Methodology for selecting segregating populations for improved N-use efficiency in bread wheat

While new lines developed by CIMMYT under intermediate levels of nitrogen are more nitrogen efficient than older lines, it is not known whether this method of selection is the most efficient. Two lines with high N up-take efficiency (UPE) were crossed to two lines with high N utilization efficiency (UTE). Their progenies were visually selected for superior agronomic type from the F-2 till the F-6 under each of five selection regimes. These were: always under low N (0 added) (LN), always under medium N (150 kg N ha(-1) added) (MN), always under high N (300 kg N ha(-1) added) (HN), alternating between low and high N starting with low N in the F-2 (ALN) and alternating between high and low N starting with high N in the F-2 (AHN). The resulting advanced lines (F-8) were tested in replicated yield trials under low, medium, and high levels of added N on an N deficient soil. The AHN selection regime resulted in the highest yields at intermediate and high N levels. The lowest yields occurred for ALN. Harvest Index (HI) did not change. Hence all effects on yield were the result of changes in biomass. Under low N all selection regimes showed progress in yield due to an improved biomass and HI, but with no differences among regimes. UPE expressed a higher correlation with yield and biomass than UTE under all levels of N.