No. 17 Article 5/July 15, 2005

Crop Recovery After Rain

Some parts of northern Illinois have received little or no rainfall from the remains of Hurricane Dennis this week, while parts of southern Illinois have received 3 to 4 inches or more. Rainfall amounts have been modest in central Illinois but have generally been enough to bring at least some life back into corn and soybean crops that have been suffering from lack of adequate water for weeks.

While rain in the middle of the growing season will probably improve crop ratings somewhat, can we expect water now to undo the damage already done to the corn crop? This is an area where, as I have said before, most previous research doesn't provide very good answers. In most studies of effects of short periods of drought on yield, stress was both imposed and relieved quickly, such that growth and size of plants were generally not affected much. That is clearly not the way things are working in the field this year.

Except in areas that received extra rain in June, the corn crop in Illinois is shorter to much shorter than usual as a result of limited water during stalk growth. Reports from many areas are of corn plants 4 to 5 feet tall with tassels out (or at least partly out) and pollen shedding under way or finished. In some of the driest areas, pollen shed may be nearly complete, but ear shoots have not appeared. In other cases, ear shoots may be present, but silks are sparse or absent. In many cases, plant growth across the field is uneven due to differences in water supply.

Corn plants that have shed pollen but have not produced silks may take on better leaf color after it rains, and some may grow a bit taller, depending on how long ago they started to shed pollen. Such plants might also "push" ear shoots and silks, though this will vary depending on how long ago they were scheduled to do so by growing degree-days. If plant growth is uneven such that some tassels continue to emerge, then emerging silks might be able to receive pollen and some kernel fertilization might take place. While silks that are struggling to emerge don't seem to be very attractive to corn rootworm adults, this could change once pollen supply becomes limited and the insects look for other food. There are hints that some fields have been sprayed for corn rootworm control when in fact there were few silks out to protect. Failure to scout is costly in such cases.

The recovery scenarios described above will not return yield potential to anything near normal in fields where drought has so limited plant growth, however. Kernel numbers are almost certain to be well below the number required for high yields. At 30,000 ears per acre, each three kernels per ear translate into 1 bushel per acre if kernels fill out to average size. Plants with smaller-than-normal leaves and shorter-than-normal stems will be challenged to fill kernels completely, though we have seen remarkable yields from short plants in some cases. We expect rainfall to result in a flush of soluble plant nutrients (mostly nitrogen) in the plant, and color could improve as a result. Lodging potential is decreased for short plants but could still be a problem if stalk diameter is small, especially if enough kernels form to draw nutrients rapidly in August.

We have clearly lost corn yield potential in Illinois to date, though the extent is difficult to assess. Some fields are being written off as kernel formation fails, and more of this is likely if it stays dry in the northern areas of the state, where the crop is not quite past the stage of full pollination. My best guess is that loss of yield potential in Illinois is in the vicinity of 15% to 20%. Unless a pattern of consistent rain returns across the state soon, that loss could go higher.

With regard to the soybean crop, this rain (where it has fallen) has arrived in time to restore crop prospects to near-normal levels. As is the case in corn, though, limited rainfall at this point does not ensure that the crop is made. Soil water content is still less than normal in much of Illinois, and where rainfall this week has totaled less than an inch, the crop could be under stress again within a week. While rainfall now is increasing the rate of pod setting in soybean, there will need to be continued water supply to ensure that more flowers will produce pods, that pods will stay on the plant, and that seeds will fill. In other words, soybean is starting to enter its most critical period, in which continued (or recurrent) lack of water will increasingly limit seed numbers and yield potential.--Emerson Nafziger

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