In-Season Optimization and Site-Specific Nitrogen Management for Soft Red Winter Wheat

application up to 70% without a reduction in grain yield compared to a grower’s practice. Site-specific N management based on an in-season assessment of Stone et al. (1996) used an on-the-go sensor measurcrop N status may offer producers increased grain yield, profitability, ing plant N spectral index to create submeter siteand spring N fertilizer use efficiency (SNUE). The goal of this study specific N management units based on an estimate of was to determine the distinct contributions of (i) in-season N rate optimization and (ii) site-specific N management. Our objective was in-season crop N status in wheat. This site-specific N to compare site-specific and field-specific N management with typical management system reduced N fertilizer by 32 and 57 growers’ practices to determine if site-specific N management (i) kg N ha 1 at two of three sites without a reduction in increased soft red winter wheat (Triticum aestivum L.) grain yield, grain yield compared with a typical grower’s practice. (ii) reduced N inputs, (iii) increased SNUE, and (iv) reduced withinThey also reported that the site-specific N application field grain yield variability. Research was conducted at eight sites in reduced spatial variation in wheat forage and grain yield 2000, 2001, and 2002. A randomized complete block design with two compared with the grower’s practice. or five N management systems was used at two and six sites, respecSimilarly, Raun et al. (2002) used a multispectral optitively. Site-specific management did not improve grain yield compared cal sensor to create 1-m2 site-specific N management with field-specific management when based on the same in-season units in wheat. A N fertilizer optimization algorithm estimation of optimum N rates. At sites where site-specific or field(NFOA) that estimates in-season crop N status and specific systems were compared with typical growers’ practices, grain potential grain yield was used to adjust N rates. They yield benefits of in-season N optimization (up to 2267 kg ha 1) were reported that by using NFOA, it might be possible to apparent. For grain yield, in-season optimization of N rate was more important than site-specific management. A large reduction in N inset more efficient and profitable fertilization levels and puts (up to 48.6%) was also attributed to in-season N rate optimizaincrease N use efficiency compared with typical growtion. After incorporating in-season optimization, a further reduction ers’ practices. in N inputs (up to 19.6%) was possible through site-specific applicaMulla et al. (1992), Bhatti et al. (1998), Stone et al. tion. Site-specific N application maximized SNUE compared with (1996), and Raun et al. (2002) compared site-specific N either field-specific or typical growers’ practices at all sites and reduced management based on either a preor in-season estiwithin-field grain yield variance at four sites. mate of the crop’s N requirement to a typical grower’s practice. Consequently, the reduction in N rates compared with growers’ practices might not have been the S N management is the adjusting of withinresult of site-specific application but could instead be field N fertilizer rates based on spatially variable due to using a preor in-season estimation of the crop’s factors that affect optimum N rate (Sawyer, 1994). This N requirement. practice may offer producers the ability to increase grain In the southeastern USA, Scharf and Alley (1993), yield, profitability, and N fertilizer efficiency by applyAlley et al. (1994), Weisz and Heiniger (2000), and ing N only where required for optimum plant growth. Weisz et al. (2001) developed a field-specific N manageSite-specific management may also be environmentally ment system for soft red winter wheat based on an inbeneficial to producers. season evaluation of the crop’s N requirement (Fig. 1). Mulla et al. (1992) created site-specific management This system first determines the whole-field tiller density units (18.3 m by 564–655 m) based on preseason soil N at Zadoks’ Growth Stage (GS) 25 (Zadoks et al., 1974). (nitrate N and ammonium N) tests and available soil When GS-25 tiller density is below a critical threshold water content. Similarly, Bhatti et al. (1998) created (540 tillers m 2), a GS-25 N application is made to insite-specific N management units based on crop produccrease tiller development (Ayoub, 1974; Power and tivity. In both cases, site-specific N reduced N fertilizer Alessi, 1978; Lutcher and Mahler, 1988; Scharf and Alley, 1993; Weisz et al., 2001). A GS-25 N application can stimulate tiller development in southeastern areas M. Flowers, USDA-ARS, Air Quality–Plant Growth and Dev. Res. because winter wheat does not enter a dormant state Unit, 3908 Inwood Rd., Raleigh, NC 27603; R. Weisz, Dep. of Crop in these southern latitudes. If GS-25 tiller density is Sci., North Carolina State Univ., Box 7620, Raleigh, NC 27695-7620; above the threshold, a GS-25 N application is not necesR. Heiniger, Dep. of Crop Sci, North Carolina State Univ., Vernon James Res. and Ext. Cent., 207 Research Rd., Plymouth, NC 27692; sary. At GS 30, a field-averaged tissue test is used to D. Osmond, Dep. of Soil Sci., North Carolina State Univ., Box 7619, optimize N application rates (Alley et al., 1994). This Raleigh, NC 27695-7619; and C. Crozier, Dep. of Soil Sci., North system resulted in an increase in estimated profit of $73 Carolina State Univ., Vernon James Res. and Ext. Cent., 207 Research ha 1 across 20 site-years (Scharf and Alley, 1993). Rd., Plymouth, NC 27692. Received 5 Dec. 2002. *Corresponding author ([email protected]). While this system (Fig. 1) has been tested and adopted Published in Agron. J. 96:124–134 (2004).  American Society of Agronomy Abbreviations: GS, growth stage; SNUE, spring nitrogen fertilizer use efficiency. 677 S. Segoe Rd., Madison, WI 53711 USA