Winter Wheat Response to Topdress Nitrogen Application Method and Source

Winter Wheat (Triticum aestivum) is the main crop grown in much of the Southern Great Plains, with roughly 1.8 million hectares being planted in Oklahoma alone in 2017 (USDA 2017). Nitrogen volatility is a significant concern when deciding on which topdressing source and method to use for wheat production in the Great Plains region. With the many options producers have for fertilizer application that is commercially available, it is of the utmost importance that research continues to be performed to develop different methods of improving nitrogen use efficiency with each option, while still remaining a viable option. This study was initiated to evaluate the impact of fertilizer source and placement on winter wheat grain yield and protein concentration. It was hypothesized that placing urea below the soil surface in season with the use of a grain drill would increase both final grain yield and protein concentration when compared to broadcasted urea, that the use of a protected urea source would have a positive effect on grain yield and protein when compared to the broadcast urea check, and that the grain drill would not have a negative effect on the wheat crop due to plant damage and disturbance. Trials were established in early January of 2017 and 2018 at the OSU Research Stations located in Perkins (central Oklahoma, Konawa and Teller Loamy fine sand), Lahoma (west central Oklahoma, Grant silt loam) , and Chickasha (South Central Oklahoma, Dale and Reinarch silt loam). Two drill types consisting of a single disk opener and a double disk opener, both applying urea, were compared to three sources of N broadcast. The sources consisting of Super U (protected urea source), untreated urea, and ammonium nitrate. All treatments received 67.25 kg ha-1 of actual nitrogen, excluding the check. Three timings were implemented, consisting of an early, mid, and late top-dress application, which were intended to represent a late January, early February, and late February applications. Out of 21 contrasts comparing grain drilled applied urea to broadcast urea, only one was found to be significantly positive. Similarly, the AN treatment was only significantly higher than broadcasted urea in one contrast while Super U was never statistically greater. While few methods statistically improved yield or protein above broadcast urea, across all locations the use of a grain drill resulted in a 5.5% higher yield than broadcast urea, while AN increased yield by 3% and Super U by 4%. The results suggested that the plant damage caused by the use of a grain drill did not have a negative impact on grain yield nor protein concentration in any of the site-years. In field observation of fertilizer placement and row closure based on drill type and soil environment suggest further research is needed to better understand the impact of grain drill type, soil conditions, and the factors controlling the probability of a positive response.Plant and Soil Scienc