Josh Lofton, Steve Harrison, and Beatrix Haggard- LSU AgCenter
As we transition from January to February and wheat begins to show spring growth, it’s time to think about applying topdress N fertilizer. Most wheat is around Feekes 5 (green-up; http://varietytesting.tamu.edu/wheat/docs/mime-5.pdf) growth stage, and it’s time to begin applying topdress N. Decisions on N fertilization rate and timing dramatically influence final yield, and N mismanagement can certainly be a limiting factor. However, N management in wheat is not as cut and dry as it can be in other crops. This is true for not only N application rates but also application timing.
Current recommendations are to apply 90-120 lbs N ac-1 throughout the growing season, split between all applications; however, these rates can vary depending on soil type, previous crop and crop condition. That means if 15 lbs N ac-1 was applied preplant, an in-season application should be between 75 and 105 lbs N ac-1. These application rates seem fairly straight forward, but attempting to manage around changing crop conditions (lush growth or stunted) or environmental conditions can be quite challenging. The major challenge for N application in wheat comes with determining how to split N applications and when those splits should occur. Currently, splitting in-season N application into two or even three topdress applications is considered the best management practice for wheat. This is due to N loss that can occur because of volatilization, leaching or denitrification. However, the most critical time for N application to have a yield benefit is late winter or early spring just prior to jointing. An initial application after jointing will limit yield potential of the crop. Furthermore, as the wheat progresses further, the benefit of N application is minimal or non-existent (by flag leaf). Therefore, N fertilizer should be applied in two to three applications between green-up and jointing with only rescue application between jointing to beginning emergence of the flag leaf.
While determining the optimum time to apply N fertilizer may seem perplexing, let your crop help you make your decision. For late-planted or wheat that did not properly develop in the fall, earlier N application (late January) is typically needed to help stimulate tillering. These applications are not needed on more developed wheat, and early topdress N application to well-tillered and advanced wheat could result in premature spring growth and subsequent yield loss if late freezes occur. Properly developed wheat in south Louisiana should probably receive an initial N application in early February, while north Louisiana should start to apply N by mid-February. However, these are just generalizations. Watch the wheat crop for signs of rapid development during warm conditions. If splitting N applications, the second application should follow the initial application by 14-28 days, allowing the crop time to green-up from the initial application and begin rapid spring growth.
While N management during in-season applications is the primary concern, managing S and, to a lesser degree, P can be critical during these stages as well. The benefit of S applications is not as wide-spread as N applications. Deficiencies are typically found on non-clay soils with lower amounts of organic matter; however, significant response can still be found on clay-textured soils with adequate organic matter content. Similar to N, S can be lost from the soil system through leaching during heavy precipitation events. Where this is an issue, the application of initial N application with urea and ammonium sulfate will minimize S issues. Managing P issues in-season for the wheat crop can be more challenging. Even if P is adequate in the soil system, cold and wet conditions can minimize the amount of P that can be taken up by the crop. If P deficiencies are present, the application of DAP as an N source cannot only help supply N needed for in-season application but also provide available P to the growing crop until conditions return where the crop can access soil-available P.
Josh Lofton, State Wheat Specialist, LSU AgCenter, email@example.com