A ‘small strip’ approach to empirically determining management class yield response functions and calculating the potential financial ‘net wastage’ associated with whole-field uniform-rate fertiliser application

Abstract Site-specific crop management requires site-specific information. The amount and pattern of variability in natural resources and production output, the reasons for variability, the agronomic implications and the management opportunities ultimately need to be identified locally. In this context it is often desirable to obtain information on any variability in production response to different application rates of inputs across a site in order to help identify if worthwhile financial gains can be made by implementing spatially variable application rates. However there has been little development and promotion of practical, considered experimental design and analysis techniques that can be applied at a commercial farm scale to test variability in optimal input rates. Here a stratified, randomised, replicated ‘small strip’ experimental design is proposed for use in commercial-scale cropping systems that employ a management class approach. The experimental design considers technical, economic, agronomic and non-spatial statistical constraints and seeks to find a compromise between these constraints such that any one constraint is not limiting to adoption or interpretation. A description and rationale for the experimental design and analysis is presented followed by the results from the application of this experimental design to 15 nitrogen and 18 phosphorus response trials on various types of crops, in 3 different regions of Australia, over 6 seasons. Under these conditions, the results show that optimum response data can be obtained using the experimental technique. A gross margin analysis for each experiment was used to calculate the ‘net wastage’ in fertiliser and yield resulting from the customary, spatially uniform input management. A median ‘wastage’ of A$39/ha for nitrogen fertiliser and A$48/ha for phosphorus fertiliser was observed. Standardising the individual results to the total, seasonal cost of uniform fertiliser application in each experiment, determined that the median ‘net wastage’ value equated to 99% of the traditional fertiliser budget for phosphorus and 97% for nitrogen. These figures suggest that the potential financial benefit to be gained over a number of seasons by knowing more about the optimum rates of fertiliser for management classes within a field could be equal to the amount of money traditionally outlaid on fertiliser. The technique does have some statistical limitations that require a practical, agronomic interpretation to be included in an assessment of the results. Although applied to nutrient response experiments in this study, it could be adapted for any crop input or management practice experiment.

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