In Search of the EONR: A Comparison Between a Static and Adaptive N Management Approaches Using Extensive Multi-Rate Field Trials.

The Economically Optimum Nitrogen (N) Rate (EONR) is the N fertilizer rate at which further increases in N rate do not result in further increases in farmer profits. The majority of N is commonly applied as a single application either at preplant or as a sidedress (SD) early in the growing season. Recently, high clearance tools have been developed making it possible to sidedress later in the season, allowing a more adaptive N management approach. We report here on the results of analyses of data from 41 multi-N rate trials conducted in Indiana, Ohio and Wisconsin during the 2013 and 2014 growing seasons. These data were used to compare the EONR rate with N recommendation from three different methods: a) the MRTN – a state-wide fixed recommendation based on soil type that is used in the Midwest US Corn Belt; b) the grower normal practice; and c) an adaptive, site specific recommendation supplied by the Adapt-N tool for two critical growing season dates: an early SD (usually conducted within 30 days of planting) and an additional late SD assumed to be 60 days from planting. Adapt-N is a web-based cloud-computing maize N management tool, driven by a dynamic simulation model that simulates the relevant crop and soil processes throughout the growing season. Our results indicate that an adaptive N management approach is better than a static one in reproducing the EONR. Adapt-N recommendations were better in reproducing the EONR rates then the MRTN or the grower N rates, which tended to overestimate the EONR (R² = 0.40, 0.21 and 0.28, respectively). Furthermore, a threshold of 120 mm of rain following the 1^st SD was identified as making a 2^nd SD beneficial for the farmer. These results highlight the advantages of adaptive, site specific N management over a static fixed rate approach.