Nitrogen (N) is an essential and often limiting nutrient to plant growth. Maize grain yields are highly responsive to supplemental N, leading to annual application of an estimated 10 million metric tons of N fertilizer to the maize crop worldwide (FAO 2004). Nearly all cultivated maize in developed countries receives some form of N fertilizer and N use is increasing in developing countries, where its impacts on raising grain yields from nutrient-poor soils are greatest. The extensive use of N fertilizer not only increases crop input costs, but also can negatively impact soil, water and air quality at both local and ecosystem scales (Tilman et al. 2002). The manufacture of N fertilizer is an energy-intensive process that is becoming increasingly costly, due to the use of natural gas as both a reactant and heat source for the conversion of atmospheric N2 to anhydrous ammonia (NH3). For these reasons, reducing the amount of supplemental N used in maize production will have significant positive economic and environmental benefits to world agriculture. Nitrogen use efficiency (NUE) can be defined in a variety of ways that emphasize different components of the soil and plant system (reviewed in Good et al. 2004) or economic returns to fertilizer use. In cereal crops like maize, agronomic NUE is most simply expressed as the ratio of grain yield to N fertilizer supplied. Comparisons of maize grain yields and N fertilizer usage on a global basis lead to estimates of maize NUE ranging from 25–50% (Raun and Johnson 1999; Tilman et al. 2002), indicating that more than half the fertilizer N applied in maize crop production is lost to the environment. Thus, there is considerable opportunity for enhancing maize NUE.