Influence of dual top-dress nitrogen applications on winter wheat (Triticum aestivum L.) grain yield

Winter wheat (Triticum aestivum) is commonly grown throughout Oklahoma and is notably the state's top cash crop. Nitrogen (N) is one of the most heavily applied nutrients in wheat by producers across the state as it is often the most deficient macronutrient. Nitrogen use efficiency (NUE) has recently been documented at 35% in cereal crop production worldwide (Omara et al., 2019). At such a low value, research must continue to explore management options to improve NUE. One way to do so is to optimize the timing of N, in order to provide the plant with the nutrients they need at the most critical growth stage. The objectives of this study were to evaluate two timings of top-dress N at different rates on winter wheat grain yield, grain N, and NUE. Nitrogen was applied in early spring before and after visible response with single and split N applications. Nitrogen rates varied from 0 kg ha^-1 to 101 kg ha^-1. Urea Ammonium Nitrate (UAN) and Urea were the two sources of N used in this study. Data was collected during the 2019 and 2020 growing seasons at Hennessey and Lahoma, OK. Split N applications commonly resulted in significantly higher grain yields when a higher rate was applied at the earlier application. This was also true for grain N results. Differences among NUE were not significant, however. It was seen that the application methods with a higher N rate early tended to have a higher grain yield and NDVI value. When N was applied later in the season, this generally resulted in higher grain N