In-Season Nitrogen Availability for Corn As Influenced By Source, Time, and Rate of Fertilizer Nitrogen.

It is hypothesized that improved nitrogen (N) use efficiency (NUE) for corn (Zea mays L.) may be obtained with in-season applications. Our goal was to identify the best application timings during the growing season to improve NUE in different environments. Twelve fields across Minnesota varying in soil and climate conditions during the 2014-2015 growing seasons received 1) pre-plant urea (0 to 202 or 315 kg N ha^-1 in 34 or 45 kg N ha^-1 increments); 2) pre-plant treatments (101 or 135 kg N ha^-1) of anhydrous ammonia with and without nitrapyrin [2-chloro-6(trichloromethyl) pyridine], polymer coated urea (PCU), and PCU-urea blends; 3) split-applied treatments with urea ammonium nitrate at planting (34 or 45 kg N ha^-1) and urea with a urease inhibitor (67 or 90 kg N ha^-1) at V2, V4, V6, V8, V12 developmental stages. Soil texture and the two-tailed log likelihood test were used to group fields by grain yield response to N rate. Group1 (loamy sand) and Group2 (silty-clay loam or finer) were linear, Group3 (loam or finer) was quadratic plateau with the plateau occurring at 182 kg N ha^-1 and 11 Mg grain ha^-1, and Group4 (loam) was non-responsive. Soil (0-30 cm) nitrate at V4 predicted yield as well or better than any other combination of N species, depth or time across all groupings. In Group1 anhydrous ammonia with and without nitrapyrin and PCU outperformed urea by 58% on average and split applications increased yields by an average 97% compared to single pre-plant application. Except for an average 21% increase with PCU-urea blends compared to urea in Group2, there were no differences in grain yield due to N source or timing for Groups2-4. Using soil N at V4 showed promise as a tool to improve N management for corn.