The challenges - and some solutions - to process-based modelling of grazed agricultural systems

Pastoral systems are characterised by a number of features that are absent in arable cropping systems. These features include: (i) pastures are biologically diverse so interactions between plant species must be considered; (ii) economic return requires the inclusion of the animal as an additional trophic level; (iii) interaction between the grazing animal and the pasture is complex, influenced by the environment, plant species and animal behaviour and this creates feedbacks that can result in vicious cycles; (iv) animals spatially transfer substantial amounts of nutrients both randomly and systematically and this creates or exacerbates soil variability; and (v) whole farm management is both more complex and more important to system function in grazed compared to arable systems and it is harder to capture in simulation models. These challenges complicate the process-based modelling of pastoral systems and present significant obstacles to model developers and users. Here we discuss these challenges, describe the range of solutions used by different models and discuss the strengths and weaknesses of these solutions. We have placed particular emphasis on the analysis of a range of possible solutions with the point of view that diversity between and within models is important to provide the flexibility needed for future uses. We find that for most challenges there is a diversity of solutions incorporated into the models and that there is the potential to capture additional diversity, if needed, from other models. We note an apparent lack of development in the modelling of extreme events such as very high temperatures, systematic animal-mediated nutrient transfers, pests, weeds and gene-environment interactions in pastoral simulation models and suggest that these subject areas should receive more attention. 2014 Elsevier Ltd. All rights reserved.