Nitrogen (N) fertilizer is routinely applied to maize (Zea mays L.) many months prior to crop uptake, often in the fall of the year and in some instances during winter and early spring. Our objectives were to: 1) determine the degree of yield loss associated with the time of N application when applied monthly from October through April; 2) determine the environmental conditions under which this loss occurs; and, 3) develop functions to predict the amount of supplemental N needed to correct for N losses. Fertilizer N was surface broadcast as ammonium sulfate at monthly intervals (October to April) at a constant rate (134 kg N/ha) to ten site-year locations in Northern IL between 1997 and 1999. Seven sites (1998-1999) also had an N titration applied in April to assess its responsiveness to N, and to estimate the degree of loss from earlier applications. Although the sites varied in their magnitude of response to N, rates above 134 kg N generally did not increase yield, suggesting that little N loss occurred from April applications. Averaged over the ten site-years, N applied from October through January significantly (P<0.05) reduced grain yield compared to April, and yield generally increased as application time was delayed. Since yield reductions could be partially explained by the time of N application and by spring precipitation, functions were developed to predict the amount of supplemental N needed to produce “April-like” yields under such circumstances. Based on these findings we suggest producers avoid October applied N entirely, and delay November through January applications as long as possible.