Responses of grain yield, biomass and harvest index and their rates of genetic progress to nitrogen availability in ten winter wheat varieties

Increased yields in winter wheat cultivars have been found to be largely attributable to improved partitioning of biomass to the grain, i.e., higher harvest index. However, there is a biological upper limit to harvest index and therefore breeders need to exploit increased biomass production as the mechanism by which yields are increased. Evidence for improved biomass was sought in experiments conducted over three years (1994 to 1996), at the Plant Testing Station, Crossnacreevy, near Belfast, with 10 varieties of winter wheat introduced over the period 1977 to 1991. Variation in grain yield was more strongly associated with variation in biomass (an increase of 0.78 t/ha in grain yield at 85% dry matter (DM) per I t/ha increase in biomass at 100% DM; R-2 = 0.71) than in harvest index (an increase of 0.1 t/ha at 85% DM per percentage point increase in harvest index; R-2 = 0.36). When age (= year of first harvest in UK National List trials) of the varieties was taken into account, yield (0.037 tha(-l)y(-1); R-2 = 0.42) and biomass (0.034 tha(-l)y(-1); R-2 = 0.31), but not harvest index (0.34%/year; R-2 = 0.001), increased as year increased. Genetic gain in yield was smaller without fertiliser N (0.021 tha(-1) y(-1); R-2 = 0.21) and at 40 kg ha N (0.025 tha(-l)y(-1); R-2 = 0.25) than at 215-250 kg/ha N (0.065 tha(-l)y(-1); R-2 = 0.39). Theoretically, if the maximum biomass (18.60 t/ha for Rialto), could have been combined with the maximum harvest index (55.3%) in Riband, yield would potentially have been increased by 2.5 t/ha compared with yields for either variety.