The challenges e and some solutions e to process-based modelling of grazed agricultural systems q V.O. Snow a, * , C.A. Rotz b , A.D. Moore c , R. Martin-Clouaire d , I.R. Johnson e , N.J. Hutchings f , R.J. Eckard e a AgResearch e Lincoln, Private Bag 4749, Christchurch, Lincoln 8140, New Zealand b USDA-Agricultural Research Service, University Park, PA 16802, USA c CSIRO Sustainable Agriculture Flagship, GPO Box 1600, Canberra, ACT 2061, Australia d INRA, UR 875, MIAT, 31326 Castanet Tolosan, France e School of Land and Environment, University of Melbourne, Vic. 3010, Australia f Aarhus University, Department of Agroecology and Environment, Tjele, Denmark article info Article history: Received 22 October 2013 Received in revised form 26 January 2014 Accepted 26 March 2014 Available online 18 April 2014 Keywords: APSIM DairyMod DIESE FASSET GRAZPLAN IFSM Pastoral farm system SGS Pasture Model Simulation model abstract 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 simu- lation models and suggest that these subject areas should receive more attention. Ó 2014 Elsevier Ltd. All rights reserved. 1. Introduction Grasslands occupy 25% of the terrestrial surface (Lemaire et al., 2011) or about 70% of the world agricultural area (FAO, 2013). They contribute to the livelihoods of over 800 million people (Reynolds et al., 2005) and so have an important role to play in satisfying the increasing demand for high-quality protein (Steinfield et al., 2006). Given their extent, grasslands are a crucial system to consider when evaluating local or global issues related to sustainable management, especially in the face of on-going land- use changes and climatic uncertainty. At the scale of the individual farmer, efficient usage of the home-grown pasture resource is also important. For example, Dillon et al. (2008) summarised data from several countries and showed that the costs of milk production decreased as the proportion of grazed pasture in the diet increased. Also Peyraud (2011) and Rotz et al. (2009) found that increasing the use of grazed pastures on dairy farms could improve their envi- ronmental sustainability by reducing leaching or gaseous emissions and energy use. Although grazing-based farming systems are often considered to be relatively environmentally benign compared to many other farming types, they have been following the general trend of q Thematic Issue on Agricultural Systems Modeling & Software. * Corresponding author. E-mail address: Val.Snow@agresearch.co.nz (V.O. Snow). Contents lists available at ScienceDirect Environmental Modelling & Software journal homepage: www.elsevier.com/locate/envsoft http://dx.doi.org/10.1016/j.envsoft.2014.03.009 1364-8152/Ó 2014 Elsevier Ltd. All rights reserved. Environmental Modelling & Software 62 (2014) 420e436