BC4 FRONT COVER REV1.indd

The need for precise and responsive management of N fertilizer in corn production is compelling for both economic and environmental reasons, but the optimum N rate remains an elusive notion. Economically optimal N rates (EONR) in much of North America may range from zero to 225 lb/A (Scharf et al., 2006; Sawyer et al. 2006), and may vary as the growing season progresses (van Es et al., 2007). Many factors impact soil N dynamics and crop N uptake in corn fi elds, and therefore should be considered in the process of determining an optimum rate recommendation. These include 1) soil texture, depth, structure, drainage, and organic matter content, 2) amount, form, placement, and timing of application of amendments like fertilizer, manure and compost, 3) crop rotational and cover crop effects, 4) tillage and crop residue management, as well as 5) weather events and 6) risk factors associated with fertilizer and grain prices. Most current recommendation systems are simple and generalized (e.g., based on yield potential or average empirical N response), while a few others incorporate some of the above attributes. Many are also static in that they give the same recommendation regardless of weather. Seasonal temperature and precipitation, notably, infl uences N gains from mineralization and losses through leaching and denitrifi cation, and are therefore highly correlated with seasonal variation in optimum fertilizer N rates (van Es et al., 2007). A meta-analysis of 51 corn N response studies in North America showed strong soil and weather effects (Tremblay et al., 2012), and fi elds that received high precipitation in the time period around sidedressing were found to have much greater N response than those receiving low precipitation. Early-season events appear to be the strongest determinant for optimum N rates (Tremblay et al., 2012), although midand late-season weather may still affect corn yields (especially in cases of drought). Static N fertilizer recommendations based on average crop response lead to excessive fertilization in some years, and inadequate fertilization in years with high N losses. From a farmer’s perspective, the uncertainty in optimum N rate poses risks for profi t losses, which is exacerbated by the asymmetric profi t response of corn to N rates. The associated higher cost of under-fertilization relative to over-fertilization drives farmers to apply higher rates, and use additional “insurance” fertilizer applications. This uncertainty can be addressed by providing more accurate locationand time-specifi c recommendations that increase accuracy and reduce uncertainty. Currently, two general approaches are pursued by scientists: canopy refl ectance spectroscopy and model-database tools. This article is focused on the latter. By Bianca Moebius-Clune, Harold van Es and Jeff Melkonian