Direct selection for grain yield in breeding programs has led to indirect improvement in secondary
traits such as nitrogen use efficiency (NUE). NUE can be defined as grain production per unit of
nitrogen (N) available in the soil, including the fertilizer, and it has two components: nitrogen
recovery efficiency (NRE=kg N uptake/kg N available in the soil) and nitrogen internal efficiency
(NIE=kg grain /kg N uptake). An experiment was conducted at Balcarce (37º45’S, 58º18’W) during
the 2009/2010 growing season in order to compare and evaluate NUE, NRE and NIE of hybrids
released in different decades. Three hybrids (released in 1965, 1995 and 2010) were sown at
75000 pl ha-1 at two N levels: 0 and 150 kg ha-1 applied at sowing. Grain yield was determined at
physiological maturity. Total aboveground biomass and N concentration in the shoot was
measured at silking and at physiological maturity. Response to N fertilization was greatest in the
2010 hybrid and resulted in reductions in NUE and its components in all cultivars. NUE was
significantly greater for the 1995 and 2010 hybrids compared to the 1965 one, at both N
availabilities. NRE did not differ among hybrids during the vegetative stages. During grain filling,
however, there was a trend toward a higher NRE in the modern hybrid at low N availability. NIE
was superior for the 1995 and 2010 hybrids than for the 1965 one. Comparing the newest hybrid
with the oldest one, both without N fertilization, NRE during the complete growing cycle increased
35% whereas NIE increased 38%. At high N availability, differences between hybrids in NUE were
mostly explained by differences in NIE. Thus, hybrids released in different decades exhibited
differences in NUE, which were most closely related to differences in NIE than to NRE.