Now soils are drying and crops are growing very fast as they rapidly accumulate lots of growing-degree days. The rapid growth of the crop requires making it a priority to apply N as soon as possible to any areas that did not receive sufficient N or that lost some of the N that was applied.
It is common to see light-green corn next to dark-green corn that is also further along in development. If N has been applied, the light-green corn is typically found in areas where water ponded.
In areas where there is adequate N, often-waterlogged soils induce symptoms resembling N deficiency. For the most part, those symptoms should have disappeared. If they persist after about a week from the time soils dry, you can be pretty certain that the crop needs more N.
Whether N-deficient areas represent many acres or just spotty zones, the question occupying farmers' minds is how to fix the problem and how to do it most efficiently.
One of the most important things to realize is that this late in the season, crops showing N deficiency have already lost some yield potential, and applying a full N rate cannot recover that lost potential. In other words, the corn crop will not be capable of using the full rate to make yield.
Another important point is that the sooner you apply N, the better response you are likely to see. This year across Illinois, corn is all over in terms of development stages. While some fields are still at early vegetative stages, others are rapidly approaching reproductive stages.
If your field fits the latter case, remember that you are very likely to obtain a yield response by applying N until tasseling. Studies have shown that even until silking, corn has a great capacity to use N and produce an increase in yield if the application is done in severely N-deficient fields.
Also keep in mind that areas needing N at this time are typically patchy, so targeted applications, rather than even applications across the field, are fundamental to reducing cost, increasing return on investment, and minimizing potential loss of N to the environment.
One way to identify the location of trouble spots is with aerial photography or observation from a vantage point above the canopy. In most fields, the corn crop is reaching heights that would make it too difficult to find problem areas by "walking the field." Aerial photographs can be converted into variable N-rate maps to guide the application.
Canopy-sensing technology can also be used to guide application rates. Research has shown that canopy sensing is most useful when plants are bigger (around V10 stage) and obviously N-deficient.
When plants are smaller, the sensor often captures too much soil area relative to leaf area and can overestimate how much N is needed. If you are using canopy-sensing technology, make sure the equipment is properly calibrated to an N-sufficient zone of the field.
High-clearance equipment is likely the only way to make the rescue N application in some fields; fortunately, it is becoming more widely available.
For rescue N, I suggest that the best alternative is between-row applications of dribbled or injected UAN solutions or urea plus a urease inhibitor such as NBPT (Agrotain).
Another option is broadcast urea with a urease inhibitor. The urease inhibitor is important to reduce the potential for volatilization losses when the product sits on the soil surface until it is incorporated by water.
I do not recommend broadcast application of UAN because of the high probability of canopy injury. Also, a slow-release product (polymer-coated urea) is not a good choice at this point because you want N to be available to the crop immediately. While the coating can protect urea from volatilization (just like a urease inhibitor does), it will take time for the coating to break down and release N, further delaying availability to the crop.
Regardless of the N source, any product that is surface-applied will require water to move it into the root system so that the plant can use the applied N, so applying ahead of rain is a good approach.
I would not apply a foliar product simply because the amount of N that can be applied is often very low and the cost per acre too high to make that profitable.