Rainfall Probability and EONR for Dryland Corn in Colorado.

Nitrogen fertilizer costs have increased 70% in the last 6 yrs in the Central Great Plains Region (CGPR). This cost increase coincides with a decrease in dryland grain yields due to drought. How does the economic optimum N rate (EONR) change with grain price and fertilizer cost? Here we evaluated 11 years of dryland corn response and 4 years of winter wheat response to applied N. For winter wheat, simply using relative yields produced a predictive relationship that was acceptabe for estimating optimal N rates for both wet and dry years. For corn a more elaborate analysis was needed that indicated some data years are more important than others in that they reflect a greater percentage of the actual weather expected based on a 100-year weather record. We used the probability of rainfall based on our long term weather record to evaluate the validity of 11 site years of N response and then used that information to determine EONR for no-till dryland corn in Colorado. Our analysis also included using math associated with the “delta yield concept”. That math summarized the 11 site years nicely into a single relationship.