Corn Response to Polymer-Coated Urea Application Timing, Nitrogen Rate, and Blending Ratio.

Escalating fertilizer prices, increasingly unpredictable spring weather conditions, and degradation of Great Lakes water quality have placed greater emphasis on improving management of nitrogen (N) fertilizer through strategies that more closely synchronize N availability with peak corn (Zea mays L.) N demand.  Although nitrogen use efficiency in corn production systems is only near 50%, polymer-coated urea (PCU) limits the amount of N available for microbial transformation after field application potentially increasing fertilizer efficiency and reducing environmental N losses. A three-year field experiment was completed in Michigan to determine the N availability of both polymer-coated urea (PCU) and conventional urea at multiple application timings, N rates, and blending ratios in corn management programs involving one N application.  The study was arranged as a 3 (application timings of pre-plant incorporated, at-plant incorporated, and sidedress surface application) x 2 (N application rates of 84 and 168 kg N ha^-1) x 5 (PCU and urea blending ratios) factorial plus non-treated control.  Data collection included R1 chlorophyll measurements, leaf firing, stalk nitrate analysis, total N concentration and biomass of grain, cob, and vegetation, residual soil nitrate after harvest, and grain yield. Multiple years of significant application timing by blending ratio interactions seem to indicate that in corn fertilizer programs involving one N application timing, a pre-plant incorporated mix of 75:25 (PCU:Urea) yielded greater than other blending ratios at this timing interval. When considering at-plant N applications, a 50:50 (PCU:Urea) blend worked well in years with a normal planting date for a specific region. When planting was significantly delayed (i.e., after May 25), shifting the blending ratio to a 25:75 mix of (PCU:Urea) seemed to improve corn yield. The greatest potential to improve corn N management still rests with application timing and minimizing nitrate availability during wet, warm conditions or during periods without active crop growth.