Can a late N topdressing help wheat yields this year?
The final yield component, kernel size, is yet to be determined at this point in the season. Weather conditions at heading and pollination will impact the number of actual kernels set per head, and rain during grain fill will produce big plump kernels. While kernel size is very important and will determine the number of kernels actually set which can be harvested and marketed, maximum kernel size is set genetically.
Unfortunately we can’t produce a wheat kernel the size of an apple. So while correcting an N or S deficiency now, as the crop is approaching or at heading, can have some potential to preserve potential yield or increase yield by maximizing the size and number of potential kernels harvested, the extent of that response is limited since the maximum number of heads and kernels per head was determined several weeks ago.
Leaf burn considerations
The potential for leaf burn is an important consideration with late applications of N. Traditional broadcast applications of 28% liquid N with flood or flat fan nozzles can cause foliar burn to the flag leaf, especially at high air temperatures. Leaf burn is a serious concern when it occurs on the upper leaves, since the wheat relies on these leaves for grain fill.
Once the head begins to emerge, the risk of injury from leaf burn from broadcast UAN far exceeds any potential response to additional fertilizer, even if the wheat is N deficient. At that stage of growth, a foliar N product is much safer to the crop.
To reduce the potential for leaf burn, there are alternative ways to apply traditional liquid N sources other than the standard spray nozzle. Streamer bars, a 10- to 15-inch long plastic bar which can be used with traditional spray booms in place of the nozzle, provide a solid stream of liquid fertilizer spaced every 5-6 inches. These streams of liquid greatly reduce foliar burn as compared to complete foliage coverage with standard flat fan spray nozzle. Broadcast granular urea also produces limited leaf burn as compared to sprayed UAN.
Foliar nitrogen products
Various foliar nitrogen (N) fertilizer products are available. These products range in analysis and can include straight nitrogen products or mixtures of N plus other macro and micro nutrients. The straight nitrogen products will typically have an analysis similar to traditional liquid N fertilizers, such as 25 to 30 percent N.
One of the main differences between traditional UAN and the foliar products is that a certain percentage of the N in the foliar fertilizers is commonly in some type of slow-release form. As a result, these specialty products are generally safer for application directly to the foliage in later stages of growth and result in less leaf burn than traditional UAN products.