Is an economically optimal corn nitrogen rate also environmentally optimal?

The economically optimal nitrogen rate (EONR), while an accepted standard as the “right rate” for corn (Zea mays L.) fertilization, does not directly account for environmental impacts. This study evaluated the effects of nitrogen (N) fertilizer application rate and timing on crop N use and N loss potential, using residual soil nitrate-N (RSN; 0- to 0.9-m depth) relative to EONR. The evaluation was conducted using 49 N response trials from eight US Midwest states from 2014 to 2016. Nitrogen rates were applied as ammonium nitrate, either all at planting or split between at planting (45 kg N ha⁻¹) and the remainder at the ∼V9 growth stage. At EONR, RSN was 42 kg N ha⁻¹ for at-plant applications and 62 kg N ha⁻¹ for split applications. However, unaccounted for N at the end of the growing season was greater for at-plant (46 kg N ha⁻¹) than for split applications (21 kg N ha⁻¹). This suggests a higher susceptibility of N loss during the early season for at-planting applications and after the season for split applications. Differences in RSN at the EONR between N timings were not explained by differences in total aboveground N uptake at R6. Residual soil nitrate did not substantially increase until N application rates exceeded the EONR by 30 kg N ha⁻¹. These findings support using EONR, at an N:corn price ratio of 5.6, as an N application sustainability standard that balances profitability and environmental concerns.