Making N rate recommendations for com has been one of the most economically important goals of publicly funded crop production and soil fertility personnel and programs over the past five decades. Changes in cropping systems, hybrids, tillage, and other management practices, along with opportunities in site-specific inputs and awareness of the need to minimize the amount of N that reaches surface and ground waters have combined to increase the interest to re-examine N rate recommendations, and to formulate new recommendations if current data support such changes.

The common N rate recommendation system used for many years in the Midwest USA was a yield-goal base factor for continuous corn, with adjustment for previous crops other than com. However, research has identified poor correlation between individual site-year corn yield and economic optimal N, and that optimal N rates on a specific soil do not change with yield (Vanotti and Bundy, 1994b). At issue also is the concern of too high or low calculated N recommendations when yields are much higher or lower than average. From this, some recommendations have shifted to approaches that don't use yield goal but instead utilize soil­ specific N recommendations based on soil productivity classification (Vanotti and Bundy, 1994a) or set ranges for specific rotations (Blackmer et al., 1997). This shift and diversity in recommendation approaches across the Midwest USA has raised questions about the reliability of the approaches currently in use and the appropriateness of N rate recommendations drived from them.

The intent of the work reported here is to analyze recent databases where optimal corn N rate was determined for many site-years, and to use results from that analysis to see whether or not recent research supports the development of an alternative N rate recommendation approach for use in Corn Belt states.