This increase had only a small effect on grain yield and did not increase soil residual N with up to 40 kg N ha 1 applied, except when N uptake was limited by low rainfall in the period after the flag leaf stage. A late N application at flag leaf stage increased grain protein content by several per cent. Nitrogen applications at tillering and the beginning of stem elongation further increased grain yield and in particular grain protein, but did not affect soil residual N, except in a year with low rainfall during stem elongation. Application of N fertiliser in February to increase soil mineral N to 140 kg N ha 1 improved the median yield to 7.8 t ha 1 but had little effect on grain protein concentration with a range of 8–10%. A median grain yield of 4.5 t ha 1 was achieved without applying fertiliser, utilising mineral soil N from previous seasons, from mineralisation and N deposition. In a simulation experiment, the APSIM Nwheat model was used with historical weather data to study the relationship between rate and timing of N fertiliser and grain yield, grain protein and soil residual N. Simulations and earlier regional yields differed, but they showed uniformity for the last decade. Simulations showed a good consistency with the higher yields of the long-term experiment, but overpredicted the lower yields. The model reproduced the general effects of N treatments on yields. Additionally, the model simulations were compared with grain yields from a long-term winter wheat experiment with different N applications, two additional N experiments and regional grain yield records. Grain yields (dry weight) and grain protein concentrations were well simulated with a root mean square deviation (RMSD) of 0.8 t ha 1 and 1.6 protein%, respectively.
The overall APSIM Nwheat model simulations of soil mineral N, N uptake, shoot growth, phenology, kernels m 2, specific grain weight and grain N were acceptable. The experiments covered two seasons and a range of N-fertiliser applications. Model outputs were compared with detailed measurements of field experiments from three locations with two different soil types. The model was applied to simulate above- and below-ground growth, grain yield, water and N uptake, and soil water and soil N of wheat crops in the Netherlands. APSIM Nwheat is a crop system simulation model, consisting of modules that incorporate aspects of soil water, nitrogen (N), crop residues, and crop growth and development.
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