In-season optimization and site-specific nitrogen management for soft red winter wheat

Site-specific N management based on an in-season assessment of crop N status may offer producers increased grain yield, profitability, and spring N fertilizer use efficiency (SNUE). The goal of this study was to determine the distinct contributions of (i) in-season N rate optimization and (ii) site-specific N management. Our objective was to compare site-specific and field-specific N management with typical growers' practices to determine if site-specific N management (i) increased soft red winter wheat (Triticum aestivum L.) grain yield, (ii) reduced N inputs, (iii) increased SNUE, and (iv) reduced within-field grain yield variability. Research was conducted at eight sites in 2000, 2001, and 2002. A randomized complete block design with two or five N management systems was used at two and six sites, respectively. Site-specific management did not improve grain yield compared with field-specific management when based on the same in-season estimation of optimum N rates. At sites where site-specific or field-specific systems were compared with typical growers' practices, grain yield benefits of in-season N optimization (up to 2267 kg ha(-1)) were apparent. For grain yield, in-season optimization of N rate was more important than site-specific management. A large reduction in N inputs (up to 48.6%) was also attributed to in-season N rate optimization. After incorporating in-season optimization, a further reduction in N inputs (up to 19.6%) was possible through site-specific application. Site-specific N application maximized SNUE compared with either field-specific or typical growers' practices at all sites and reduced within-field grain yield variance at four sites.