Organic spring wheat (Triticum aestivum L.) breeding programs have been initiated, yet yield efficiency and N economy research is limited. We evaluated the performance of advanced lines selected from an organic breeding program initiated in 2003. Fourteen F₈ and F₉ lines in 2009 and 11 lines in 2010 were compared with commercial (check) cultivars. Field experiments were conducted under organic management at four site–years in Manitoba and Saskatchewan. Combined analysis showed no difference in biomass accumulation between organic lines and check cultivars; however, harvest index and grain yield were greater in organic lines compared with the checks. Organic lines were shorter than check cultivars, but yield efficiency, defined as kernel number per unit of crop biomass at anthesis, was higher (P < 0.05). Kernel mass was also greater for organic lines. Biomass N uptake was similar for organic lines and check cultivars, although total uptake of N into grain was greater for organic lines. The average grain protein content of organic lines was significantly lower than the check cultivars. This study demonstrated that improved yield under organic management was because of better assimilate partitioning, both at anthesis and crop maturity, for organically selected genotypes.