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Venturing Into the Fields

July 26, 2002
Crop conditions have improved considerably over the past 2 weeks in those fields that received rain; but many fields didn't receive rain, and conditions continue to deteriorate. As of July 21, only 32% of the corn crop in Illinois was listed as being in good condition and only 3% as in excellent condition, making this one of the lowest-rated crops for this time of year that we have seen recently. Rains early this week were not nearly as widespread as we had hoped they would be, but they might move the ratings up a bit.

Producers vary considerably in how much they want to know about individual fields when crop conditions are generally not very good. Those who want to sell crop on weather rallies often need to have some idea about how much they will have to sell. On the other end of the spectrum, we can't generally do anything to help fields with weather-related problems, so some people choose to stay out of such fields often (I suspect) so that they can remain optimistic that there might be more there to harvest than they had imagined. From that standpoint, those who think their crop is in good shape may have more to learn (on the downside, unfortunately) than those who are pretty sure their crop is in trouble and want to wait to know if their optimism, faint as it might be, is misplaced.

If you are one of those who likes (or whose job it is) to go to the field, this is a good time to look. Stand, plant height, leaf color, and foliar diseases are things that are easy to observe and that often have a lot of influence on eventual yield. This is also a good time to start getting a handle on the success of the pollination process. Silking was listed as having occurred in 59% of fields as of July 21, which is well behind the 83% in 2001 but less than 10 percentage points behind the 5-year average of 68%. Growing degree-day accumulations suggest that all corn planted before late May should be silking by now; in general, silking occurs at about 50% of the seasonal requirement for GDD in early hybrids, up to about 60% of the seasonal requirements for late-maturing hybrids. For late-planted corn, the reduction in GDD required to reach maturity comes mostly in a reduction in GDD after, not before, silking. As expected, a reduction in GDD during grain filling translates into lower yield. If a field is not yet silking even though it has accumulated enough GDD, then it is likely that dry soils are restricting growth.

A considerable amount of plant variability exists in size and emergence time this year, and this will show up as variability in silking date. Variable water availability can also affect sillking time. Because viable pollen needs to fall on silks in order to initiate the fertilization process, the nick (degree of synchrony between pollen shed and silk appearance) is critical, especially this year. We have had reports this year of both early silk appearance (which we do not expect to be a problem in most fields) and early pollen shed. In some fields with early pollen shed, silks had not appeared by the time pollen shed had ended. One sure way to have no yield from a field is to have pollen no longer available once silks emerge. It is almost certain that some fields will be like this in Illinois in 2002. In most fields, probably some "straggler" tassels will shed pollen long after most other plants have shed all of theirs, and this will help fertilize some late-emerging silks. Some pollen usually blows into fields as well, so total barrenness is rare unless it's so dry that silks never emerge. This may also be a year when the practice of planting two hybrids in strips across the field pays off by extending the pollination period.

If silking took place several weeks ago and silks have turned brown, it is usually possible to check the success of pollination by removing husks and pulling gently on any still-attached silks. If they remain attached to their kernel, then fertilization has not taken place. By the time silks turn brown, those kernels that were fertilized and that avoided abortion (which almost always occurs during dry weather and affects kernels on the tip of the ear) are starting to increase in size, and it may be possible to count kernels per ear. Once we can count "likely" kernels (they can still abort into the late milk stage), then we can make yield estimates, though the sooner we do these after pollination the less accurate they tend to be.

To make a corn yield estimate, count the number of ears (only those that have set more than a few dozen kernels) in 1/1000 of an acre (17 feet, 5 inches in 30-inch rows; 522.72 divided by actual row spacing for feet of row at other row spacings). Count kernels (number of rows times kernels per row) on three ears, using every fifth ear down the row to avoid ear size bias. Calculate the average kernel number per ear, then multiply this times the number of ears you counted to get estimated number of kernels in this 1/1000 of an acre. Normal expected kernel size is 90,000 per bushel, in which case we divide kernels per 1/1000 of an acre by 90, to give expected bushels per acre (the thousands cancel out). If reason exists (and there usually isn't one until at least a month after pollination, by which time we might be able to judge how grain filling is proceeding), we can use a different number from 90, down to perhaps 80 or less, if we think kernels will be larger than normal, or up to 100 or above if we think kernels will turn out smaller than normal.

To use a handy calculator for yield estimation in corn, see the Web-based Illinois Agronomy Handbook at http://web.aces.uiuc.edu/aim/IAH/ch2/ecy.html.

Soybean fields are also suffering from lack of water in many areas, with the primary consequence that the plants are short. In wider rows, the crop is also not filling the canopy very well. Most fields have flowered by now, but under very dry conditions early flowers may produce low pod set. Most fields still have the ability to respond to increased moisture supply by making more growth, including adding nodes, flowers, and pods. That ability will decline sharply once leaves start to lose their green color, which may be happening in some sandy soils where there has been little rain. August weather will determine the fortunes of this year's soybean crop, as usual, but with greater-than-usual sensitivity given the need to make up for decreased growth from dry conditions in many areas.

Wheat variety test results from the 2002 University of Illinois trials are available at http://www.cropsci.uiuc.edu.--Emerson D. Nafziger

Author: Emerson Nafziger


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