Fields that usually get nitrogen applied prior to planting and incorporated may have been planted this year without any nitrogen in order to speed up the process. Post planting N application of UAN (28%) is a good option in these situations but there are a few things to consider. Nitrogen application using UAN will cause some corn leaf burn but the impact on yield is usually negligible. Guidelines for post-emerge UAN application:
- UAN (fan nozzle) and herbicide – max 2 leaf corn
- UAN (fan nozzle) no herbicide – max 3 leaf corn,
- UAN (streamer nozzle) – max 6 leaf corn.
Broadcast applications of urea, done when corn leaves are dry, will have minimal burn but may tend to have more nitrogen volatilization losses than UAN if rainfall does not come within 7-10 days.
None of these techniques may be quite as N efficient as true injected sidedressing, but they get the job done quicker and if a rain comes soon after application concern over N loss disappears. Remember that if corn is at or beyond the 3-4 leaf stage and has received neither weed control nor nitrogen, weed control must be the priority.
On the subject of UAN sidedress injection we did conduct some trials in 2010 where we contrasted the amount of N volatilization from UAN that was properly covered with soil at about 3 inches deep to an injector that left the UAN uncovered. The results indicated much more N loss than expected if the uncovered UAN was left in the bottom of the slot and did not receive rain for two weeks after application. The message from the trials was clear: take the time to ensure that your sidedresser is doing a good job of injecting and covering the UAN.
Finally, some producers will be worried about how much of their N (applied prior to planting) has been lost because of the abundant May rainfall. Indications are that for Ontario conditions the loss of N is a much greater risk on the heavy soils that sit saturated and N is lost through denitricfication than it is from the lighter soils that drain more rapidly. Growers wishing to evaluate the N status of their fields can use the soil nitrate test to give them some guidance as to the need for additional nitrogen. See the Agronomy guide for more details on soil nitrate testing.
Figure 1. UAN Burn Damage on Corn