Euphytica 119: 223–230, 2001. © 2001 Kluwer Academic Publishers. Printed in the Netherlands. 223 Methodology for selecting segregating populations for improved N-use efficiency in bread wheat M. Van Ginkel, I. Ortiz-Monasterio, R. Trethowan & E. Hernandez International Maize and Wheat Improvement Center (CIMMYT), Mexico Key words: Nitrogen, NUE, N-uptake efficiency, N-use efficiency, UPE, UTE, wheat Abstract 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. Introduction Research has shown that modern semi-dwarf cultivars respond more to available nitrogen than the old, tall cultivars, which translates into higher returns to farm- ers (Ortiz-Monasterio et al., 1997). In addition, semi- dwarf wheats do not necessarily require more nitrogen than older cultivars at lower levels of fertility (Jain et al., 1975; Fischer, 1981; Austin et al., 1993; Ortiz- Monasterio et al., 1997). At CIMMYT, the Bread Wheat Breeding program has selected breeding lines under intermediate nitrogen levels for many years. The emanating lines have shown improved levels of N up- take efficiency (UPE), N utilization efficiency (UTE), and N use efficiency (NUE) (Ortiz-Monasterio et al., 1997). However, it is unclear whether this method of managing segregating populations is effective in identifying the most nitrogen use efficient wheats. NUE, defined by Moll et al. (1982) as grain yield per unit of available N (from the soil and/or fertilizer), can be partitioned into two key components. UPE, the ability of the crop to extract the nutrient from the soil, is defined as plant N per unit of available N (either soil + fertilizer N or only fertilizer N), and UTE, the capacity of the plant to convert the already absorbed N into grain yield, as grain yield per unit of N in the plant. NUE is the product of these two components (1). Nutrient use efficiency (NUE) = Uptake efficiency (UPE) × Utilization efficiency (UTE) Grain yield Nutrient supplied = Total nutrient in the plant Nutrient supplied × Grain yield Total nutrient in the plant (1) UTE can again be divided into two sub-components (Moll et al., 1982) (2). UTE = Harvest Index × Biomass Production Effi- ciency Grain yield Total nutrient in the plant = Grain yield Total plant biomass × Total plant biomass Total nutrient in the plant (2) Dhugga and Waines (1989) examined UPE and UTE in both bread and durum wheats at three levels