No. 16 Article 5/July 10, 2009

Pollination 2009

The NASS report this week indicates that 5% of the corn in Illinois was silking by Sunday, July 5. That's not a fast start to pollination--the 5-year average by that date is 31%--but it's a little bit ahead of 2008. By this date in 2008, 63% of the Illinois corn crop was rated as good or excellent, while this year that number is only 57%. Soil moisture conditions this year are better than they were in 2008, with only 3% rated as deficient (short or very short) this year, compared to 15% in 2008.

As we know, rainfall was plentiful in July 2008, and the crop condition improved steadily, to more than 70% good or excellent by the end of July. We've had a good start to plentiful July rainfall in most of Illinois this year, so we can hope for a similar recovery. One difference between 2008 and 2009, however, is the obvious, and in some cases serious, compaction problems created during tillage and planting this year, as well as excessive rainfall before and after emergence. Visible tractor tire patterns remain in many fields, along with low areas that have yellow, stunted corn. Good weather is not likely to fully correct these problems.

In our planting date study here at Urbana, the earlier (110-day) hybrid planted on April 9 reached R1 (silking) on July 3. The same hybrid planted on April 26 reached silking on about July 7. We received rain on July 4 and again on July 7, so soil moisture is not limiting in these fields now. Nor is it likely to be for the next two weeks, during which much of the crop in Illinois will reach silking.

As we have seen in several other recent years, silks of many of the hybrids we grow today tend to emerge early and grow aggressively. While we have normally described the pollination process as one in which silks emerge a day or two after pollen shed begins, it is clear that this period can be as short as zero, and in some cases silks are beginning to emerge slightly before the first pollen is shed from the same plant.

Fresh silk growth in late afternoon. Much of the length of silk likely emerged during this day.

Tassels emerging, with some pollen shedding under way.

This early silk emergence is likely a result of selection in newer hybrids for reduced "silking interval," the period between the time pollen shed begins and first silks emerge. Silking interval has been identified as a factor in stress tolerance of hybrids, particularly tolerance to inadequate water. Silk tissue is especially sensitive to lack of adequate water in the plant, and dry conditions can mean that silks are delayed by several days. Pollen shed is less sensitive to lack of water, and it might even be speeded up when conditions are dry, especially when temperatures are high. In cases of serious drought, pollen shedding can end before the first silks appear.

So it stands to reason that selection for the ability to pollinate under stress means earlier appearance of silks. It is not clear if silks are simply less affected by low water availability--that is, if they can "draw" water better--or if there is something else at work. But when we have good pollination conditions, as we have had the past three years, silking can seem early and aggressive.

As far as we can tell, there is no drawback to this early and rapid growth of silks. Silks don't have much dry weight so don't "cost" the plant very much to produce. Rapid silk growth may provide enough material to dilute the effect of insect feeding, and rapid growth means that silks that are eaten off by an insect should recover more quickly. Pollen grains can land on, and germinate on, any part of the length of the silk, so as long as some intact silk is exposed when pollen grains land, the pollination process should proceed normally.

While we do gain considerable advantage in higher kernel number from normal to above-normal rainfall in July, it can bring some related problems that detract to some extent. June was relatively cloudy, and continued cloudiness associated with rainfall means less photosynthesis and less sugars in the plant to fuel the processes of pollination and kernel set. Wetness also can encourage leaf disease development, though we are seeing little of that so far. On balance, though, when we say that "rain makes grain" we usually refer to rain in July. This year, we hope rain continues into August, since the late planting of the crop will mean pollination later than normal in many fields.--Emerson Nafziger

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