Polymer-Coated Urea Application Ratios and Timings Affect Wheat and Double-Crop Soybean Yields.

Effective management of soil N fertilizer amendments, including the use of enhanced efficiency N fertilizer sources and optimal timing of fertilizer application, may be important for improving agronomic performance of double cropped winter wheat (Triticum aestivum L.) with soybeans [Glycine max (L.) Merr.] under no-till management.  In the no-till double cropping system, N fertilizer applications may affect wheat yields and production of surface wheat residues as well as subsequent soybean emergence and yields.  The objectives of this research conducted over three growing seasons from 2008 to 2010 in northeastern Missouri were to evaluate N availability from polymer-coated urea (PCU) compared to conventional non-coated urea (NCU) and ammonium nitrate (AN) when applied at different application timings, and to assess the effects of fall to spring application timings of PCU and ratios of PCU with NCU on wheat and double-crop soybean growth response. The experimental design was a 2 (N fertilizer application rates at 84 and 112 kg N ha^-1) x 7 (application timings in Oct., Nov., Dec., Jan. Feb. March, and April) x 5 (N fertilizer source/blends) factorial and a non-treated control.  During all three growing seasons, there were significant two-way interactions of N rate with timing or source for wheat production (yield and test weight), and timing by N source for wheat yields.  In 2008 and 2010, wheat yields significantly (P < 0.05) increased with increasing N rates at each application timing.  Treatments of 100% PCU in Nov. had yields similar to those of NCU applications in Mar. and Apr., and an Apr. application of AN.  Based on these results, PCU at 100% could be applied from Nov. to Jan. and ratios should shift to 75:25 from Jan. to Mar. followed by a 50:50 blend for an Apr. application timing. Soybean yields generally increased with lower N rates and earlier applications, and were positively correlated with winter wheat yields.