Pre-Plant and Split Nitrogen Applications: When and What Nitrogen Source?.

Improving nitrogen (N) fertilizer use efficiency of corn (Zea mays L.) under ongoing climate change is an important goal for economic and environmental-protection reasons.  In 2014, six fields were chosen from across Minnesota ranging in soil and climatic conditions to better understand the role of N source and of split-N application timing on grain yield and soil-N availability. Lower N rates are for corn-soybean and higher rates are for corn-corn as follows: 1) pre-plant treatments of urea (0 to 202 or 314 kg N ha^-1 in 34 or 45 kg N ha^-1 increments), and at one rate (101 or 135 kg N ha^-1) anhydrous ammonia with and without nitrapyrin [2-chloro-6-(trichloromethyl) pyridine], polymer coated urea (ESN), and ESN-urea blends; and 2) 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, and V12 development stages. The first year of this multi-year study was excessively wet. Excessive N loss resulted in a linear response of yield to N in three locations.  In most locations, pre-plant urea was outperformed by the other N sources, though sometimes it was only a trend. Overall, the five fine-textured soils (comparisons at one-rate 101 or 135 kg N ha^-1) split applications before V8 were similar to any of the pre-plant treatments. Unlike in fine-textured soils, pre-plant urea in a coarse-textured soil reduced yield 51% relative to the mean of V6 or later split-applications. In the coarse-textured soil, pre-plant ESN and blends with urea did not improve yield relative to urea and only pre-plant anhydrous ammonia with nitrapyrin had similar yields to V6 or later split-applications. In-season soil N availability measurements closely followed grain yield results showing that split-applications are most useful for coarse-textured than fine-textured soils.