DairyMod and EcoMod: biophysical pasture-simulation models for Australia and New Zealand

Abstract

DairyMod and EcoMod, which are biophysical pasture-simulation models for Australian and New Zealand grazing systems, are described. Each model has a common underlying biophysical structure, with the main differences being in their available management options. The third model in this group is the SGS Pasture Model, which has been previously described, and these models are referred to collectively as ‘the model’. The model includes modules for pasture growth and utilisation by grazing animals, water and nutrient dynamics, animal physiology and production and a range of options for pasture management, irrigation and fertiliser application. Up to 100 independent paddocks can be defined to represent spatial variation within a notional farm. Paddocks can have different soil types, nutrient status, pasture species, fertiliser and irrigation management, but are subject to the same weather. Management options include commonly used rotational grazing management strategies and continuous grazing with fixed or variable stock numbers. A cutting regime simulates calculation of seasonal pasture growth rates. The focus of the present paper is on recent developments to the management routines and nutrient dynamics, including organic matter, inorganic nutrients, leaching and gaseous nitrogen losses, and greenhouse gases. Some model applications are presented and the role of the model in research projects is discussed.

Modelling the seasonal and geographical pattern of pasture production in New Zealand

“…2B). In the model, the effect of a high-temperature event on plant growth was incorporated as described by Johnson et al (2008). In this simulation, high temperature started to restrict plant growth when daily maximum temperature reached the onset temperature of 28 8C and to fully restrict plant growth when it reached 35 8C.…”

“…These changes included a dynamic partitioning rule for new plant growth between shoots and roots, which allocated less new growth to roots in spring (approximately during the plant reproductive period) and more in autumn, based on experimental evidence (Sheehy et al 1979;Parsons & Robson 1981). Water and N balance, as well as their deficit effects on plant growth, were calculated in a similar but different way to that used by Johnson et al (2008). Briefly, plant water demand was calculated using the PenmanÁMenteith approach (Micromet).…”

“…When pasture level GLF n B 1, an explicit partition of soil N among species was performed. The symbiotic N fixation (SNF) of legume species was estimated first based on the formulation of Schwinning & Parsons (1996) and Johnson et al (2008):…”

“…For these reasons, a new pasture module (AgPasture) was developed based on the plant physiological model described by Thornley & Johnson (2000) and as implemented in EcoMod (Johnson et al 2003. The performance of the EcoMod pasture model in simulating pasture growth rates has been extensively evaluated Johnson et al 2008;White et al 2008). Cullen et al (2008) showed that the model simulated the monthly growth patterns and seasonal yields of ryegrass-based pasture at various sites with reasonable accuracy, including the growth rates over 2 years at two sites in New Zealand.…”

Modelled effects of rising CO2 concentration and climate change on native perennial grass and sown grass-legume pastures

“…Here we used a detailed biophysical pasture simulation model (EcoMod; Johnson et al 2003Johnson et al , 2008 to forecast potential future states of grasslands in southeastern Tasmania. EcoMod uses simple empirical and mechanistic equations based on generally accepted biophysical relationships to describe processes within the pasture system, and is the only model, to our knowledge, that attempts to integrate all facets of the pasture system: soil water, soil nutrients, animal metabolism, pasture management and plant growth.…”