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Issue No. 14, Article 11/June 29, 2007

Crop Conditions and What to Watch as Corn Enters Pollination

The return of rainfall to most of Illinois during the past week has provided great benefit to the crop, and it means that you can forget nearly everything I have written about dry soil effects over the past month--except the part about dry weather in June sometimes being an advantage; we think it will turn out to have been a boost as well, given the lack of disease and the generally good crop color. The corn crop has come back well from its period of leaf curling and nonproductive days, though it's too early to know if, in areas that have been dry for a month or more, there might be lower yields as a result. Even with the dry weather, though, the crop is farther along in its development than normal for the end of June. Our corn planted on April 2 here at Urbana was fully in stage R1 (silking) on the morning of June 27, and that planted on April 20 is fully tasseled (stage VT), with a few silks showing. Plants with silks are showing them in abundance and with good length, which is a very positive consequence of the recent rainfall.

There have been some reports that plants are shorter than normal as they approach pollination, with concern that such shortened plants might not be able to yield as well as they might have without such stress. We know that there are some high-yielding hybrids that tend not to grow very tall, but there is also a general idea that tall corn means high yields. I think that the key linking all of this is not how tall the plants end up, but rather how complete the canopy ends up being. Shorter plants have some tendency to have less-complete canopies, in part because shorter internodes mean less ability to have leaves "flex" to cover the row middles.

But the biggest potential problem associated with shorter plants is that the same water deficit that limited stalk growth might also have limited leaf area expansion. Corn does not have an overabundance of leaf area--its leaf area index (LAI) at full canopy is usually only about 4, meaning that there are four acres of leaf for each acre of ground, or about 5.8 square feet of leaf per plant at 30,000 plants per acre. That's enough if the leaves are oriented so that light passing by one is captured by the leaf below or by a leaf from a neighboring plant. But reduced leaf expansion means a lower LAI, and that often leads to a decrease in the ability of the canopy to intercept all of the light. It is possible to have stalks shortened without much loss in leaf area, but because these processes are happening at the same time, both are usually reduced when soils are dry.

The crop canopy refers to both leaf area and how the leaf area is oriented. The quality of the canopy when leaves are healthy is best measured as the percentage of light falling on a field that is intercepted by leaves. Under drought stress in recent weeks, when leaves were curled up, most of the light was reaching the ground, meaning that the canopy was not very good, even where the LAI of larger plants was between 2 and 3. The LAI reaches its maximum at about the time of silking, when the top leaf is fully expanded. This should also be the time of maximum light interception, which is important because it's also the time when photosynthetic rates need to be at a maximum in order to feed sugars into the plant so it can successfully complete the pollination process.

Looking at light interception at or just after the time of pollination can thus provide a good idea of the potential of the crop to produce top yields. Providing that leaf area is healthy, yield will often reflect directly the percentage of light that the canopy intercepted during grainfill. Very good canopies can intercept as much as 98% of sunlight in the visible range that falls on the field (radiation outside the visible range is not used by the crop for photosynthesis). The best way to judge this is to look down the rows at about noon, when the sun is high, and see how much sunlight is hitting the ground. A "prime" canopy will let almost no light through, so if you'd need a flashlight to read a newspaper beneath the canopy on a sunny day in late July, you have a very high-quality canopy.

Most of the sunlight that passes through canopies will end up as "sun flecks," or little patches of bright light on the soil surface. Leaves that are light-colored due to nitrogen deficiency do not very well absorb the light that falls on them, so they will appear light underneath even if most of the light hits leaves. On the other hand, badly diseased leaves intercept light but don't use it very well. It is thus not always easy to assess canopy quality with great accuracy, but just looking down rows will usually be enough to show how well the canopy is doing. If the pattern shows a streak of sunlight down the row middles, then there's a good chance that the rows are too far apart to allow for full yield, at least for that hybrid. When light spots are numerous and distributed more uniformly across the row, this often means that plant population is not as high as it should be. Plants with leaves made smaller by drought stress would show a similar effect.

With the rainfall has come great improvement in the prospect for successful pollination, and there's little need to go into problems to watch for with regard to normal plant development. The synchrony of pollen shed and silk appearance is shown by how early silks appear after the tassel has appeared (two to three days is good), and based on this the crop is in great shape to pollinate with few problems. A return to slightly cooler temperatures during pollination, with highs in the 80s, will be favorable as well, especially if night temperatures fall into the uppers 50s. More sunshine will also mean more photosynthesis and will be favorable as well.

One problem that a number of people have noted is the uneven plant height in some fields. Some of this started with uneven emergence due to differences in soil condition and moisture at planting. The problem was made worse by differences in root growth and water availability caused by varying degrees of soil compaction and changes in soil texture across the field. Whether this will be a problem for pollination will depend on how uniform the growth stages are, as opposed to differences in size among plants that emerged at about the same time. Those that emerged at different times have retained the difference in development, and unless those differences were evened out by dry conditions (that is, older plants were held back more), these are likely to pollinate at different times. Plants that emerged at the same time but then grew under different amounts of water stress might stay different in size but might pollinate more or less at the same time.

There isn't anything to do to prevent different parts of the field from pollinating at different times, but you need to watch such fields very closely to make sure that insect damage doesn't further compromise pollination. Insects like Japanese beetles and corn rootworm adults move readily from less attractive to more attractive parts of fields, and so can sometimes do more damage, over a longer period, in uneven fields than in uniform fields. In addition, because those plants that are developing late have more stress from the competition from older plants near them, they will have greater difficulty in pollinating successfully. They will also, depending on how much larger their neighbors are, have difficulty filling grain if they do pollinate successfully.

Many soybean plants in central and southern Illinois are now showing flowers and are getting a boost from the rain. An early start to flowering is preferable to a late start, but the success of the flowering period, measured by the number of pods that set, will be determined mostly by weather conditions over the next month. A regular supply of water will help, and average temperatures, perhaps a little above average, will be favorable as well. Unless July turns out to be wet, we should be able to avoid the excessive growth and root injury that we saw in 2003.--Emerson Nafziger

Emerson Nafziger

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