Variability in Optimum Nitrogen Rates for Maize

Maize (Zea mays L.) grain yield levels and the response to fertilizer nitrogen (N) are expected to change from year to year and from location to location. Because yield level and N response have been documented to be independent and are known to influence N demand, optimum N rates at the same location vary each year due to unpredictable changes in the environment. The objective of this study was to further analyze maize grain yield levels and optimum fertilizer N rates from published data in maize growing regions of the United States. Optimum N rates were determined by calculating the difference in N uptake between the highest-yielding plot and the check plot (no N applied [0-N]). The difference in grain N uptake between the fertilized plot and the 0-N check plot was then divided by 0.33 (the assumed average N use efficiency) to estimate optimum N rate by site and year. For the 213 site-years of data included in this study, grain yields in both the high N rate and check (0-N) plots were highly variable. Optimum N rates fluctuated from year to year at all locations. Optimum N rates were not highly correlated with the high-N rate yield (R-2 = 0.20) or 0-N check yield (R-2 = 0.16). The wide range in optimum N rates observed in all maize experiments suggests the need to adjust N rates by year and location. A potential solution is to use midseason sensor-based technologies that can accurately predict yield potential and simultaneously encumber N responsiveness known to be independent of yield.