Does Corn Still Need Rescuing?

The 2015 Illinois corn crop continues to develop on schedule, with 75% of the crop having reached silking by July 19. But the crop condition rating continues its steady downward trend, with the good + excellent percentage now at 55%, down from its high of 79% at the end of May. Virtually all of this decline is due to standing water, past or present.

Rainfall frequency and amount has moderated some, but parts of western Illinois have received more than 6 inches so far in July. 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.

Soil samples pulled at tasseling at several sites in our NREC-funded N-tracking project show that soil N levels have fallen considerably in recent weeks, to levels at or below 50 lb N per acre in the top two feet. We’re finding that soil N levels tend to go down to 40 to 50 lb and not much below that. That’s in part because roots don’t “find” all of the N that’s there, and in part because the mineralization process continues to release N into the soil. When soil N levels are this low, we’re finding much of the N to be in the ammonium form, which is the form produced by mineralization.

For the crop that is well past pollination, the number of kernels developing per ear is an indicator of maximum yield potential. 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.

In some cases the crop is only now starting to show loss of green coloryellowing, 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, so where we expected the crop to be starting to recover. This still seems to be occurring mostly in lower parts of fields, where water may have stood temporarily, perhaps several weeks ago.
It seems logical to think that N deficiency developing now in fields where standing water hasn’t been a big concern is due to loss of N finally showing up as the plant continues to take up N. There certainly might be some of that happening. Another possibility is that limitations of the root system from earlier damage are only now showing up, especially as the 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 N applied to the surface, as long as the N 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. 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 N to a field, the rate should be restricted to no more than 40-45 lb and the N should be applied as soon as possible. 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 N (urea-formaldehyde solution) is another possible source of N. The lower rate of N applied (usually 10 lb per acre or less) might be enough to carry the crop until its roots can take up more N from the soil. 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 N deficiency can be relieved by supplemental N applied this late, N will not prove to be the most limiting factor; yield prospects will depend mostly on the ability of the root system to continue to take up water, and along with it some N, through the next month to 6 weeks. Some of the developing N 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.

Things are looking better in parts of the state or fields where water hasn’t stood recently and where roots are getting some oxygen. Outside 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. It’s not 2014, when temperatures this time of year were well below normal. But pollination and kernel set should at least be normal in undamaged parts of fields. Let me know if you see something different.