The Sustainable Grazing Systems Pasture Model: Description, philosophy and application to the SGS national experiment

A biophysical simulation model (the Sustainable Grazing Systems Pasture Model) was developed as an integral part of the Sustainable Grazing Systems National Experiment. It was developed to meet the needs of the researchers both for analysing data and processes at individual sites, and for simulating the outcome of these processes operating in generic pasture systems on a range of soil types, under specific grazing managements. The model was designed to reside on the desktops of individual researchers and for those researchers to be part of its development process. The Sustainable Grazing Systems Pasture Model incorporates the following: a physiological model of pasture species herbage accumulation in response to climatic conditions; the water balance including evapotranspiration, runoff (surface and subsurface), infiltration and drainage; pasture utilisation by grazing animals; a metabolisable energy-based animal growth model; and organic matter and inorganic nutrient dynamics (for nitrogen, phosphorus, potassium and sulfur) including plant uptake, adsorption, leaching, nitrogen fixation by legumes, and atmospheric nitrogen losses. A range of grazing options (set-stocking, rotational grazing and continuous grazing at a variable rate) is available for ewes and lambs, and wethers. Each of the main modules (water, nutrients, pastures and animals) is interconnected. To avoid bias in the influence of any one module, each is described at about the same level of complexity, with the description of any process being restricted to about 5 parameters. The model is hierarchical in structure and most processes are described in terms of a series of fluxes (or, more specifically, flux densities) that have dimensions of amount per area per time. The model can be closely linked to a database specifically developed for the Sustainable Grazing Systems National Experiment to allow easy importing and exporting of climate and experimental data for comparison with model output. This paper gives an overview of the model structure and its output, the process that was used for its development within Sustainable Grazing Systems, and its use by the Sustainable Grazing Systems sites and themes. Comments are provided on the implementation of the development process to assist future programs using a similar approach.