Here's what Emerson has to say.

https://farmdocdaily.illinois.edu/2019/06/dealing-with-very-late-planting.html

Q: How will yields respond to very late (June) planting?

A: We don’t have recent data on corn planted after early June, but in a recent paper from Iowa State [Baum et al., Agronomy Journal 111:303-313 (2018)], researchers reported yield losses of about 25% (55 bushels) by June 10, 40% (88 bushels) by June 20, and 61% (133 bushels) by June 30. So planting corn on the last insurable date (June 25) would be expected to produce about half the normal yield. There’s a good chance that yields could be better than that, but for now that’s the best guess we have.

Q: What do we do about nitrogen fertilizer for corn?

A: This is one of the really tough challenges, with such a wet May following a wet fall with limited N application and limited ability to apply N this spring. As with other things, we’re in uncharted waters here: we have done a lot of N research over the past five years, but have had no conditions like this, and so we need to do a lot of guessing, of which only some will be “well-educated.”

If a producer was “lucky” enough to get be able to apply N last fall, how much is still in the soil to be available for this year’s crop, once it’s planted? We can be sure that nearly all of the N applied last fall or before May this spring is in the nitrate form by now, whether or not N-Serve was used. Nitrate moves with water, and with so much rainfall and no chance for the soil to dry to halt movement, there’s a good chance that most of the N has moved down into the soil, well below where the crop needs it to be once growth begins. While some of this N has likely reached the lines by now, we would expect that most of it is still in the soil, and capable of moving back up towards the roots zone when (if) water stops moving down. Where water has stood in fields for a week or more, we can expect that, at least by the time the soils there dry out, as much as half of it may be gone, lost to leaching and to denitrification. Some of the most persistent water holes won’t likely be planted, which will make the need for additional N there moot.

On the positive side, with soil temperatures in the 60s and 70s now, mineralization has kicked in, and this will be an important source of N as crop growth gets underway. That takes some of the urgency away from having to apply N right at planting. Even with the soil N supply kicking in now, it will be important to get fertilizer N on by the time growth gets underway, in order to avoid deficiency. Broadcast UAN used as herbicide carrier or N applied with the planter will boost the short-term N supply and postpone the need to apply in-season N. It will help a great deal if soils dry out at some point in order to stop the downward movement of N and to allow the roots to grow and take up N, and the plants to take on the darker green color that says they are not lacking in N. This won’t happen until the water coming into the plant carries with it more N than it is carrying today.

Any corn planted will likely benefit from some additional N, regardless of what’s been applied up to now. With so much water moving into and through the soil, we can’t count on full availability to the crop even of N applied a month ago in preparation for planting that didn’t happen then. How much N we should apply once the corn crop is planted, including how much to credit already-applied N, are tough questions.

If we applied 180 lb N in the fall following soybean harvest, let’s guess that 150 of it is still in the soil, and we’ll further guess (hope) that rainfall will slacken enough that, at some point before tasseling, soils will dry enough so that roots can take up some of this 150 lb of N. In such a case, we should still consider applying 10 gallons of UAN (about 30 lb N) at or after planting; if not before emergence, then dribbled near the row soon after emergence. The idea is to sustain the crop as it establishes itself, with additional N coming from mineralization until the roots begin to reach and take up the N from the fall application.

If less than the full amount of N planned for application was applied in the fall or early spring, then we might similarly discount that amount—by a fourth to a third—when determining how much more to apply. If corn was planted in mid-May with only 25 or 30 lb N applied then, we should apply the rest of the N as soon as we can reasonably do so in order to ensure a good N supply as growth gets underway. How we apply this N may not matter much, as long as we get it on in a way that keeps it relatively safe from immediate loss.

Although many may be shying away from anhydrous ammonia due to the need to apply before soils dry out, remember that ammonia injection provides better retention of applied N than does any other form or method of application. The addition of stabilizer to ammonia for in-season application is not recommended. If ammonia can be applied before the crop is more than a few inches tall, injecting it closer to the row than 15 inches can help improve access by the roots to the N. Once the crop has more than 4 leaves, application should be moved to the row middle to avoid damage to the roots.

If the main source of fertilizer N will be UAN, it should be applied using method other than surface broadcast. That means shallow injection; broadcast followed by incorporation by tillage; or dribbled, streamed, or surface-banded (all mean more or less the same thing) on the soil surface. The idea with streaming UAN is to concentrate the application so that some of the UAN moves down into the soil, where any ammonia released from urea (by urease enzyme) is captured by dissolving in soil water. Even so, applying a band of UAN in the surface leaves some of it without much protection from loss. Using a urease inhibitor may limit volatilization some, but when surface soils are warm, this won’t prevent all loss. Injection is a safer method, and dribbling UAN near the row, especially once the crop has 4 or more leaves, can get the N into the crop more quickly than if the UAN is midway between the rows. If rainfall continues, the concern will shift from urea volatilization loss to movement of the N deeper in the soil.

Broadcast urea can also be a good source of N, with the advantages of rapid application and the ability to broadcast by ground or by air onto the emerged crop without damage. A disadvantage is the potential for volatilization loss, as we discussed above. Urease inhibitors like Agrotain can be used to limit such loss, but rain within a few days effectively limits losses. Controlled-release forms should be used with caution, as we are interested in having the N from urea get into the soil and to the root quickly once the crop starts to grow.

If we get lucky and rainfall returns to more normal patterns in June, we should be OK using the MRTN rates for N, which in round numbers are about 180 lb N/acre for corn following soybean in southern and central Illinois, and about 170 lb N/acre in northern Illinois, with ammonia at $600 per ton and corn at $4.00 per bushel. Rates for corn following corn are 200 to 210 lb N per acre. If June is wet like May, then we may want to go back with another 30 to 50 lb N in the two weeks before tasseling. I don’t know of a good way to be sure whether this will be needed or not, but as a general guideline, having the crop take on a dark green color in mid-vegetative stages would tend to indicate that it isn’t needed, while getting a lot of rain during June but still having good crop growth and apparent yield potential might be a signal to go ahead.