Does Corn Still Need Rescuing?

Jul 24, 2015
The 2015 Illinois corn crop continues to develop on schedule, with 75 percent of the crop having reached silking by July 19. But the crop condition rating continues its steady downward trend, with the good plus excellent percentage now at 55 percent, down from its high of 79 percent at the end of May, said a University of Illinois crop scientist.
“Virtually all of this decline is due to standing water, past or present,” said Emerson Nafziger. “Rainfall frequency and amount has moderated some, but parts of western Illinois have received more than 6 inches so far in July.”
Nafziger added that growing degree accumulations since May 1 are running close to average for the whole state. “With planting a little ahead of normal this year, we can expect the crop to reach maturity beginning in early September or even late August,” he said.
Soil samples taken at tasseling at several sites from a nitrogen tracking project (funded by the Illinois Nutrient Research and Education Council) show that soil nitrogen levels have fallen considerably in recent weeks to levels at or below 50 lb nitrogen per acre in the top two feet of soil.
For the crop that is well past pollination, the number of kernels developing per ear is an indicator of maximum yield potential. According to Nafziger, crops with badly damaged root systems have, in most cases been showing pale or yellow leaves, death of lower leaves, and stunting, and either show no ears at all or have very low numbers of kernels.
“Their yield potential is low and can’t be improved,” he said.
In some cases, Nafziger said that the crop is only now starting to show loss of green color or yellowing, including lower leaf firing that may be moving up the plant. Some of this is occurring in parts of Illinois that have started to dry out where crops are expected to start recovering. This still seems to be occurring mostly in lower parts of fields, where water may have stood temporarily, perhaps several weeks ago, he noted.
“It seems logical to think that nitrogen deficiency developing now in fields where standing water hasn’t been a big concern is due to loss of nitrogen finally showing up as the plant continues to take up nitrogen,” Nafziger said. “There certainly might be some of that happening.”
Another possibility, he added, is that limitations of the root system from earlier damage are only now showing up, especially as surface soils start to dry and roots start to draw water from a little deeper in the soil.
Can we know which is the case, and is there anything we can still do about it?
“Either of the situations might mean the crop could respond to nitrogen applied to the surface, as long as the nitrogen can get into the plant. Before spending the equivalent of about 10 bushels of corn to apply urea by air, though, it would be good to try to assess the yield potential and to try to figure out if the crop is in good enough condition with enough kernels to make a return on the investment likely,” Nafziger said. “There’s no sure indicator of the potential of the crop to respond, but as a start, it should have most leaves still intact and around 450 to 500 kernels developing per ear.”
If a decision is made to apply more nitrogen to a field, the rate should be restricted to no more than 40-45 lb and the nitrogen should be applied as soon as possible, he added. “Protecting urea with Agrotain should help reduce volatility, but if urea stays on the surface long enough (without enough rain to move it into the soil), it is not going to get into the plant to do much good anyway.”
Foliar nitrogen is another possible source of nitrogen. The lower rate of nitrogen applied (usually 10 lb per acre or less) might be enough to carry the crop until its roots can take up more nitrogen from the soil, Nafziger explained. “With any source, the idea is to keep deficiency from advancing. The amount of green leaf area is closely tied to grainfilling rate. Dribbling UAN could work, but getting it into tall corn without delay in fields with wet spots might be difficult.”
Even if temporary nitrogen deficiency can be relieved by supplemental nitrogen applied this late, Nafziger added that nitrogen will not prove to be the most limiting factor in most fields. “Yield prospects will depend mostly on the ability of the root system to continue to take up water, and along with it some nitrogen, through the next month to 6 weeks,” he said. “Some of the developing nitrogen deficiency that we are seeing now may be the result of root systems that are damaged more than we think. In that case, even moderate stress as surface soils dry out might send plants into a downward spiral.
“There’s no good diagnostic for root system intactness, but pulling a few plants might provide a hint. If root systems seem small, shallow, or have darkened areas and dead root tips, the prognosis isn’t great,” he added.
“Things are looking better in parts of the state or fields where water hasn’t stood recently and where roots are getting some oxygen. Aside from the downpours, the weather continues to be good, without high temperatures so far. More low-humidity days would help, but humid days with sunshine are still helping the crop,” Nafziger said. “It’s not 2014, when temperatures this time of year were well below normal. But kernel set should at least be normal in undamaged parts of fields. That means they have the potential for good yields at least
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