EVIDENCE FOR DIFFERENCES BETWEEN WINTER-WHEAT CULTIVARS IN ACQUISITION OF SOIL MINERAL NITROGEN AND UPTAKE AND UTILIZATION OF APPLIED FERTILIZER NITROGEN

The response of cultivars to applied nitrogen was examined in 11 seaso ns, 1982-92, in two experiments per year, normally testing seven culti vars at seven rates of fertilizer nitrogen. In all, 27 cultivars were tested in 22 experiments throughout Nottinghamshire, Lincolnshire, Nor thamptonshire and Suffolk. Cultivars ranged in their date of introduct ion from Maris Huntsman (1969) to Hereward (1988). For each cultivar i n each experiment, the economic optimum yield (Y-opt), the amount of f ertilizer N needed to produce it (N-opt), the grain % N at N-opt, the offtake of N in the grain at nil N (N-off(N0)) and N (N-off(opt)) and the estimated recovery of fertilizer in the grain at N-opt (AFR(opt)) were estimated by fitting linear plus exponential curves to data for g rain yield and two-straight-line models to data for grain N offtake. F rom cross-site analysis, normalized cultivar means were calculated for each variate. Over the 20-year period relating to the cultivars in th e trial, the contribution of new genotypes to grain yield improvement was 1.92 t/ha, Y-opt increasing by 96 kg/ha per year. There was no cha nge in grain % N at N-opt. The effect of changes through breeding from 1969 to 1988 was to increase N-off(opt) by 42 kg/ha (2.1 kg/ha per ye ar), that was associated with a decrease in N-off(N0) (equivalent of s oil N offtake) of 15 kg/ha (0.77 kg/ha per year). Part of the increase d requirement for fertilizer N was fulfilled by an increase in AFR(opt ) of 18% over the 20-year period. The net effect was for N-opt itself to increase by 56 kg/ha (2.8 kg/ha per year). Since survey evidence in dicates no general increase in N use on wheat by farmers since the mid -1980s, it appears that current fertilizer use by farmers may be under estimating the requirement for N now. Alternatively in previous years N requirements may have been overestimated. The change in N available for loss to the environment, from the balance of grain N-off(opt) and N-opt, was from 11 kg N/ha in 1969 compared to 25 kg N/ha in 1988. It seems possible that the potential increase in nitrate levels in ground water associated with plant type may not have been realised because fa rmers have conserved the amount of N they use.