Late-season nitrogen application: Is it worth it?
Some farmers wonder about nitrogen applications for corn late in the season, even after the tassel emerges, as a way to increase yield. The basic thinking is that the crop still needs to take up half of its total nitrogen at this point in the growing season. The truth is, in locations with a shorter growing season like Minnesota, we have no evidence to suggest that post-tassel nitrogen applications provide a yield benefit.
Further, recent research evaluating nitrogen application timing and its effect on different soil types across Minnesota showed that, except for irrigated sandy soils, applications around the V12 development stage never produced better yields than a pre-plant only or pre-plant plus sidedress application by V8.
In some situations, the yield with the later application was similar, but in some fields, a late-season nitrogen application actually ended up reducing corn yield compared to earlier applications. This yield reduction was likely the result of either the yield potential of the crop being compromised due to insufficient nitrogen earlier in the growing season or the late sidedress nitrogen did not get to the roots because of a lack of moisture.
The only situation where a post-tassel application before R1 may be beneficial to increase yield is if the crop is severely limited by a lack of nitrogen. In those cases, however, one should carefully evaluate the potential return on investment, as the nitrogen application cannot make up for lost yield potential.
That said, there was generally low potential for nitrogen loss during the spring this year, so if an adequate amount of nitrogen was applied earlier in the season, additional nitrogen applied now will likely result in no yield benefit.
Applying nitrogen now will likely result in the crop not showing yellowing (firing) of the lower leaves until later in the reproductive stages. Having dark green lower leaves as the crop progresses through reproductive stages is generally a sign of over-application of N.
While it might be visually satisfying to have a dark green crop from top to bottom, having green lower leaves later in the season is not advantageous because those leaves transpire and use plant resources but conduct very little photosynthesis since they are mostly shaded by the upper canopy.
Lower leaves yellowing or even falling off as the crop progresses through the reproductive stages can actually be beneficial because nitrogen stored in those lower leaves moves to the grain. On the other hand, if the leaves are yellowing all the way to the ear or the whole plant is getting yellow that would be an indication of insufficient nitrogen.
However, as mentioned earlier, if an adequate amount of nitrogen was applied earlier in the season, it is highly unlikely that a deficiency is going to develop this year since nitrogen loss potential was low.
Fernandez is an associate professor at the University of Minnesota whose research and extension education programs are focused on soil nutrient management and plant mineral nutrition.