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How Much Is Dryness Hurting Corn and Soybean?

July 20, 2001
As of July 15, more than 60% of the corn crop in Illinois was silking, and that number will move up quickly with the warm weather, probably to near 90% by this weekend. Even though rainfall since the crop was planted continues to be close to or even above normal in many areas, the recent lack of moisture is starting to take a toll on yield potential in many cornfields. This may not be very obvious at this pointmost fields are showing silks at about the proper time compared to tassel emergence, and the earlier hybrids have made it reasonably well through the pollination process.

But in lighter soils, areas with little or no rain for the past month, or where plant populations are high, the number of kernels per ear is likely to be reduced. The warm nights and insects that feed on silks aren't helping the situation. We're not talking about 1988-style disaster here (the drought that year started much earlier, and soil water supplies were badly depleted by mid-July), but it's safe to say that many fields will not have kernel numbers or the kernel size potential that we saw last year. Even those areas that have received rain are likely to have lower kernel set due to the temperatures this year; even though cool temperatures last week were quite favorable in decreasing transpiration and night respiration, they did not persist long enough. I expect kernel abortion to be higher due to warm nights this week than if it had stayed cooler.

One indicator of how much dryness has affected the crop is plant height. In areas where there has been little lack of moisture in the past 6 weeks, plants are at least as tall as, and maybe taller than, average. Plants that are shorter than average by silking show evidence of lack of water during rapid stem elongation, which begins when plants are about 18 inches tall. Where on the stem dryness limited growth can be seen by looking at the internodes to see which ones are shortened. Shortened internodes at 3 or 4 feet above the soil surface are probably not a problem, but if the uppermost internode is short, and especially if the tassel emerged slowly or did not completely clear the flag leaf, then the plant is signaling that it is short of water as silking begins. Unless it has rained, such fields are likely candidates for decreased kernel set.

For those anxious about kernel set, the number of developing kernels can be checked by the time silks are dried and the ear has started to increase in size. Strip back the husks carefully, and count kernel rows and the number of kernels per row that are starting to increase in size. It will often be possible to see kernels on the end of the ear (or other places on the ear, especially if insects ate silks) that are not developing; many of these are probably aborted, meaning that they were pollinated, but failed to develop. As a general guideline, fewer than 35 to 40 kernels per row probably means less than full kernel set when plant populations are 30,000 per acre or less.

How well kernels fill will, of course, depend on filling conditions during the next 6 weeks. Some idea of potential for this may be gained by looking down row middles when the sun is high. If almost all of the sunlight is being intercepted and it is dark on the soil surface, potential for fill is high. Things that decrease canopy completeness and grain-filling potential include low populations; hail damage; insect and disease damage (disease damage can limit leaf function without decreasing light interception); and lack of water either before pollination, causing leaves to be small, or after pollination, causing leaves to curl or lower leaves to fire due to loss of nitrogen.

The soybean crop has also been affected by dryness, though it has a better chance for recovery than does corn that has pollinated. The most obvious symptom of dryness effects so far is the width of the canopy. If plants are not covering the row middles in 30-inch rows by now, then they may never have a complete canopy and yield potential will be lost. Remember that the height will increase and the canopy will continue to develop for another few weeks, so we can't yet assess the canopy. In our soybean planting date study here, we had some serious Japanese beetle damage until we sprayed the field earlier this week. If we get rain and the canopy continues to develop, the leaf area loss from this insect feeding may not have much effect, but if it continues to be dry, yield potential may be decreased by this damage.--Emerson Nafziger

Author: Emerson Nafziger

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