RESPONSIVE IN-SEASON NITROGEN MANAGEMENT FOR CEREALS

Authors: Shanahan, John; Kitchen, Newell; RAUN, W - OKLA STATE U/STILLWATER; Schepers, James

Submitted to: Computers and Electronics in Agriculture
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 6/4/2007
Publication Date: 4/1/2008
Citation: Shanahan, J.F., Kitchen, N.R., Raun, W., Schepers, J.S. 2008. Responsive in-season nitrogen management for cereals. Computers and Electronics in Agriculture. 61:51-62

Interpretive Summary: This paper is a review of current nitrogen (N) management strategies for cereals like corn and wheat, problems associated with these strategies, and how computer and electronic technologies can be employed to address these problems. Briefly, the major problem is shown to be low N use efficiency (NUE), leading to economic losses and environmental contamination. In this paper, NUE is defined as the percent of fertilizer N recovered in the aboveground crop biomass during the growing season. We identify the primary causes for low NUE in current systems, which are: 1) poor synchrony between soil N supply and crop demand, 2) field uniform applications to spatially-variable landscapes that commonly have spatially-variable crop N need, and 3) failure to account for temporal variability and the influence of weather on mid-season N needs. In retrospect, it is not surprising that current N management schemes have resulted in such low NUE values given that current practices typically utilize a suspect approach in estimating crop fertilizer N requirements, make use of large pre-plant N applications (i.e., lack of synchrony), and ignore within-field variability in N fertilizer need. The key to optimizing the tradeoff amongst yield, profit, and environmental protection for future N management practices is to achieve synchrony between soil N supply and crop demand, and account for landscape spatial variability in soil N supplies and crop N uptake. This means less dependence on large pre-plant applications of uniformly applied N and greater reliance on a “reactive approach” that involves in-season estimates of crop N needs with the ability to adjust for both temporal and spatial variability effects on soil and crop N dynamics. To accomplish this task it will be necessary to utilize various precision agriculture tools like on-the-go soil and crop sensors that have the ability to remotely sense soil N supply and crop N status in “real-time”, and deliver spatially-variable N applications based on crop N need. We present our vision for development of alternative strategies, involving use of both soil-based management zones (MZ) and crop-based remote sensing of crop N status for in-season variable N applications. Successful deployment of these “responsive approaches” will rely heavily upon utilizing emerging Precision Agriculture technologies like on-the-go soil and crop sensors, as well as data communication protocols between sensors, controllers, computers and databases. We show how our proposed approach addresses the fundamental problems associated with current N management strategies. Since these cereals provide a significant portion of human dietary calories, and they account for a majority of global fertilizer N use, adoption of these technologies should lead to significant savings in N fertilizer costs and dramatically reduced environmental impacts worldwide.

Technical Abstract: In this review, we illustrate how traditional nitrogen (N) management schemes for worldwide cereal production systems have resulted in low N use efficiency (NUE), environmental contamination, and considerable debate regarding appropriate use of N fertilizers in crop production. Hence, development of alternative strategies that improve NUE and minimize environmental impact is crucial to sustaining cereal-based farming. The major causes for low NUE of traditional N management practices are: 1) poor synchrony between soil N supply and crop demand, 2) field uniform applications to spatially-variable landscapes that commonly have spatially-variable crop N need, and 3) failure to account for temporal variability and the influence of weather on mid-season N needs. Poor synchronization is mainly due to large pre-plant applications of fertilizer N, resulting in high levels of inorganic soil N before rapid crop uptake occurs. Uniform applications within fields discount the fact that N supplies from the soil, crop N uptake, and crop response are spatially variable. Failing to account for N mineralized in warm, wet years ignores indigenous N supply. When N fertilizers are applied in these ways or ignoring these factors it is at considerable risk for environmental loss. The key to optimizing tradeoffs amongst yield, profit, and environmental protection is to achieve synchrony between N supply and crop demand, while accounting for spatial and temporal variability in soil N. While some have advocated a soil-based management zones (MZ) approach as a means to direct variable N applications and improve NUE, it appears that this strategy is inconsistent in characterizing spatial yield variation across variable climatic conditions. Thus, it seems unlikely that the static soil-based MZ concept alone will be adequate for variable application of crop inputs like N. We advocate using remote sensing to assess crop canopy N status and direct in-season spatially-variable N applications, utilizing emerging computer and electronic technologies. One such technology that shows promise for determining crop N need is ground-based active-light reflectance measurements converted to NDVI or other like indices. This approach for on-the-go sensing and fertilizer application addresses the issues of spatial variability and synchronizing N inputs to match timing of crop N uptake. We propose this approach may be improved by first delineating a field into MZ using soil or other static field properties to modify the decision associated with ground-based reflectance sensing. We expect N use efficiency can be greatly enhanced using this responsive strategy for N management in cereals.