By Ruth Beck
The excess moisture and limited available field days have made it difficult for producers to add the nitrogen they need to wheat fields this year. This could be a concern as nitrogen contributes to both yield and protein. Low protein is not a desirable selling quality for winter wheat. Even in fields that had adequate nitrogen applications, there could be concern as some nitrogen may have been leached or denitrified with heavy rains.
It may pay off this year to take tissue and soil tests from those questionable wheat fields. Knowing levels of total and nitrate N in plant tissue and what is available to the plant in the soil could help with nitrogen application decisions.
Nitrogen Application Considerations
Nitrogen applications made to wheat after tillering can still contribute to protein and yield. There are a number of options available, however it is not recommended to add nitrogen during the anthesis (flowering) period.
Stream-bar applications of liquid N sources or broadcast dry fertilizer can be effective if the plants are not too mature and rainfall moves the fertilizer into the root zone. Foliar applications, where nitrogen is applied over top of the plant and absorbed by the plant leaves, is often considered a fast way to get nitrogen into the plant. However, there is concern that foliar nitrogen application can result in burning the leaves. Reduced leaf area due to tissue burn can reduce yield potential.
In 2017, research performed at the Dakota Lakes Research Farm, compared leaf burn from nitrogen applications of urea ammonia nitrate (UAN 28%) and liquefied urea (16%). The rate of total N used in all treatments was 27 lbs. of N/acre. The nitrogen was applied to the field when the wheat was at Feekes 7-8 growth stage. Leaf burn was rated and was higher with the UAN product. However, in this study there was no significant differences in yield between treatments. This may have been because nitrogen was applied prior to flag leaf emergence (Feekes 10) and therefore the leaf burn occurred on lower leaves. All treatments that received the foliar nitrogen did have higher protein levels. The increased protein ranged from 0.4-0.9%.
Foliar N application after pollination can also increase grain protein. Multiple studies have been conducted by SDSU in the past, which concluded that 30 lbs. N/a as UAN mixed 50/50 with water and applied after pollination could effectively increase protein. If the mixture is applied during cool, dry conditions, leaf burn is minimized. This post pollination application should only be made if the planned yield goal is predicted to be exceeded. The planned yield goal is determined by past field history and following the SDSU nitrogen recommendations for wheat. The SDSU wheat nitrogen recommendations are 2.5 lbs. X field average yield, subtract soil test nitrate-N (0-2ft) and legume credits. If this plan has been followed and the wheat in the field is predicted to exceed the yield goal, the post pollination application of 30 lbs. N/a as previously discussed should increase grain protein. Yield goals can be determined by counting stems that will produce a head (or heads if the timing allows). Calculate heads per acre, assume 22 kernels per head, and 16,000 kernels per lb. and 60 lbs. /bu. (iGrow Wheat: Best Management Practices for Wheat, p. 287)
There is one difference between winter wheat fields this year and those in 2016 that may be worth taking in to consideration. In 2016, in general, winter wheat had more tillers/plant than I have noted in fields this year (personal observations). Tillers do contribute to yield. A higher number of tillers will result in available nitrogen being divided among more heads. This year available nitrogen will be spread across fewer tillers and theoretically should result in more nitrogen available for protein. This would happen in a perfect world where nitrogen losses were “0” and all applied nitrogen went to the crop.