From use efficiency to effective use of nitrogen: A dilemma for maize breeding improvement.

Maize (Zea mays L.) breeding is continuously moving forward yield gains for many fields crops, increasing dependency to technology such as high input seed costs and high use of nitrogen (N) fertilizers. For this crop, breeding improvement led to concomitantly enhancing N recovery and uptake but following a similar ratio relative to the plant biomass (W) and nitrogen nutrition index (NNI, as actual to critical N concentration) levels. The aim of this review is to provides new insights related to the true gains of N use efficiency (NUE) for maize over time and to propose new direction to target improvement on the effective use of N. Thus, the increase in fertilizer N for modern hybrids to attain greater yields lead to a greater dependency to N fertilization and potentially increasing the overall environmental risks for N losses associated to this practice. Contrarily to the improvement based on NUE, improving the intrinsic N uptake capacity (more N uptake per unit of biomass) is needed to maximize yield and the effective use of N. These results highlight that crop breeding should refocus to directly target an increase on the effective use of N, increasing the efficiency on using environmental resources while seeking for improving attainable yields. SYNOPSIS: Enhancing resilience of our production systems is critical for food security goals. This review highlights the need of investment on directly targeting improvement of the effective use of N not only to improve efficiency but to reduce the dependency to fertilization and environmental risks.