Corn Nitrogen Calibration in South Dakota.

South Dakota (SD) corn nitrogen (N) rate recommendation was last established in 1991 and is based on the coefficient of 1.2 lbs N/bu of the yield goal.  The South Dakota corn N rate calculator subtracts the pre-plant soil test NO3-N (0-2ft) and legume credits and adds 30 lbs N/a for recently established no-till.   Much improved corn genetics, shifting farming practices, and climate changes warrant for re-calibration of corn N rate.  During 2013-2105, N rate studies were conducted at 20 sites in SD.  Six N rates (0, 40, 80, 120, 160, 200) applied as SuperU were randomized and applied to small plots (15 x 30 ft) in four replications at each location prior to seeding corn.  An additional 80 lbs/a N rate plot was included in each replication and applied as urea.  Chlorophyll (SPAD) and Greenseeker readings were obtained at three times during the growing season (V6, V10 and R2 growth stages).  Grain yield was significantly influenced by N rate at all locations.  Pre-plant soil nitrate-N ranged from 32 to 126 lbs N/a. The grain yields ranged from 99 to 260 bu/a. Even though SuperU gave numerically higher grain yield when compared to urea treatment at twelve of the twenty sites, the values were statistically significant at only two sites.  Estimated N coefficients ranged from 0.76 to 1.55 lbs N/bu.  Relative grain yield and available N comparison showed a linear/plateau relationship with minimal yield gains from added nitrogen above 170 lbs N/a.