Can a late N topdressing help wheat yields this year?
K-State has tested many different types of foliar N fertilizer products over the years. Foliar N fertilizer products are just as effective as traditional N fertilizers on a pound-for-pound basis, but they are not more effective than traditional N fertilizers. They can be applied in a broadcast spray application at later growth stages of wheat growth than traditional N fertilizer products without damaging the wheat.
One of the reasons the foliar products have not been found to be more effective than traditional soil application is that only a small portion of the N applied as a foliar application to wheat actually moves into the plant through the leaf tissue. An excellent study done in Canada a few years ago found that when care was taken to prevent foliar applied N from reaching the soil, only 8-12% of the applied N was recovered by the plant, compared to 35 to 70% of soil applied N being taken up by the plant. Thus it is very likely that many foliar applied fertilizers are actually taken up through the roots once they wash off the plant.
Invariably, the foliar products will be higher in terms of cost-per-pound-of-N than the traditional N fertilizers. In unusual situations (well after jointing or when trying to increase protein levels), the foliar N products would have some premium value since traditional N products could not safely be used in a broadcast spray application.
The bottom line
The bottom line is, there are limited opportunities to increase wheat yields in most fields this late from N applications.
The last opportunity for improving yield is maximizing grain fill with larger berries. However, current research by K-State has shown that flag leaf emergence is the last growth stage that has marginal reliability for getting yield and protein responses to N applications in Kansas.
Foliar N products can be used for later applications, but the limited amounts of N which can be applied based on the labels of many of these foliar products limits their use in situations where large amounts of N are needed, and the potential for yield increase is limited physiologically.
The potential for yield response to an N application at this point in the season is very low. Additional N applications now will likely result in reduced profit per acre.