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

While new lines developed by CIMMYT under intermediate levels ofnitrogen are more nitrogen efficient than older lines, it is not knownwhether this method of selection is the most efficient. Two lines with highN up-take efficiency (UPE) were crossed to two lines with high N utilizationefficiency (UTE). Their progenies were visually selected for superioragronomic type from the F₂ till the F₆ under each of fiveselection regimes. These were: always under low N (0 added) (LN), alwaysunder medium N (150 kg N ha^-1 added) (MN), always under highN (300 kg N ha^-1 added) (HN), alternating between low and highN starting with low N in the F₂ (ALN) and alternating between highand low N starting with high N in the F₂ (AHN). The resultingadvanced lines (F₈) were tested in replicated yield trials under low,medium, and high levels of added N on an N deficient soil. The AHNselection regime resulted in the highest yields at intermediate and high Nlevels. The lowest yields occurred for ALN. Harvest Index (HI) did notchange. Hence all effects on yield were the result of changes in biomass.Under low N all selection regimes showed progress in yield due to animproved biomass and HI, but with no differences among regimes. UPEexpressed a higher correlation with yield and biomass than UTE under alllevels of N.