|Yield Goals and Prediction of Yield Goals|
Before 1957, most N rate recommendations were based
on soil criteria and crop management. Since 1970, the yield goal approach has
been a popular method for maize in the Mid-West; it converts the expected
yield into N rate recommendations using fixed factors (Fernandez et al.,
2009). Yield goals are determined from a recent 5-year crop yield average,
increased typically by 10-30%, and that assures adequate N for above-average
growing conditions (Johnson, 1991). Maximum Return to Nitrogen (MRTN) is a
procedure for estimating economically optimum N rates. It has been used in the
Midwest across the Corn Belt, and determines preplant N rates by estimating
the yield increase to applied N using current grain and fertilizer prices
(Sawyer et al., 2006). This approach provides generalized N rate
recommendations over large areas and years. However, it fails to address the
issue of year-to-year variability in temperature and rainfall (Shanahan, 2011;
Van Es et al., 2006) and does not provide site-year recommendations.
Although optimal N rates can vary substantially within and between fields, most US maize producers still apply the same rates to entire farms (Scharf et al., 2005). Limiting application rates is the most important factor in reducing environmental impacts; nonetheless, inappropriate methods and poor timing continues to pose the risk of N loss to the environment (Ribaudo et al., 2012). Additionally, the inability to accurately estimate optimum N rates results in over-fertilization for some years and fields and under-fertilization in others and a lower NUE (Shanahan, 2011). Consequently, there is an urgent need to improve N fertilizer management. As such, the accurate estimation of optimum N rates, year-to-year and field-to-field remains elusive (Van Es et al., 2006).
Yield goal was defined by Dahnke et al. (1988) as the 'yield per acre you hope to grow.' They further noted that what you hope to grow and what you end up with are two different things. Yield goals can vary all the way from past average yield to potential yield (Dahnke et al., 1988). They defined potential grain yield as the highest possible yield obtainable with ideal management, soil, and weather. In our work, what they define as potential grain yield would be maximum grain yield, since 'potential' yield is associated with specific soil and weather conditions that can change annually. For most farmers, North Dakota State University recommends that the grain yield goal is the highest yield attained in the last four to five years and that it is usually 30 to 33% higher than the average yield (personal communication, R. J. Goos, 1998).
Rehm and Schmitt (1989) noted that with favorable soil moisture at planting it would be smart to aim for a 10 to 20% increase over the recent average when selecting a grain yield goal. They also indicated that if soil moisture is limiting, use of history and past maximums (used to generate averages) may not be the best method for setting a grain yield goal for the upcoming crop. Use of farm and/or county averages was not suggested for progressive farmers concerned with high farm profitability (Rehm and Schmitt, 1989). Black and Bauer (1988) reported that the grain yield goal should be based on how much water is available to the winter wheat crop from stored soil water to a depth of 1.5 m in the spring plus the anticipated amount of growing season precipitation. Combining grain yield goal, soil test NO3-N and a simple estimate of nitrogen use efficiency can be used to estimate N fertilization requirements. Several states recommend that farmers apply 33 kg N/ ha for every 1 Mg of wheat (2 lb N/ac for every bushel of wheat) they hope to produce, minus the amount of NO3-N in the surface (0-15 cm) soil profile (Johnson et al., 1997). Therefore, when grain yield goals are applied, it explicitly places the risk of predicting the environment (good or bad year) on the producer. University extension (e.g., soil testing), fertilizer dealers and private consulting organizations have historically used grain yield goals, due to the lack of a better alternative.
and A. Bauer. 1988. Setting winter wheat yield goals. In J.L. Havlin (ed.)
Central Great Plains Profitable Wheat Management Workshop Proc. Wichita, KS.
Aug. 17-20, 1988. Potash & Phosphate Institute, Atlanta, GA. |
Dahnke, W.C., L.J. Swenson, R.J. Goos, and A.G. Leholm. 1988. Choosing a crop yield goal. North Dakota State Ext. Serv. SF-822. Fargo, North Dakota.
Fernandez, F. G., E. D. Nafziger, S. A. Ebelhar, and R. G. Hoeft. 2009. Managing nitrogen. Illinois agronomy handbook. Univ. Illinois Coop. Ext. Serv. Urbana-Champaign. 113-132.
Johnson, G.V. 1991. General model for predicting crop response to fertilizer. Agron. J. 83:367–373.
Rehm, George, and Michael Schmitt. 1989. Setting realistic crop yield goals. Minnesota Ext. Serv. AG-FS-3873, Univ. of Minnesota, 55108.
Sawyer, J., E. Nafziger, G. Randall, L. Bundy, G. Rehm, and B. Joern. 2006. Concepts and rationale for regional nitrogen rate guidelines for corn. PM 2015. Iowa State Univ. Extension, Ames.
Scharf, P. C., N. R. Kitchen, K. A. Sudduth, J. G. Davis, V. C. Hubbard, et al. 2005. Field-scale variability in optimal nitrogen fertilizer rate for corn. Agron. J. 97(2), 452-461.
Shanahan, J. 2011. Determining optimum nitrogen rates for maize. Crops Insights, 21(2), 1-5 Pioneer Hi-Bred, Johnston, IA.
Shanahan, J.F., N.R. Kitchen, W.R. Raun, and J.S. Schepers. 2008. Responsive in-season nitrogen management for cereals. Computers and Electronics in Agric. 61:51-62.
Van Es, H. M., B. D. Kay, J. J. Melkonian, and J. M. Sogbedji. 2006. Nitrogen management for maize in humid regions: Case for a dynamic modeling approach. In Managing Crop Nitrogen for Weather: Proceedings of the Symposium “Integrating Weather Variability into Nitrogen Recommendations,” Indianapolis, IN.15:6-13.
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