The Delta Yield Concept for Dryland Winter Wheat in Eastern Colorado.

A calculation of the economic optimum nitrogen (N) rate (EONR) for dryland winter wheat is greatly influenced by soil water stored at planting time, by precipitation received during the growing season and by residual and mineralized N from the soil. Our objective is to determine how to combine rainfall probability distributions with the Delta yield concept to identify which years and equations of N response (developed during those years) are most useful for making N management decisions. Twenty seven site years of data were evaluated and used to develop EONR equations and to develop a Delta yield relationship from fitted quadratic equations of N response. The relationships come from N rate field experiments conducted in 1977 through 2009. EONR values ranged from 0-90 kg N per ha and averaged 48kg N/ha. The magnitude of the calculated EONR value is linked to the probability of receiving a given amount of precipitation and soil water storage during the summer fallow period. The Delta yield concept helped us summarize the 27 site-years of data into one relationship. However, the fitted relationship was not able to accurately capture extremely high yielding and corresponding high EONR years. In this exercise, we were able to identify which EONR values have a high probability and a low probability of occurrence.