Effects of nitrogenous fertilizer on the growth, grain yield and grain protein concentration of wheat

The responses of wheat to applications of nitrogenous fertilizer were examined between 1988 and 1990 at 10 sites in South Australia which were considered to be marginally deficient in N. Nitrogen rates ranged from 0 kg N/ha to 150 kg N/ha and the experiments were sown after a range of crops and pastures. Nitrogen often increased early crop vigour and subsequent vegetative growth but significant increases in grain yield occurred at three of the 10 sites only; at the remaining sites there was no significant response or there was a reduction in yield at the highest rates of N. Kernel weights fell and grain protein concentration increased at most sites as the rate of N increased. The total amount of N per kernel was relatively constant across the N treatments at each site and across the 10 sites it varied less than the starch content per kernel. Grain protein concentration therefore was affected more by the amount of starch deposited in the grain than by the total amount of nitrogen. The amount of dry matter remobilized post-anthesis, calculated from changes in dry weight, was high and at the majority of sites was increased with applications of nitrogenous fertilizer. Despite the generally large amount of dry matter remobilized, this appeared to be used inefficiently during grain filling and there was little evidence that it greatly contributed to grain growth and grain protein concentration. The relationship between starch content per kernel and N content per kernel varied between sites: in some cases starch and N were negatively correlated, while in other instances there was a positive correlation or no correlation. The data suggest that high grain protein concentration at high levels of N are not a direct consequence of increased mobilization of dry matter and greater translocation of N to the grain. Dry matter production at anthesis was correlated with the amount of growth after 10 weeks but generally this increased dry matter production was of no benefit to yield. It is concluded that in the medium rainfall areas of the state, there is no advantage to be gained from improved early vigour, except perhaps where poor early growth is due to inadequate management.

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