No. 13 Article 9/July 2, 2010

Corn Roots Under Stress

Last week I noted concern about the effects of flooding on corn and whether the crop could come back with good yields. Crop ratings dropped only slightly from last week, and the crop in many areas remains in good shape. The percentage of the crop that is silking jumped to 15%, following above-average growing degree-day accumulations. Our plots planted on April 5 at Urbana reached silking late last week, after accumulating about 1,350 GDD.

Though we normally expect corn in mid- to late vegetative stages to be able to withstand a week or so of flooding, the corn that stood in water in mid-June suffered greater and faster decline than I had expected. The photo here was taken near Champaign, after heavy rainfall the previous night. In the center of the ponded area the corn had died earlier or was submerged. The corn near the edges of the pond, though, clearly died after reaching a height of 4 feet or so, so fairly soon after water accumulated. It's not difficult to find areas in fields that are not flooded now but where the crop died in the past two weeks.


Plants near Champaign following rapid death in standing water.

There are several reasons why the crop died so quickly after growing well into June. The fact that plants look like they "dried up" is because they did: as I have said before, roots without oxygen can do very little, including taking up water for more than a few days. High temperatures meant high demand for water and hastened the demise. Finally, the warm water standing in the low spots carried little dissolved oxygen, so the roots failed quickly.

It's likely that roots of plants that did not stand in water also suffered some damage from saturated soils, even if soils were very wet for only a day or two. The soil compaction that we have had since last fall, and that we added to this spring, likely restricted roots to an unknown extent, but probably less than did lack of oxygen. As a result, we expect that the root system of the crop is shallower, less extensive, and less active than we would like. This is at a time when the plant needs all of the resources (sugars) it can muster to complete vegetative growth and begin the pollination process. This lack of additional sugars will reduce the amount of new root growth.

During this recovery process, we need to let the crop tell us how it's doing. If it regains and maintains upright, fresh-looking leaves, this means that it's taking up water well; if its leaf color deepens, then the plant is taking up the nitrogen it needs. Such processes obviously need good roots, so we'll be anxious to see if the plant can juggle its needs for roots, tassels, silks, and the last additions of top growth, all with a more or less fixed supply of photosynthesis and the sugars it produces.

Highs dropping into the upper 70s and lower 80s this week will greatly assist corn in its recovery from excessive wetness and what may be less-than-ideal root systems. Low night temperatures reduce the loss of sugars to respiration, and lower daytime temperatures will reduce water loss rates slightly, while bright sunshine means higher rates of photosynthesis (which increases water use).

As soils continue to dry out, helped greatly by water uptake by roots, roots will be recharged with oxygen, and they might grow some more if there is enough sugar to go around. Roots usually reach their maximum size at about silking, though, so we can't expect a lot of additional root growth. This means that our best hope is that it not stop raining for very long over the next two months.--Emerson Nafziger

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