By Ed Lentz and Laura Lindsey
Producers are tempted to apply nitrogen (N) in February because it is easy to drive across the field and there is no competition with other farm activities.
However, research has shown that this may be a costly operation in lost nutrients and a potential environmental concern (nutrients may leave the field and move into streams and other waterways).
A three year OSU study showed that when N is lost from a pre-greenup application (February), yields will be significantly reduced to where they are only slightly better than areas that receive no N. The outcome was the same regardless of the N source.
In this same study, when nitrogen loss occurred on the pre-greenup applications, urea ammonium nitrate (28% solution) did not “burn” into the soil, ammonium sulfate did not “stabilize” into the soil, and urea did not stay put until a rain.
All three of the sources moved off the plots with water as the soil thawed out, much like losses from manure on frozen ground. Even though polymer-coated products were not tested in this study, it would be expected that these types of N products would also be lost in a similar fashion under the same conditions.
As with phosphorus, the 4R nutrient management philosophy also applies for N. The right time to topdress spring N on wheat is between greenup and early stem elongation (Feekes Growth Stage 6). Rapid uptake of N by wheat generally does not begin until the latter part of April (early stem elongation).
Depending upon the source, the risk of N loss increases for every week that applications are made prior to early stem elongation. Since fields may be unfit for application at early stem elongation, a practical compromise is to topdress N any time fields are suitable for application after initial greenup to early stem elongation.
Research has shown that significant yield losses generally do not occur from delayed topdress until about late stem elongation to boot stage (Feekes Growth Stage 8 to 9). Boot stage refers to when the wheat head is fully developed and can be easily seen in the swollen section of the leaf sheath below the flag leaf.
A producer may get away with applying N in February on wheat. However university data has not shown a yield advantage for February applications, but results have shown a major N loss and yield reduction from pre-greenup applications. Why take the risk, just wait until greenup; the wheat does not need most of the N until April and May anyway.