No. 12 Article 5/June 25, 2010

Can Flooded Corn Be Salvaged?

After a fast start to the corn-growing season and the return to warm temperatures in May, we should be watching a great corn crop developing in Illinois fields. It's not a bad crop--the latest report indicates that 69% is in good or excellent condition, down only slightly from the low 70s during the past month. But we've had to downgrade our dreams to some extent.

The rains that began in May have continued in many areas of Illinois, and they have gone well past the amounts required to be considered excessive in some areas. Parts of the state have received more than 20 inches of rainfall since May 1, and there is still more to come in some places. The latest forecasts are for dry weather to return during the last week of June, but some similar recent forecasts haven't materialized.

Such excessive amounts of rainfall have led to real concerns about nitrogen loss. Some fields have not received all of the N they were scheduled to get, and some have had little or no herbicide applied. Attempts to apply N or herbicide by ground have been stymied by wet soils, and in some places aerial applications are underway. I had one report of glyphosate application over the top of corn 4 to 5 feet tall, with the weeds nearly as tall as the corn. Not only has there been a considerable loss of yield when weeds compete for this long, but corn plants this size are also in danger of yield loss from glyphosate, which can sometimes affect developing ears.

We've gotten a lot of questions about attempting to revive corn that has been swamped with excessive rainfall. These questions often concern the low parts of fields, where plants have spent way too much time with their root systems (if not their tops) submerged, or at least in saturated soils. Where it's been wet the longest, plants that are still alive in the low spots have been severely stunted, often to one-fourth or less of the height of plants in the better-drained parts of the field. In fields that have not received intended N amounts after planting, the color of the whole field may be light green, especially in corn following corn. But in many such fields, plant heights have not been reduced as much by N deficiency as from standing in water, so height differences will still be present, even if color differences are less pronounced.

We all understand that N deficiency, either from lack of application or from excessive N loss, needs to be corrected for the crop to have much chance to produce good yields. Fabián Fernández has written about this in the Bulletin, and here I'll only address the question of yield loss briefly. Corn that is head-high has typically taken up about half of its N, and total uptake is about 1.2 lb N per bushel of grain yield if pollination is successful. The amount of N available from the soil, which should be substantial in a year like this with warm soils, has been reduced by high rainfall, both through leaching and, where soils have been saturated, by denitrification.

If a field has not received the full rate of N fertilizer and has been pale since plants were small, then growth rates have been reduced and some yield potential has been lost. We can use plant height and leaf color as indicators of possible yield loss; small, pale plants at V10 to V12 are unlikely to yield much more than half to two-thirds what they might have with adequate N from the start. Plants that are pale but have reached close to full height with fairly good stalk diameter have better potential, and getting N to them quickly might prevent serious yield loss.

Corn will respond to added N right up to the time of pollination, but how much N should be applied this late if the yield potential has been reduced? Answering this question involves a fair amount of guessing, but in general we would not want to apply more than 90 to 100 lb of N in late vegetative stages if plant size is reduced and leaves are small and pale. Such plants are simply not going to be able to turn green and healthy in time to produce good stalks, leaves, and ears.

If plants are larger and pale, this indicates that they had N early but ran out, in which case the same 90 to 100 lb N should restore them to good productivity, especially if root systems are good. Plants that seem normal except for color slightly paler than normal might need even less N to restore productivity. They need more N than would be applied in fertilizer, but you can count on some N being available from the soil organic matter once soils dry out some. Plant growth will speed the drying of the soil, as long as it stops raining at some point.

What about those plants left standing in the mud, with stunted growth and very light leaf color, shortened internodes, and symptoms of deficiency not only of N but of other nutrients as well? Besides being much shorter, such plants are usually behind others in the same field in their development, sometimes by three leaf stages or more. Internode lengths have usually been shortened as well, and once internodes are "set," with hardened (lignified) cell walls, no more cell expansion is possible, meaning that these internodes will grow no more. It's the same story with leaves; small leaves will stay small once two or so leaf collars appear above them.

If we could magically dry up soils and get N to such stunted plants, we could green up the existing leaves and increase growth rates of the rest of the plant, restoring some yield potential if the developing ears have not been compromised too badly. Our competition removal studies have shown that ear size can increase when a plant gets more light and space into late vegetative growth stages, so restoring color will help some. But having a healthy plant add more kernels late in vegetative growth is not the same as doing this with sickly, injured plants. The prognosis for stunted, yellow plants that are standing in water today is not very good.

The overwhelming deficiency suffered by plants that are standing in wet or saturated soils is the lack of oxygen in the roots, not of nitrogen in the plant. It is simply impossible to cure N deficiency while oxygen deficiency exists. Attempts to do so using aerial applications of foliar forms of N or dry forms such as urea cannot succeed until the water goes away and the roots start to take up oxygen. In other words, roots do more than just take up nutrients like N; they produce plant hormones, they grow into the soil to reach more nutrients, and they anchor the plant. They do none of these things, or none of them well, when they are sitting in saturated soils unable to take up oxygen. In addition, they release carbon dioxide, which builds up in saturated soils and can poison the roots.

In the lowest parts of fields, in the wettest areas, and where N has not been applied, plants that are severely stunted now and standing in water that will be slow to go away, applying N is likely to produce little benefit. If such areas are only small parts of whole fields that still need N, applying N to the whole field makes sense, and it should be done as soon as possible. If large areas in the field are stunted and standing in water, it might make sense to wait to apply N in these areas until the water is gone and plants start to show signs of greening up. If the rest of the field has started to pollinate by then, the flooded plants will be much reduced in yield potential, and applying N may not produce much return.

I do not recall ever having seen plants with severe stunting and nutrient deficiency from standing in saturated soils come back to produce much yield. That's not proof that it can't happen, but the deck is really stacked against such plants, and applying more inputs may be a demonstration of more hope than is justified.--Emerson Nafziger

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