As the corn starts growing, we’re approaching the time where we need to think about whether supplemental N will be necessary. While anyone who applied N in the fall probably lost a lot from the warm, wet weather during late fall and early spring, spring applications should be pretty well-protected due to lower than normal rainfall so far. Weather data from Houston, Fillmore, and Mower counties (found here, courtesy of the Root River Field-to-Stream Partnership) shows that while growing degree days so far this season are right around average, most of southeastern Minnesota is several inches below average on rainfall. Most rain events so far have been less than half an inch in most locations, making it unlikely that much leaching has occurred.
As of now, it looks like supplemental N should not be necessary for those who applied their nitrogen in the spring. I’ll keep my fingers crossed that it stays that way. Nonetheless, since they are for research purposes, we will still be taking presidedress nitrate tests at our U of M nitrogen rate response trials here in southeastern Minnesota. This test isn’t perfect under Minnesota conditions, but we do know that when nitrate levels are above 20 ppm, you won't get a yield response to added N. That’s the main value of the test. Below 20 ppm is still hard to interpret- hopefully having more data on presidedress nitrate tests from rate response trials will help us refine this tool.
A more valuable tool at this point in time is the Supplemental Nitrogen Worksheet, found here or as an online calculator here. It’s good at helping you think through whether you had the right weather conditions to cause large nitrogen losses, and whether your corn looks nitrogen deficient. If necessary, the best time to sidedress is right around V6.