Proceedings 2004

Indiana Crop Adviser Conference

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N Fertilization

Why Revise N Rate Recommendations?

Though most of us have at least a vague idea that current N rate recommendations resulted from N rate trials, the process of forming such recommendations, and of changing them once we have them, is not simple. Part of the problem is that the word "recommendation" implies that we have some number (lb N per acre) that will, without fail, provide just the amount of N that the corn crop needs. It just doesn't work that way. Assembling a lot of N response data over locations and years usually shows that: a) optimum N rates vary among site-years, from very low or zero N to the highest N rates used in the trials; and b) that there is no known way to predict beforehand what the rate response in a certain field will actually be in a given year. There is a surprising number of different ways to look at collections of N response data, but one way that we think has promise is to look at a "distribution" of optimum N rates from trials, then to assess how high the rate needs to be to assure there's enough N a certain percentage of the time. As an example, we might be able to say, "While 140 lb N per acre is adequate (at or above the economic optimum N rate) 50 percent of the time, it takes 160 lb to have adequacy 90 percent of the time." We can evaluate the economic penalties for having N rates too high or too low, and can thereby define risk-based N rate "guidelines" that should serve us better than "recommendations" have. We'll look at some different data sets to see what answers they provide.

Emerson Nafziger Professor of Agronomic Extension
University of Illinois

Biography: Emerson Nafziger grew up on a farm in Northwestern Ohio and earned degrees in Agronomy at Ohio State, Purdue, and the University of Illinois. He is Professor of Crop Production Extension at the University of Illinois, where he conducts programs in applied research and extension in management of corn, soybean, and wheat.