Real-world recommendations: Do limits to validation constrain model usefulness?

Abstract: This paper describes efforts to validate a model of glyphosate resistance in awnless barnyard grass (Echinochloa colona (L.) Link.) in the sub-tropical northern grains region of Australia. Possible constraints to the perceived usefulness of the model due to hidden or sparsely documented system factors are discussed. The evolution of herbicide resistance in real weed populations becomes apparent only after years or decades of selection. Modelling is therefore particularly useful in investigating and describing the change in the weed population and the relative importance of different factors pushing towards or mitigating against that change. A key role of modelling in farming systems research is to improve understanding of issues such as the evolution of herbicide resistance. However, model results are increasingly important for formulating or adjusting recommendations, including for resistance prevention, to be used by land managers. Importantly, the research projects under which models of agricultural systems are produced are likely to include the development or refinement of recommendations for change as part of their expected outcomes. The comprehensiveness of model validation can be thought to equate generally to the strength of an argument in favour of a model’s hypotheses. By extension (and by convention), more comprehensive validation is often given to equate to a stronger argument in favour of practical recommendations that arise more or les