented on the excellent color of the corn crop. This probably is most striking in those areas that have not had excessive rainfall and where the favorably dry winter and spring weather have resulted in little N loss. The dark green color is an additional safety net under the crop, in that it translates to maximum photosynthetic rates now, when the crop most needs energy. Where it's been very wet and where water has stood, we're seeing the crop lose its green color. Once that happens, it will be difficult for the crop to regain its color, though a period of dry weather should allow the roots to take up additional nitrogen. Unfortunately, where it's been very wet, some--perhaps much--of the N has been lost, and plants in such areas may well remain deficient through the rest of the season.

As I've said before, the early start to pollination should be an advantage for the crop the rest of the season. A sudden onset of hot, dry weather can still cause some damage, especially to those crops that haven't yet tasseled. But if grainfilling starts early and takes place during warmer weather, it will also occur more rapidly, and physiological maturity should come early. As a general rule of thumb, it takes about 55 to 60 days from silking to physiological maturity. Where silking is early, it should need fewer days because of the higher temperatures. Unfortunately, warm night temperatures during grainfill are also more likely when pollination is early, and warm nights tend to reduce the weight of kernels. An ideal situation would be for temperatures to stay moderate for July and August and to average 2 or 3 degrees below normal for both months.--Emerson Nafziger

Author: Emerson Nafziger

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