IRRIGATED SPRING WHEAT AND TIMING AND AMOUNT OF NITROGEN-FERTILIZER .1. GRAIN-YIELD AND PROTEIN-CONTENT

To improve N-use efficiency in wheat (Triticum aestivum L.), this study determined the latest stage of development at which supplemental N fertilizer would elicit a full grain yield response. Three experiments were carried out in 1986 with early-June-sown irrigated semidwarf spring wheats in a low fertility field at Griffith in New South Wales (34-degrees-S). N was supplied as NH4NO3 top dressings and crop management was otherwise optimal. The grain yield response to N given at seeding was very high (in the main experiment the initial slope was 38 kg grain/kg N, with 180 kg N/ha increasing yield from 1812 kg/ha to 7113 kg/ha). Single N dressings given in this and an adjacent second experiment indicated no significant decrease in yield response until dressings were delayed beyond the onset of stem elongation. There was no evidence of synergism between early and late split N dressings. The apparent N recovery in the crop at maturity ranged from 44-77% (mean 63%) and did not decline strongly until all N was given after stem elongation began. N-14 recovery in crop from labelled fertilizer applied to microplots ranged from 40 to 56% and increased slightly with later N dressings, while losses decreased. With delay of single N dressings, grain protein content only increased with dressings after the onset of stem elongation. Excluding those treatments where all N was given after this stage, protein responses were quantitatively related inversely to grain yield responses (r2=0.49), with negative responses accompanying the large yield responses ( > 3 5 kg grain/kg N) generated by initial increments of N. Examination in a third experiment of possible predictors of the optimum amount of supplemental N to be given at the onset of stem elongation suggest that stem base NO3-N concentration and culm number m-2 could serve this role.