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

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. Two dryland experiments were established in 2011 - at Western Triangle Agricultural Research Center (WTARC) near Conrad, MT, and Jack Patton (Chouteau County, MT), and three in 2012 – WTARC, Patton, and Lindsay Martin (Teton County, MT).  Choteau spring wheat variety was used for all site-years. A combination of 4 preplant N rates (0, 40, 80, and 120 lbs N ac^-1), 3 topdress N rates (0, 40, and 80 lb N ac^-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 prills were manually broadcasted and UAN was applied as a foliar spray using backpack sprayers. At maturity, grain yield and protein content were determined. Treatment effect (preplant N rate, topdress N source, rate, and application time) on spring wheat grain yield, grain total N (protein) were evaluated. In most site-years evaluated, the rate of preplant N fertilizer rate affected spring wheat grain yields. For example, at WTARC in 2011 and 2012, and at Martin in 2012, preplant application of 80 lb N ac^-1 resulted in significantly higher grain yields. On the other hand, increasing preplant N fertilizer rate to 120 lb N ac^-1 did not further increase grain yields at most site-years, except for WTARC in 2012. The plots that received topdress N as broadcasted urea yielded the same as those that were foliar sprayed with UAN solution. There were no significant differences in spring wheat grain yield associated with topdress N fertilizer source (urea vs UAN) at any of the 5 site-years. There were not statistically significant differences in grain yields associated with the time of topdress N fertilizer application (before flowering vs after flowering) were observed at any of the site-years. Practically the same yields were achieved whether the topdress was applied prior to or after the anthesis. Grain protein response to preplant N was only observed in the second year of the study. However, increasing the preplant rate from 40 to 80 to 120 lbs N ac^-1 did not further increase protein. Similarly, topdress N resulted in significantly higher protein in the second year, but no response to topdress N was observed in 2011. Protein was significantly higher for the UAN treatments compared to urea in 3 of 5 site-years. As with yield, there were not statistically significant differences in grain protein content associated with the time of topdress application.