Implementing Adaptive Nitrogen Management Practices for Corn on a Large Scale: A case study in Iowa.

Predicting nitrogen (N) fertilizer requirements for corn are difficult because the dynamic nature of processes that supply N to the plants from the soil, fertilizer, and manure and because of variable effects of weather and rapid changes in economic situation. To reduce these and other uncertainties, adaptive N management promises to identify the best performing practices and rapidly adapts them by constantly monitoring and evaluating their outcomes. We report a case study conducted in Central Iowa with a group of 30 growers who used precision farming technologies within 56 fields planted to corn in 2007.  Replicated two-treatment N trials  (Normal minus 50 and Normal plus 50), the late-spring soil nitrate test, aerial imagery of corn canopy, and the guided corn stalk nitrate test were used to detect fields and management practices that showed corn N stress and potential yield losses. The replicated trials included different N rates, timing, method, forms of N fertilization, and previous crops. The yield data were analyzed to detect possible interactions between the treatment effect and management practices or site-specific factors. The corn stalk nitrate data were analyzed to estimate the probability of a given practice to be in a specific corn stalk nitrate category. Both yield and corn stalk nitrate data confirmed significant losses of N due to excessive spring rainfall. A good positive relationship was found between cumulative spring rainfall and yield differences between the treatments with normal and normal minus 50 fertilizer rates in trials collected between 2001 and 2007, but this relationship was poor for the 2007data. The study suggested, however, the need to collect an appropriate sample size of fields with different management practices a given geographic area for identifying critical factors that affect N management outcomes.