Sensor-based diagnostic tools can improve nitrogen use efficiency (NUE) in maize improving system profitability. There is an optimal time to detect N deficiencies with sensor-based tools and to prescribe N rates obtaining high yield response and NUE. This time is established when more accurate sensing capability of detection is gained later in the season; but it is compensated with a reduction in the application window and yield response. Four experiments (EXP) were conducted during 2003-04 in Paraná, Argentina (31.5ºS). Experiments included combinations of N rates applied at planting (0, 70, 140 y 210 kg N ha-1) with: i) timing of late fertilization (LF), 70 kg N ha-1 at V8, V10, V12 and V14 in EXP1; and ii) N rates applied at V12 (0, 20, 40 and 80 kg N ha-1) with soybean (EXP2) or maize (EXP3) as previous crop. EXP4 included combinations of N rates (0, 69 and 138 kg N ha-1) at planting and V6. In EXP 1, LF outyielded controls irrespective of timing when N rates at planting were 0 or 70 kg N ha-1, whereas no yield differences of LF were found at higher N rates (i.e. interaction between factors was significant). Except to 210 kg N ha-1 at planting, NUE was higher than 50% in LF treatments. In EXP2 and EXP3, significant yield responses to LF were found irrespective of N rates at planting. Pooling all experiments, normalized difference vegetative index (NDVI) obtained from V8 to V14 was associated to N uptake and crop yield variations. Better relationships were obtained when NDVI readings at V8 and V14 were excluded. LF, in a wider than traditional window, showed a high yield response and NUE. Sensor-based N prescription tools will be validated and available for Argentinean farmers soon.