Challenges of Predicting Year Variation in Corn Nitrogen Fertilizer Requirement.

Considerable effort has been aimed at optimizing nitrogen fertilizer application rates so as to maximize profitability while limiting potential negative externalities associated with nitrogen fertilizer.  Maximum economic rate of nitrogen (MERN) have been shown to vary spatially within and across fields, but much less effort has been made to quantify temporal variation.  The objective of this research was to 1) assess using a long-term nitrogen trial established in 2009 at the Elora Research Station, University of Guelph, the year to year variation in MERN for a given field, and 2) assess whether existing nitrogen recommendation systems can effectively predict yearly variation in MERN for a given location. A long-term nitrogen trial was established in 2009 at the University of Guelph, Elora Research Station (43⁰39' N 80⁰25' W, 376 m elevation), Elora, Ontario, Canada.  The trial consists of 5 nitrogen fertilizer rates (0, 28, 57, 115, and 188 kg-N ha^-1) injecting as urea-ammonium-nitrate (28%) solution at approximately the 6^th leaf stage mid-row to a depth of 5-10 cm.  Annual yield response to N rate was characterized and MERN estimated by fitting a polynomial quadratic response model.  Nitrogen response curves as well as MERN values varied considerably across years even though management (N history, hybrid, tillage, and weed control) and soil type were constant across years.  Neither the Ontario General Maize Nitrogen Recommendations, which accounts for soil type, previous crop, heat unit zone, maize:nitrogen price and expected maize yield, or the Pre-sidedress Soil Nitrate test were effective in predicting the  rate of nitrogen required.  Temporal factors related to environment are a significant cause of variation in maize N requirement.