Effect of Nitrogen Sources, Rates, and Application Time on Spring Wheat Yield and Grain Protein.

Nitrogen (N) is a primary constituent of proteins; thus, N nutrition directly and radically impacts spring wheat production profitability. Numerous important questions remain unanswered regarding the optimum fertilization strategies for maximizing spring wheat yield and grain protein. The major objective of this study is to determine the most efficient N fertilizer source, rate, and application time combination for optimizing Montana spring wheat yield while maximizing grain protein and N use efficiency (NUE). This project was funded by the Montana Fertilizer Tax Advisory Committee, initiated in 2011, continued in 2012 at at three dryland locations. Three dryland experiments were established: one at Western Triangel Agricultural Research Center, Conrad, MT, and two in cooperating producers’ fields (Jack Patton, Chouteau County, and Lindsay Martin, Teton County) using Choteau spring wheat variety. A combination of 4 preplant N rates (0, 40, 80, and 120 lbs N ac), 3 topdress N rates (0, 45, and 90 kg N ha-1), 2 topdress N fertilizer sources (granual – urea, 46-0-0, and liquid – urea ammonium nitrate (UAN), 28-0-0), and 2 topdress application times (before flowering and after flowering) were evaluated. Urea was manually broadcasted and UAN was applied as a foliar spray using an ATV-mounted sprayer. Treatment effect (preplant N rate, topdress N source, rate, and application time) on spring wheat grain yield, grain protein, N uptake, and N use efficiency (NUE) were evaluated. The wide range of yielding environments allowed us to examine the way crop responds to N application. Results clearly showed that in a higher yield potential situation, the benefit of applied N is greater, where as in a lower yield potential environment application of N, especially at seeding, is does not pay off. Application of all N prior to or at seeding is not advisable, especially in dryland wheat production systems, where crop’s yield potential and environmental conditions are not yet known. Providing just enough N to allow good crop stand establishment and then topdressing according to mid-season crop status evaluation and taking into account environmental conditions is a more efficient approach to nutrient management. Not being able to accurately predict precipitation and temperature later in the season that might substantially affect grain yield and quality is still a big challenge. However, having a clear picture of growing conditions for the first part of the season and being able to evaluate crop’s nutrient status using tissue sampling, chlorophyll meters or precision sensors, will allow us to make more informed nutrient management decisions.